Dr. Jeff Masters' WunderBlog

Little Change to 97L in Central Caribbean

By: JeffMasters, 12:16 PM GMT on September 30, 2013

A low pressure system (Invest 97L) over the Central Caribbean is generating heavy rains from Jamaica to the coast of Panama, and is moving northwest at about 8 mph. Satellite loops show 97L has a modest area of disorganized heavy thunderstorms. The broad area of spin 97L had on Sunday is less apparent today. Dry air covers the Northwest Caribbean, and this dry air is slowing development. Ocean temperatures are a very warm 29°C (84°F), and wind shear has fallen since Sunday, and is now a moderate 10 - 20 knots.


Figure 1. Latest satellite image of Invest 97L.

Forecast for 97L
Marginally favorable conditions for development are expected for the next five days, according to the latest SHIPS model forecast, with moderate levels of wind shear (10 - 20 knots), but an atmosphere that will grow drier as 97L approaches the Northwest Caribbean and Southeast Gulf of Mexico. None of the reliable computer forecast models develop 97L into a tropical depression over the next five days. These models (the GFS, ECMWF, and UKMET) predict a northwesterly track for 97L over the next three days, bringing the center into the Yucatan Channel between Mexico's Yucatan Peninsula and Western Cuba by Thursday. On this path, 97L will bring heavy rains of 2 - 4" to Jamaica and Southwest Haiti on Monday, to the Cayman Islands and Central and Eastern Cuba on Tuesday and Wednesday, and to Western Cuba and Mexico's Yucatan Peninsula on Wednesday and Thursday. It is uncertain if 97L will then turn northeast and affect Florida late in the week, or stay in the Central Gulf of Mexico and head north into Louisiana. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 97L 2-day odds of development of 10%, and 5-day odds of 30%.


Figure 1. Latest satellite image of TD 11.

Little change to Tropical Depression 11 in the Central Atlantic
Tropical Depression Eleven continues to wander slowly across the Central Atlantic, far from land. Wind shear from an upper-level just northwest of TD 11 is currently keeping the storm disorganized, with just a small area of heavy thunderstorms displaced from the center of circulation. However, the storm is more organized than on Sunday, and just a small reduction in shear should allow TD 11 to intensify into Tropical Storm Jerry. TD 11 is not a threat to any land areas.

Jeff Masters

Hurricane

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97L in Caribbean Will Spread Heavy Rains to Jamaica and Cuba by Monday

By: JeffMasters, 3:59 PM GMT on September 29, 2013

A low pressure system (Invest 97L) over the Central Caribbean is generating heavy rains from Haiti to Panama, and is moving northwest at about 7 mph. Satellite loops show 97L has a modest area of disorganized heavy thunderstorms and a broad area of spin. Dry air covers the Northwest Caribbean, and this dry air is slowing development. Ocean temperatures are a very warm 29°C (84°F), and wind shear is a high 20 knots.


Figure 1. Latest satellite image of Invest 97L.

Forecast for 97L
Marginally favorable conditions for development are expected for the next five days, according to the latest SHIPS model forecast, with moderate levels of wind shear (10 - 20 knots), but an atmosphere that will grow drier as 97L approaches the Northwest Caribbean and Southeast Gulf of Mexico. None of the reliable computer forecast models develop 97L into a tropical depression over the next five days. These models (the GFS, ECMWF, and UKMET) predict a northwesterly track for 97L over the next three days, followed by a more northerly track over Western Cuba and just west of the Florida Keys late in the week. On this path, 97L will bring heavy rains of 2 - 4" to Jamaica and Southwest Haiti on Sunday and Monday, to the Cayman Islands and Central and Eastern Cuba on Monday and Tuesday, to Western Cuba on Tuesday and Wednesday, and to South Florida on Wednesday and Thursday. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave 97L 2-day odds of development of 20%, and 5-day odds of 40%.


Figure 2. MODIS satellite image of TD 11, taken at 10:30 am EDT on September 29, 2013. Image credit: NASA.

Tropical Depression 11 develops in the Central Atlantic
Tropical Depression Eleven formed from a tropical wave in the Central Atlantic on Saturday night, but the storm is far from land and is not likely to threaten any land areas. Wind shear is currently keeping TD 11 disorganized, with just a small area of heavy thunderstorms displaced from the center of circulation. By Monday, the upper-level low bringing the shear should move away, allowing TD 11 to intensify into Tropical Storm Jerry.

Jeff Masters

Hurricane

Updated: 7:52 PM GMT on September 29, 2013

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Landmark 2013 IPCC Report: 95% Chance Most of Global Warming is Human-Caused

By: JeffMasters, 10:50 AM GMT on September 27, 2013

"Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased." Thus opens the landmark 2013 Intergovernmental Panel on Climate Change (IPCC) report issued today. Working without pay, hundreds of our most dedicated and talented climate experts have collaborated over a six-year period to create the most comprehensive and authoritative scientific document on climate change ever crafted. The first 31 pages of what will be a 4,000-page tome was released this morning after an all-night approval session that stretched until 6:30 this morning in Stockholm, Sweden. This "Summary For Policymakers" lays out a powerful scientific case that significant climate change with severe impacts is already occurring, humans are mostly responsible, the pace of climate change is expected to accelerate, and we can make choices to cut emission of carbon dioxide and other heat-trapping gases that will limit the damage.

Q: How much has the planet warmed, and what has caused the warming?
The report documents that Earth's surface temperature warmed by 0.85°C (1.5°F) between 1880 - 2012. Two-thirds of this warming (0.6°C, 1.1°F) came after 1950. Human-emitted heat-trapping gases likely were responsible for 0.5 - 1.3°C of this post-1950 warming, while human-emitted aerosol particles reflected away sunlight and likely caused cooling (-0.6° - 0.1°C change in temperature.) Climate change due to variations in solar energy, volcanic dust, and natural sources of heat-trapping greenhouse gases were likely responsible for a small -0.1° - 0.1°C change in temperature since 1950. The sun was in a cool phase between 1978 - 2011, and the report estimates that lower solar output cooled Earth's climate slightly during this period. The influence of cosmic rays on climate over the past century was to weak to be detected, they said. In short, the report shows little support for a significant natural component to global warming since 1950. In fact, natural effects may well have made Earth cooler than it otherwise would have been. The report says that "The best estimate of the human-induced contribution to warming is similar to the observed warming over this period." In other words, close to 100% of the observed warming is due to humans.


Figure 1. The changing view of the IPCC's assessment reports on the human contribution to climate change.

Q: How have the IPCC reports changed through time?
1990: The report did not quantify the human contribution to global warming.

1995: "The balance of evidence suggests a discernible human influence on climate."

2001: Human-emitted greenhouse gases are likely (67-90% chance) responsible for more than half of Earth's temperature increase since 1951.

2007: Human-emitted greenhouse gases are very likely (at least 90% chance) responsible for more than half of Earth's temperature increase since 1951.

2013: Human-emitted greenhouse gases are extremely likely (at least 95% chance) responsible for more than half of Earth's temperature increase since 1951. This is the same confidence that scientists have in the age of the universe, or that cigarettes are deadly, according to an excellent AP article published this week by Seth Borenstein.

Q: Did the new report change the plausible range of global warming?
A. Yes. The "climate sensitivity" is defined as how much the planet would warm if the amount of atmospheric CO2 doubled. A variety of studies have arrived at very different estimates of the exact CO2 sensitivity of the climate, and the 2007 IPCC report gave a range of the most plausible values: 2 to 4.5ºC, with 3ºC deemed the most likely value. Recent research indicates that a sensitivity as low as 1.5ºC may be possible, so the IPCC widened the range of the most plausible values: 1.5 to 4.5ºC. The new lower limit of 1.5ºC is a best-case scenario that appears no more likely than the high end of 4.5ºC. Furthermore, even the lowest sensitivity scenario would not negate the need for emissions reductions. Current trends show that emissions are on track to increase far beyond doubling, which would create dangerous temperature rise even in a low-sensitivity climate. (Note that they give a small but worrisome possibility--0 to 10% chance--that the climate could warm by more than 6ºC for a doubling of CO2.)


Figure 2. Average of NASA's GISS, NOAA"s NCDC, and the UK Met Office's HadCRUT4 monthly global surface temperature departures from average, from January 1970 through November 2012 (blue), with linear trends applied to the time frames Jan '70 - Oct '77, Apr '77 - Dec '86, Sep '87 - Nov '96, Jun '97 - Dec '02, Nov '02 - Nov '12. Climate change skeptics like to emphasize the shorter term fluctuations in global temperatures (blue lines) and ignore the long-term climate trend (red line.) The global surface temperature trend from January 1970 through November 2012 (red line) is +0.16°C (+0.29°F) per decade. Image credit: skepticalscience.com.

Q: What does the IPCC say about the "speed bump" in surface global warming over the past 10 - 15 years?
Much attention has been given in the press to the fact that the rate of surface warming over the past fifteen years has been slower than during previous decades. The report notes that due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over the past 15 years (1998–2012) of 0.05 °C per decade, which begins with a strong El Niño, is smaller than the rate calculated since 1951 of 0.12 °C per decade. However, the recent slow-down in surface warming is likely to be a mere "speed bump" on the highway of global warming, caused by natural variability. We have seen such "speed bumps" before, as well as short, sharp downhill stretches where surface warming speeds up. For example, climate scientist Stefan Rahmstorf writes at realclimate.org that "the warming trend of the 15-year period up to 2006 was almost twice as fast as expected (0.3°C per decade), and (rightly) nobody cared. We published a paper in Science in 2007 where we noted this large trend, and as the first explanation for it we named “intrinsic variability within the climate system”. Which it turned out to be." Physics demands that the massive amounts of heat-trapping carbon dioxide humans have dumped into the atmosphere must cause significant warming, but the chaotic complexity of the system is expected to obscure the magnitude of the long-term trend on time scales of a few years to a decade. The attention being to this latest "speed bump" on the highway of global warming is a direct result of a well-funded PR effort by the fossil fuel industry. One has to look at the total warming of the atmosphere, oceans, land, and ice to judge the true progress of global warming, not just the surface temperature. There has been no slowdown in total global warming when we regard this entire system, as I argued in a post earlier this year. More than 90% of the energy of global warming goes into the oceans, and the reason for the relative lack of surface warming this decade is that more heat than usual is being stored in the oceans. That heat will be released to the atmosphere at some point, removing the "speed bump".

The new IPCC report says that there is medium confidence that the "speed bump" in surface warming is due in roughly equal measure to natural multi-year unpredictable variability in the weather, and to changes in the amount of sunlight reaching the surface due to volcanic eruptions and the downward phase of the current solar cycle. Most of the climate models do not reproduce this lower surface warming rate during the past 10 - 15 years. There is medium confidence that this difference between models and observations is due to natural climate variability that is impossible to predict (for example, the El Niño/La Niña cycle), with possible contributions from the models' inadequate handling of volcanic eruptions, changes in solar output, and changes in light-reflecting aerosol particles, and, in some models, a too-strong response to heat-trapping gases. For an explanation of why arguments about the global warming “slowdown” are misleading and should not offer any consolation, see this explainer from Skeptical Science and this one from the Union for Concerned Scientists.

Q: What does the IPCC say about drought?
A: Drought and reduction in water availability due to decreased mountain snow and ice is the greatest threat civilization faces from climate change, since it attacks the two things we need to live--water and food. Unfortunately, the report makes no mention of drought in the text, and we will have to wait for the March 2014 release of the "impacts" portion of the report to hear more about the threat drought poses to society. Today's report does mention drought in one of their two tables, giving “low confidence”--a 20% chance--that we have already observed a human-caused increase in the intensity and/or duration of drought in some parts of the world. This is a reduction in confidence from the 2007 report, which said that it was more likely than not (greater than 50% chance.) However, the forecast for the future is the same as in the 2007 report: we are likely to see dry areas get dryer due to human-caused climate change by 2100. In particular, there is high confidence (80%) in likely surface drying in the Mediterranean, Southwest U.S., and Southern Africa by 2100 in the high-end emissions scenario (RCP8.5), in association with expected increases in surface temperatures and a shift in the atmospheric circulation that will expand the region of sinking air that creates the world's greatest deserts.

Q: What does the IPCC say about sea level rise?
A: Global average sea level has risen 7.5" (19 cm) since 1901. Sea level has accelerated to 1.5" (3.2 cm) per decade over the past 20 years--nearly double the rate of rise during the 20th century. The report projects that sea level will rise by an extra 0.9 - 3.2' (26 to 98 cm) by 2100. While the maximum sea level rise expected has gone up since the 2007 report, when the IPCC did not even consider melt from Greenland and Antarctica because of the primitive state of glacier science then, the new upper bound (3.2') is still is a very conservative number. IPCC decided not to include estimates from at least five published studies that had higher numbers, including two studies with rises of 2 meters (6.6 feet.) This is in contradiction to NOAA's December 2012 U.S. National Climate Assessment Report, which has 6.6 feet (2.0 meters) as its worst-case sea level rise scenario for 2100. Even this number may be too low; at a presentation Thursday in New York City for Climate Week, glaciologist Dr. Jason Box, who knows as much about Greenland's ice sheets as any person alive, explained that Greenland's contribution to global sea level rise doubled over the past ten years. If Greenland's melt rate continues to double every ten years until 2100, Greenland alone will contribute 4.6' (1.4 meters) of global sea level rise, he said. If the doubling time becomes every nine years, then Greenland will cause 16.4' (5 meters) of sea level rise by 2100. His best-guess number for global sea level rise by 2100 is 4.7' (1.5 meters), but warns that our models used to predict melting of ice of Greenland have large unknowns.

Long-term sea level rise is expected to be much greater. The IPCC report states with "very high confidence" that 119,000 - 126,000 years ago, during the period before the most recent ice age, sea levels were 16 - 33 feet (5 - 10 meters) higher than at present. Melting of Greenland "very likely" contributed 1.4 - 4.3 meters of this rise, with additional contributions coming from Antarctica. Temperatures at that time weren't more than 2°C warmer than "pre-industrial" levels during that period. Two of the four scenarios used for the report project we will exceed 2°C of warming by 2100, with "high confidence", raising the possibility that we could see sea level rises of many meters over time scales of 1,000 years or so. The report expects sea level rise reach 3.3 - 9.8' (1 - 3 meters) by 2300, assuming CO2 levels rise above 700 ppm (close to what the higher-end RCP6.0 scenario prescribes.)

Q: What does the IPCC say about ocean acidity?
A: The world's oceans have seen a 26% increase in acidity since the Industrial Revolution, as the average pH has dropped from 8.2 to 8.1. Under all report scenarios, the acidification of the world's oceans will increase, with the pH falling by another 0.06 - 0.32 units. According to a 2012 study in Science, the current acidification rate is likely the fastest in 300 million years, and "may have severe consequences for marine ecosystems."

Q: How about hurricanes?
A: The new report gives “low confidence”--a 20% chance--that we have observed a human-caused increase in intense hurricanes in some parts of the world. This is a reduction from the 2007 report, which said that it was more likely than not (greater than 50% chance.) The IPCC likely took note of a landmark 2010 review paper, "Tropical Cyclones and Climate Change", authored by ten top hurricane scientists, which concluded that the U.S. has not seen any long-term increase in landfalling tropical storms and hurricanes, and that "it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes." The 2013 IPCC report predicts that there is a greater than 50% chance (more likely than not) that we will see a human-caused increase in intense hurricanes by 2100 in some regions; this is a reduction from the 2007 report, which said this would be likely (66% chance or higher.)

Q: How about extreme weather events?
"Changes in many extreme weather and climate events have been observed since about 1950. It is very likely that the number of cold days and nights have decreased and the number of warm days and nights has increased on the global scale. It is likely that the frequency of heat waves has increased in large parts of Europe, Asia, and Australia. There are likely more land regions where the number of heavy precipitation events has increased than where it has decreased. The frequency or intensity of heavy precipitation events has likely increased in North America and Europe." The report made no mention of tornadoes and severe thunderstorms, since the uncertainties of how they have behaved in the past and how climate change might affect them in the future are too great.

Q: What does the IPCC say about a "Day After Tomorrow" scenario?
A: In the disaster movie "The Day After Tomorrow", the Atlantic Meridional Overturning Circulation (AMOC)--the ocean current system of which the Gulf Stream Current is a part of--collapses, causing a rapid and extreme change in climate. A collapse of the AMOC is very unlikely (0 - 10% chance) before 2100 according to the report, but cannot be ruled out beyond the 21st century. A weakening of the AMOC by about 11 - 34% by 2100 is expected in the moderate RCP4.5 scenario, where CO2 levels reach 538 ppm in 2100. However, these odds assume that Greenland will dump a relatively modest amount of fresh water into the North Atlantic by 2100. If the higher-end sea level rise estimates that the IPCC did not consider as plausible come true, the AMOC will likely slow down much more, with a higher chance of collapse this century. No slow-down in the AMOC has been observed yet, according to the report.

Commentary
As I read though the report, digesting the exhaustive list of changes to Earth's atmosphere, oceans, and ice that have occurred over the past few decades, I was struck by how the IPCC report reads like lab results from a sick hospital patient. The natural systems that civilization depends upon to thrive have been profoundly disturbed, and the forecast for the future reads like a medical diagnosis for an overweight smoker with a heart condition: unless the patient makes major lifestyle changes, the illness will grow far worse, with severe debilitation or death distinct possibilities. We can and we must make the huge effort to turn things around. Oil and natural gas are the energy technologies of the 20th century. Coal is the energy technology of the 19th century. We have countless innovative and dedicated people ready to move us to the energy technology of the 21st century; I heard three of them speak last night at the Climate Week event I am at, and they really gave me some needed hope that we can turn things around. We must elect new leaders and pressure our existing leaders to take the strong actions needed to advance us into a new, 21st century energy economy. You can all help make it so!

Jeff Masters

Climate Change

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New Blockbuster IPCC Climate Report: Comprehensive, Authoritative, Conservative

By: JeffMasters, 11:31 AM GMT on September 26, 2013

Comprehensive. Authoritative. Conservative.
Those words summarize the world's most rigorous and important scientific report in history: the 2013 Intergovernmental Panel on Climate Change (IPCC) climate assessment, due to be released at 4am EDT Friday in Stockholm, Sweden. The Nobel Prize-winning IPCC has put together an amazingly authoritative and comprehensive report on a subject crucial to the future of civilization, a report that will guide policymakers worldwide as they struggle to cope with the growing chaos generated by the Great Climate Disruption that is already upon us. The first 31 pages of the report, called the "Summary For Policymakers", is what will be released Friday, and this summary will lay out a powerful scientific case that significant climate change with severe impacts is already occurring, humans are mostly responsible, the pace of climate change is expected to accelerate, and we can make choices to cut emissions of carbon dioxide and other heat-trapping gases to limit the damage.

Q: What is the IPCC?
A: In 1988, 300 scientists and high-ranking government officials at an international conference convened by the United Nations Environmental Programme (UNEP) and the World Meteorological Organization (WMO) concluded that changes in the atmosphere due to human pollution “represent a major threat to international security and are already having harmful consequences over many parts of the globe.” Immediate action was needed, they said, to negotiate a set of strict, specific targets for reducing greenhouse gas emissions. But who should coordinate such an effort? The conservative Reagan Administration and some other governments were wary of control by any group that was part of the United Nations structure. These governments proposed formation of a new, fully independent group under the direct control of representatives appointed by each government—that is, an intergovernmental panel. Responding to this pressure, the WMO and UNEP collaborated in creating the Intergovernmental Panel on Climate Change (IPCC) in 1988. The IPCC was neither a strictly scientific nor a strictly political body, but a unique hybrid. It could issue reports only with the firm agreement of a great majority of the world’s leading climate scientists, plus the unanimous consensus of all participating governments. Importantly, it would put policy options on the table, but would not make explicit policy recommendations. Given these requirements, the IPCC reports tend to be quite conservative, but have unimpeachable authority.

Q: What is an IPCC report?
A: Every 5 - 6 years, the IPCC issues a massive 3,000+ page report summarizing the current state of knowledge on climate change. These "assessment reports" have been issued in 1990, 1995, 2001, 2007, and now, 2013. The latest assessment will be released in four parts:

"The Physical Science Basis" (September 2013) will describe the observed and predicted changes to Earth's climate.

"Impacts, Adaptation and Vulnerability" (March 2014) will document the dire consequences associated with the path that we’re on.

"Mitigation of Climate Change" (April 2014) will outline what it will take to get us back on track to avoid the worst impacts of climate change.

The "Synthesis Report" (October 2014) will summarize all of the other reports.

The scientists who prepare the 3,000+ page report cite over 9,200 peer-reviewed scientific articles, but present no original research of their own. At least 259 authors from 39 countries drafted the part of the report being released this week, and the report was subjected to two rounds of review by 1089 experts in 55 countries beginning in December 2011. None of the scientists were paid for their work. The report was also reviewed by government representatives from 38 nations, and the final report that is being debated in Stockholm this week was revised based on the over 54,000 review comments received. The most important part of the report is the "Summary for Policy Makers", a 31-page document that summarizes the key scientific findings, used by governments to make policy decisions on how to respond to climate change. The "Summary for Policy Makers" for "The Physical Science Basis" portion of the 2013 IPCC report is being released on September 27. The actual 1,000+ page scientific report that the "Summary for Policy Makers" summarizes is being released the following Monday (September 30.) While the "Summary for Policy Makers" is drafted by the scientists who serve as the lead authors for the IPCC report, the summary is subject to approval by the governments of the 195 member nations of the IPCC. During the final week of the approval process, politicians can weigh in and demand changes to the summary drafted by the scientists, since the final "Summary for Policy Makers" requires unanimous approval by all of the IPCC nations. The IPCC reports have the most elaborate review and approval process for any scientific report in the world. In 2007, the IPCC won the Nobel Peace Prize. In short, three words summarize the IPCC reports:

Comprehensive.
Authoritative.
Conservative.


FIgure 1. IPCC lead authors gather for a group photo at the four most recent meetings for drafting of the 2013 IPCC assessment report. Image credit: IPCC.

Q: Do errors in the IPCC reports undermine confidence in the science?
A: No. Two small errors have been found in the 3000+ pages of the 2007 IPCC report. Neither has anything to do with the basic conclusions that the globe is unequivocally warming and that human activity is the primary cause (one error was simply a typo.) The mistakes have been acknowledged and corrected and review procedures are being strengthened to avoid future errors. In a report of over 3,000 pages by hundreds of authors, it is not unusual that there would be a few minor errors. Contrarians seeking to discredit climate science, and some in the media, have blown these errors out of proportion, claiming the errors invalidate the entire IPCC report. It's like saying we need to throw out an entire phone book because two misspellings were found in it.

Q: What are some of the weaknesses of the IPCC report?
1) The report is already out-of-date, since papers had to be submitted for publication by July 2012 and published by March 2013 in order to be cited.

2) The report is tedious, complex, and difficult to read, making this vital science difficult to access. Little regard was given by the IPCC to communicating the results of the report. Science has little value if it is not understandably communicated to those who need the information. Where are the accompanying explanatory videos? Why was the report issued on a Friday, the worst day of the work week to get attention? The IPCC has devoted a very small portion of its budget to communication and outreach, leaving the interpretation of the report to others. I can understand the reluctance of the IPCC to provide a more slick and showy interpretation of the report, since they might be accused of "spinning" the science, and one of the great strengths of the IPCC report is its great science and the impartiality of the content. But the assumption that the science will speak for itself is wrong. The most powerful and richest corporations in world history--the oil companies--are waging very well-funded PR campaigns to deny the science, play up the uncertainties, and question the character of the scientists who write the report. The world's most rigorous and important scientific report in history is being kicked apart by powerful special interests whose profits are threatened by the findings.

3) Since the "Summary for Policymakers" is subject to unanimous approval by politicians, the science is potentially compromised, and the conclusions will tend to be conservative. Naomi Oreskes, in Chapter Six of her book, "Merchants of Doubt", recounts the haggling that led up to the approval of the 1995 Summary for Policy Makers. Government delegates for Saudi Arabia, Kuwait, and other major oil exporting nations demanded a change to the statement the scientists had drafted, "The balance of evidence suggests an appreciable human influence on climate." For two whole days, the scientists haggled with the Saudi delegate over the single word "appreciable". Nearly 30 different alternatives were discussed before IPCC chair Bert Bolin finally found a word that both sides could accept: "The balance of evidence suggests a discernible human influence on climate." The term "discernible" established a middle ground by suggesting that human-caused climate change was detectable, but the level of that influence was subject to debate. This sentence would go on to become one the most famous scientific statements ever made about climate change, but it was more conservative than what the scientists wanted.

4) The lower-end emissions scenario, called RCP2.6, which assumes that CO2 concentrations will reach 421 ppm by the year 2100, is highly unlikely. Earth reached 400 ppm of CO2 earlier this year, and CO2 has increased by over 2 ppm per year during the past decade. CO2 emissions are accelerating, and CO2 levels will surpass 421 ppm by the year 2023 at the current rate of acceleration. RCP2.6 requires that we slash emissions of CO2 by 50%, relative to 1990 levels, by 2050. We are currently on a pace to match or exceed the worst-case scenario considered by the IPCC (RCP8.5), where CO2 levels reach 936 ppm by the year 2100.

Commentary
The two higher-end emission scenarios of the four considered by the IPCC will very likely warm the planet more than 2°C (3.6°F) over pre-industrial levels. Two degrees Centigrade represents a "dangerous" level of warming for civilization that we must avoid, according to the 2009 Copenhagen Accord, signed by world leaders including President Obama. We will have to work very hard, and very soon, to keep warming below this 2°C "danger" level. As climate writer Elizabeth Kolbert says, holding the global temperature increase to “only” two degrees Celsius, though, is like limiting yourself to “only” a few rounds of Russian roulette: unless you’re uncommonly lucky, the result is not likely be happy. The 0.9°C warming we've experienced since 1900 has already caused a destabilization of global weather patterns, resulting in unprecedented extreme weather events and accelerating melting of polar ice caps. As a group of climate scientists wrote in 2009 at RealClimate.org,

"Even a “moderate” warming of 2°C stands a strong chance of provoking drought and storm responses that could challenge civilized society, leading potentially to the conflict and suffering that go with failed states and mass migrations. Global warming of 2°C would leave the Earth warmer than it has been in millions of years, a disruption of climate conditions that have been stable for longer than the history of human agriculture."

I'll have a full analysis of the new IPCC report Friday morning, and will be offering expert commentary live on The Weather Channel beginning at 7:10 am EDT on Friday. The 2013 Summary For Policymakers will be available on the IPCC website beginning at 4 am EDT Friday.


Video 1. I did a live interview with http://www.democracynow.org Thursday morning during their 8am - 9am EDT news hour, discussing the upcoming IPCC report.

Jeff Masters

Climate Change

Updated: 4:10 PM GMT on September 26, 2013

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Quiet in the Atlantic

By: JeffMasters, 1:15 PM GMT on September 25, 2013

In the Northeastern Gulf of Mexico, a stalled stationary front is bringing heavy thunderstorms to west-central Florida, where heavy rains of up to six inches have caused flooding problems. A weak area of low pressure along this front will move over the coastal waters several hundred miles offshore of North Carolina by Friday, when an extratropical storm is expected to develop. Ocean temperatures off the North Carolina coast are 26 - 27°C, which is warm enough to help give the storm some extra energy and moisture. However, wind shear will be high, and this storm is expected to stay non-tropical as it heads north-northeast, potentially bringing rainy weather to New England and the Canadian Maritime Provinces on Sunday and Monday. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave no odds that anything tropical would spin up in the next five days. None of the reliable computer models for tropical cyclone genesis predicts development over the next five days, and the Atlantic is dominated by dry air and high wind shear. The next area to watch for development might be the Western Caribbean or the area between the Bahamas and Bermuda next week. However, chances of development will be below average for this time of year, due the fact we are in the suppressed phase of the MJO. This suppressed phase may end by mid-October, increasing the odds of development in about two weeks' time.


Figure 1. All quiet in the Atlantic: The Atlantic remains welcomely quiet at 8:15 am EDT on September 25, 2013, with an unusual lack of heavy thunderstorm activity for this time of year. Image credit: NOAA.

Join me in New York City on Thursday for Climate Week
I write a lot about billion-dollar weather disasters like Hurricane Sandy, the coming great climate disruption, and other "doom-and-gloom" topics. These are important to discuss, but too much talk of disaster can turn people off and make them feel hopeless. Social science research shows that including a positive message along with your science will make people more inclined to believe your science, and it is important to emphasize some of the remarkable solutions on how we can lessen and adapt to climate change that technology and entrepreneurship are coming up with. This Thursday afternoon, I am moderating a panel discussion in New York City on some innovative ways to combat climate change. It's part of Climate Week, which culminates Friday with the release of the latest Intergovernmental Panel on Climate Change (IPCC) report, which is released only once every six years. You can register to attend Thursday's free event here. The session begins at 2 pm with remarks by David Kenny, CEO of The Weather Company, followed by guest speaker Mayor Bloomberg. My "New Frontiers" panel is at 5:40 pm.



Jeff Masters

Hurricane

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Disturbed Weather Over Florida May Develop Into Weekend U.S. East Coast Storm

By: JeffMasters, 2:01 PM GMT on September 24, 2013

In the Northern Gulf of Mexico, a stalled stationary is bringing heavy thunderstorms to northern and central Florida, where heavy rains of up to six inches have caused isolated flooding problems. A weak area of low pressure along this front will move over the coastal waters several hundred miles offshore of South Carolina on Thursday, where the ECMWF and Canadian GEM models predict that development into a tropical or subtropical depression could occur by Friday. Ocean temperatures off the South Carolina coast are just warm enough for development, 26 - 27°C, so this scenario is plausible. However, the other two reliable models for tropical cyclone genesis, the GFS and UKMET models, are not predicting development. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave no odds that anything would spin up in the next five days. Out of respect for the ECMWF model, which has the lowest incidence of false alarms for predicting genesis of tropical cyclones, I put the 5-day odds of development at 20%. The ECMWF and GEM models predict that the storm will head slowly northwards, and the ECMWF model predicts that sustained winds of 30 - 40 mph will affect much of the mid-Atlantic coast on Sunday and Monday.


Figure 1. Predicted winds for 11 am EDT Sunday, September 29, 2013, from this morning's 00Z Tuesday, September 24, 2013 run of the ECMWF (European) model. The model is predicting that winds of 30 - 40 mph (light orange colors) could affect a large portion of the mid-Atlantic coast.

Jeff Masters

Hurricane

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Earth has its 4th Warmest August on Record, and 6 Billion-Dollar Weather Disasters

By: JeffMasters, 1:37 PM GMT on September 23, 2013

August 2013 was the globe's 4th warmest August since records began in 1880, according to NOAA's National Climatic Data Center (NCDC). NASA rated it the 5th warmest August on record. The year-to-date period of January - August has been the 6th warmest such period on record. August 2013 global land temperatures were the 11th warmest on record, and global ocean temperatures were the 5th warmest on record. August 2013 was the 342nd consecutive month with global temperatures warmer than the 20th century average. Global satellite-measured temperatures in August 2013 for the lowest 8 km of the atmosphere were 14th or 11th warmest in the 35-year record, according to Remote Sensing Systems and the University of Alabama Huntsville (UAH), respectively. Wunderground's weather historian, Christopher C. Burt, has a comprehensive post on the notable weather events of August 2013 in his August 2013 Global Weather Extremes Summary. The big stories that he highlights are the intense heat waves that hit Central Europe and East Asia, which brought all-time national heat records to Austria, Slovenia, and Japan. Ljubljana, the capital of Slovenia, passed its all-time heat record a remarkable five times during the month.



Figure 1. Departure of temperature from average for August 2013, the 4th warmest August for the globe since record keeping began in 1880. Most of the world's land areas experienced warmer-than-average monthly temperatures, including Australia, northern South America, western North America, Europe, and much of eastern Asia. Far eastern China, part of eastern Russia north of Japan, and part of northeastern South America were record warm for the month. The southeastern United States, Far East Russia, part of South Africa, Paraguay, and Bolivia were cooler than average. No regions of the globe were record cold. Image credit: National Climatic Data Center (NCDC) .


The six billion-dollar weather disasters of August 2013


Disaster 1. The most damaging billion-dollar weather disaster of August was in Northeast China, where the Nei River overflowed, killing 54 and leaving 97 missing in Fushuan. The flooding killed 118 people and cost $5 billion. In this photo, workers use an excavator to clean up mud after heavy rain hit on August 19, 2013 in Fushuan, in the Liaoning Province of China. Photo by ChinaFotoPress/Getty Images.


Disaster 2. Super Typhoon Utor killed 70 people in China and did $2.6 billion in damage. Utor also did $33 million in damage in the Philippines. This video taken by storm chaser James Reynolds shows debris flying as Typhoon Utor hits Zhapo, China on 14th August 2013.


Disaster 3. Torrential rains, due, in part, to moisture from Typhoon Trami, fell in the Philippines August 18 - 21, causing massive flooding on Luzon Island that cost $2.2 billion. Twenty-seven people were killed, and 60% of metro Manila was under water at the peak of the flood. According to EM-DAT, the International Disaster Database, this was the most expensive natural disaster in Philippine history. In this photo, pedicabs and makeshift rafts ferry office workers and pedestrians through flood waters that submerged parts of the financial district of Makati on August 20, 2013 in Makati City south of Manila, Philippines. Image credit: Dondi Tawatao/Getty Images)


Disaster 4. In Pakistan, torrential monsoon rains caused significant flooding that affected 5,739 villages. At least 208 people were killed, 63,180 homes were damaged or destroyed, and 1.4 million acres (567,000 hectares) of crops were submerged. The government estimated economic agricultural losses alone at $1.9 billion. Pakistan's four most expensive weather-related disasters in its history have been floods that occurred in the past four consecutive years. In this photo, Pakistani residents hold onto a rope as they evacuate a flooded area in Karachi on August 4, 2013. Image credit: Asif Hassan/AFP/Getty Images)


Figure 5. Russia experienced its costliest flood disaster in history beginning on August 4, when the Amur and Zeya rivers in the far east of the country along the Chinese border overflowed, flooding 1.7 million acres, damaging or destroying over 11,500 buildings. It was the 4th most expensive natural disaster of any kind in Russian history. These false-color infrared satellite images of Russia's Amur River taken a little over a year apart show the extent of the extreme flooding that affected the Komsomolsk-on-Amur area (population 500,000) in August 2013. Image credit: NASA.


Disaster 6. A severe weather outbreak in the U.S. Plains and Midwest August 5 - 7 brought baseball sized hail and thunderstorm wind gusts over 80 mph to Minnesota and Wisconsin. Two people were killed, and damage was estimated at $1 billion. In this photo, a severe thunderstorm closes in on Edgemont, South Dakota, on August 7, 2013. Image credit: wunderphotographer ninjalynn.

The world-wide tally of billion-dollar weather disasters so far in 2013 is 25, and the U.S. total is six, according to the August 2013 Catastrophe Report from insurance broker Aon Benfield. This excludes the September Colorado flood, whose damages are preliminarily estimated at $2 billion. Ranked in term of cost, here are the 25 disasters:

1) Flooding, Central Europe, 5/30 - 6/6, $22 billion
2) Drought, Brazil, 1/1 - 5/31, $8.3 billion
3) Drought, Central and Eastern China, 1/1 - 7/31, $6.0 billion
4) Flooding, Calgary, Alberta Canada, 6/19 - 6/24, $5.3 billion
5) Flooding, China, 8/9 - 9/5, $5.0 billion
6) Tornado, Moore, OK, and associated U.S. severe weather, 5/18 - 5/22, $4.5 billion
6) Flooding, China, 7/7 - 7/17, $4.5 billion
8) Flooding, Indonesia, 1/20 - 1/27, $3.31 billion
9) Super Typhoon Utor, China and Philippines, 8/12 - 8/15, $2.6 billion
10) Flooding, Australia, 1/21 - 1/30, $2.5 billion
11) Flooding, Philippines, 8/18 - 8/21, $2.2 billion
12 Tornadoes and severe weather, U.S., 5/26 - 6/2, $2 billion
12) Severe weather, Midwest U.S., 3/18 - 3/20, $2 billion
14) Flooding, Pakistan and Afghanistan, 8/3 - 8/31, $1.9 billion
15) Winter weather, Europe, 3/12 - 3/31, $1.8 billion
16) Drought, New Zealand, 1/1 - 5/10, $1.6 billion
16) Severe weather, U.S., 4/7 - 4/11, $1.6 billion
18) Flooding, Toronto, Canada, 7/8, $1.45 billion
19) Flooding, China, 6/29 - 7/3, $1.4 billion
19) Flooding, China, 7/21 - 7/25, $1.4 billion
21) Flooding, Argentina, 4/2 - 4/4, $1.3 billion
22) Flooding, India and Nepal, 6/14 - 6/18, $1.1 billion
23) Winter weather, U.S. Plains, Midwest, Northeast, 2/24 - 2/27, $1.0 billion
23) Severe weather, U.S. Plains and Midwest, 8/5 - 8/7, $1.0 billion
23) Flooding, Russia, 8/4 - 8/31, $1.0 billion

Neutral El Niño conditions continue in the equatorial Pacific
For the 17th month in row, neutral El Niño conditions existed in the equatorial Pacific during August 2013. NOAA's Climate Prediction Center (CPC) expects neutral El Niño conditions to last though the winter of 2013 - 2014, and the large majority of the El Niño models also predict that neutral conditions will last through the winter. Temperatures in the equatorial Eastern Pacific need to be 0.5°C below average or cooler for three consecutive months for a La Niña episode to be declared; sea surface temperatures were 0.0°C from average as of September 16, and have been +0.1 to -0.4°C from average since April 1, 2013.

Arctic sea ice falls to 6th lowest August extent on record
Arctic sea ice extent during August was 6th lowest in the 35-year satellite record, according to the National Snow and Ice Data Center (NSIDC). This was the largest August extent since 2009, and a nice change of pace from last year's all-time record retreat. The Arctic sea ice reached its minimum extent for the year on September 13, and has now begun re-freezing. I'll have a dedicated post on this, probably on Tuesday.

Quiet in the Atlantic
In the Gulf of Mexico, the tail end of a cold front off the coast of Texas has developed a few disorganized heavy thunderstorms. This disturbance has some modest spin to it, thanks to absorbing Invest 95L on Saturday. However, wind shear is high, 20 - 30 knots, and I don't expect this disturbance will develop. The disturbance is expected to bring 1 - 3" of rain to Florida later this week, and on Saturday, the Army Corps of Engineers has re-opened the flood gates on Lake Okeechobee to dump water out of the lake in anticipation of the heavy rains. None the reliable models for tropical cyclone formation is predicting development during the coming five days.

In the Western Pacific, Typhoon Usagi has dissipated after hitting China about 100 miles east-northeast of Hong Kong. The storm is being blamed for at least 25 deaths in China and 2 in the Philippines. Preliminary damage estimates are over $500 million.

Jeff Masters

Climate Summaries

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Category 2 Usagi Hits China; Hong Kong Misses the Storm's Worst

By: JeffMasters, 4:50 PM GMT on September 22, 2013

Typhoon Usagi made landfall near Shanwei, China, about 90 miles east-northeast of Hong Kong, near 6 pm local time (6 am EDT) on Sunday. At landfall, Usagi--the Japanese word for rabbit--was a powerful Category 2 typhoon with top sustained winds of 110 mph. Shanwei recorded a sea level pressure of about 941 mb at landfall. As of noon EDT, the top winds recorded at the Hong Hong Airport were sustained at 40 mph, with gusts to 53 mph. Hong Kong's Cheung Chau Island recorded sustained winds of 54 mph, gusting to 76 mph. Since the typhoon made landfall well to the east of the city, Hong Kong was on the weaker (left) side of the storm, and missed Usagi's strongest winds and most significant storm surge. Hong Kong had a 0.7 meter (2.3') storm surge at the Kwai Chung measurement site. Shantou, located on the strong (right) side of the storm, experienced sustained winds of 49 mph, gusting to 67 mph. Two people were killed by a falling tree in China near Usagi's landfall location, and the typhoon is also being blamed for two deaths in the Philippines and nine injuries in Taiwan. Satellite images show that Usagi is weakening quickly as it moves inland, and the storm should dissipate over China by Tuesday morning.


Figure 1. Radar image of Usagi as it approached landfall showed that the typhoon had multiple concentric eyewalls. Image credit: weather.com.cn.


Figure 2. MODIS satellite image of Typhoon Usagi, taken at approximately 02:30 UTC on September 22, 2013. At the time, Usagi was a Category 3 typhoon with 115 mph winds. Image credit: NASA.

Usagi Links
Wunderground's weather historian, Christopher C. Burt, has a new post, Hong Kong's typhoon history.

Landfall radar loops from three radars, put together by Brian McNoldy

Southeast China radar

#usagi at Twitter

Webcams in Hong Kong

The University of Wisconsin CIMSS Satellite Blog has a nice animation showing the trochoidal (wobbling) forward motion characteristic of intense tropical cyclones.

Vulnerability assessment of storm surges in the coastal area of Guangdong Province, a 2011 journal article by Li and Li.


Video 1. Typhoon chaser James Reynolds caught video of some impressive surf from Typhoon Usagi impacting Hong Kong on September 22, 2013. His Twitter feed is here.

Quiet in the Atlantic
In the Gulf of Mexico, the tail end of a cold front off the coast of Texas has developed an area of concentrated heavy thunderstorms. This disturbance has some modest spin to it, thanks to absorbing Invest 95L on Saturday. However, wind shear is high, 20 - 30 knots, and I don't expect this disturbance will develop. The disturbance is expected to bring 2 - 3" of rain to Florida later this week, and the Army Corps of Engineers has re-opened the flood gates on Lake Okeechobee to dump water out of the lake, in anticipation of the heavy rains. None the reliable models for tropical cyclone formation is predicting development during the coming five days.

Jeff Masters

Hurricane

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Hong Kong Braces for Category 3 Typhoon Usagi

By: JeffMasters, 4:25 PM GMT on September 21, 2013

Dangerous Category 3 Typhoon Usagi is charging through the Luzon Strait between Taiwan and the Philippines on its way towards China, where landfall is expected early Monday morning local time (near 2 pm EDT on Sunday) near Hong Hong. The typhoon battered the northern Philippine Batanes Islands overnight with wind gusts of up to 155 mph (250 kph), ripping down power lines and damaging crops, according to the South China Morning Post. Torrential rains of over a foot (305 mm) have fallen in 24 hours over eastern Taiwan, where Usagi's counterclockwise flow of moist air rode up over the high mountains of the island. Usagi reached its peak strength on Thursday, taking advantage of low wind shear and very warm waters 30°C with high heat content, to intensify to a Category 5 super typhoon with 160 mph winds. On Friday, Usagi began an eyewall replacement cycle that the typhoon is still attempting to complete. This process, where the inner eyewall collapses and a new, larger-diameter eyewall forms from an outer spiral band, typically causes a reduction in intensity by one Saffir-Simpson category, but spreads out the storm's hurricane-force winds over a larger area. Satellite images show that Usagi has lost its symmetry and the cloudtops have warmed, indicating weakening; this weakening is likely due to disruption of the low-level inflow by the high mountains of Taiwan. Wind shear is low, near 10 knots, but ocean temperatures have cooled to about 29°C, and the heat content of the waters is much lower than on Friday.


Figure 1. MODIS satellite image of Typhoon Usagi, taken at approximately 04:30 UTC on September 21, 2013. At the time, Usagi was a Category 4 typhoon with 140 mph winds. Image credit: NASA.


Figure 2. Rainfall in Taiwan for the 24-hour period ending at 12 pm EDT (midnight in Taiwan) on Saturday, September 21, 2013. Rainfall amounts in excess of 300 mm (11.81", yellow colors) fell in less than 24 hours in several regions. Image credit: Central Weather Bureau.

Forecast for Usagi
Continued slow weakening of Usagi is likely as the storm tries to complete its eyewall replacement cycle as ocean temperatures cool to 28°C and the heat content of the water diminishes. By the time Usagi reaches the coast near Hong Hong in the early morning hours on Monday (local time), the storm should be at Category 2 strength. This is still strong enough to pose a formidable storm surge, wind, and heavy rain threat to China, and Usagi is likely to be one of the five strongest typhoons to affect Hong Hong in the past 50 years. If the eye of the storm hits just west of Hong Kong, a large storm surge capable of causing over a billion dollars in damage will inundate portions of the coast along the bay that Hong Kong, Macau, and Shenzhen share. Wunderground's weather historian, Christopher C. Burt, has a new post, Hong Kong's typhoon history, detailing the most notable storm's in Hong Hong's history. The most notable typhoon to hit Hong Kong in the past 50 years was Typhoon Rose of 1971, which sank over 300 boats and killed 110 people.


Figure 3. Super Typhoon Usagi, as seen by the VIIRS instrument on the Suomi satellite on Thursday, September 19, at 1635 UTC (375-m I-band 5). At the time, Usagi was a Category 5 super typhoon with 160 mph winds. The eye is at its narrowest at the surface, and slopes outward with altitude (like being inside a stadium), so that the cloud temperatures measured in infrared light in this image change dramatically surrounding the eye. The warmest brightness temperature inside the eye in this image is -11°C, which suggests that we are seeing a mid-level cloud deck in the center of the eye. Image credit: Dan Lindsey, NOAA/RAMMB/CIRA.


Figure 4. Visible light image from VIIRS taken under the full moon at the same time as the above infrared-light image. Image credit: Dan Lindsey, NOAA/RAMMB/CIRA.

Usagi Links
Southeast China radar

The University of Wisconsin CIMSS Satellite Blog has a nice animation showing the trochoidal (wobbling) forward motion characteristic of intense tropical cyclones.


Video 1. Typhoon chaser James Reynolds caught video of some impressive surf from Typhoon Usagi impacting Taiwan on September 21, 2013. His Twitter feed is here. He is now in Hong Hong to document the storm's arrival there.

Vulnerability assessment of storm surges in the coastal area of Guangdong Province, a 2011 journal article by Li and Li.

Quiet in the Atlantic
There are no tropical cyclone threat areas in the Atlantic to discuss today, and none of the reliable models for tropical cyclone formation is predicting development during the coming five days.

Jeff Masters

Hurricane

Updated: 2:59 AM GMT on September 22, 2013

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95L Remains Unorganized in the Gulf of Mexico

By: shauntanner , 6:15 PM GMT on September 20, 2013

Invest 95L, located in the southwest Gulf of Mexico, is around 100 miles east of Tampico, Mexico Friday morning and is struggling to organize based on satellite imagery. Visible satellite shows the strongest thunderstorm activity has been displaced to the south of what would be the center of circulation. Only a few thunderstorms are associated with this disturbance currently, with the strongest of these on the southern and eastern edges.


Satellite image showing Invest 95L and a lack of thunderstorm development

The National Hurricane Center has decreased the likelihood of 95L developing over the next two days from 70% on Thursday to 30% heading into the weekend.  Some models suggest this disturbance could track north toward the Texas coast, where it could strengthen. This northward jog would be guided by a cold front that is forecast to track across the U.S over the weekend.  Regardless whether this disturbance develops, significant rain is expected from southern Texas through much of the Southeast as it will couple with the aforementioned cold front to draw considerable moisture northward. Upwards of 5 inches of rain (see image below) could fall along coastal areas from Texas through Mississippi even if the disturbance does not develop.


WPC forecast showing the QPF forecast for Friday through Monday.  Note the significant rain expected through the Southeast due to the combination of the tropical disturbance in the Gulf of Mexico and a cold front moving through the eastern U.S.


Model tracks showing some possible paths of Invest 95L

Super Typhoon Usagi

Usagi maintained Super Typhoon strength on its path just south Taiwan and north of Philippines.  With a wind strength of 150 mph, Usagi is taking a reasonably good path as the official forecast takes the typhoon into the South China Sea and weakens it into a Category 2 storm before eventual landfall near Shenzen on Sunday.  This will still be a strong storm with the capability of doing considerable damage and significant flooding.


Tracking map of Super Typhoon Usagi.

Hurricane

Updated: 6:17 PM GMT on September 20, 2013

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Manuel Hits Mexico Again, 138 Dead or Missing; Cat 4 Usagi a Threat to Hong Kong

By: JeffMasters, 12:56 PM GMT on September 19, 2013

Hurricane Manuel made landfall in the Mexican state of Sinaloa almost due east of the tip of the Baja Peninsula this Thursday morning near 8 am EDT, as a Category 1 storm with 75 mph winds. It was Manuel's second landfall this week in storm-weary Mexico. Manuel hit the coast on Monday as a tropical storm with 70 mph winds northwest of Acapulco, bringing torrential rains of up to foot in the coastal mountains. The combined one-two punch of Manuel on Mexico's Pacific coast and Hurricane Ingrid on the Atlantic coast this week have caused flooding blamed for at least 80 deaths. Another 58 people are missing and probably dead in a landslide that hit La Pintada, several hours north of Acapulco. The double blow by Manuel and Ingrid this week was the first time since 1958 that two tropical storms or hurricanes had hit both of Mexico's coasts within 24 hours. Acapulco received 7.41" of rain from Manuel September 12 - 16, and up to a foot of rain fell in the surrounding mountains. Manuel promises to bring 5 - 10" of rain to the Mexican state of Sinaloa, which is likely to cause additional flash floods and mudslides over the next few days. Mexican radar shows that Manuel is moving very slowly inland, and should weaken significantly as long as the eye stays ashore. Satellite images show that Manuel is a small storm, and its heavy rains are affecting a relatively limited area. With such a large death toll, it is possible that the name Manuel will be retired from the list of active Eastern Pacific hurricanes; only three other Pacific storms that hit Mexico have had their names retired.


Figure 1. Hurricane Manuel at landfall in the Mexican state of Sinaloa at 7:15 am EDT September 19, 2013. Image credit: Conagua.


Figure 2. Rainfall for the 9-day period September 9 - 18, 2013, as measured by NASA's TRMM satellite. Rainfall amounts of 12" inches (red colors) affected portions of Mexico near Acapulco. Image credit: NASA.


Figure 3. A soldier wades through the water at the airport of Acapulco, after floods from Tropical Storm Manuel hit on September 17, 2013. Image credit: Pedro Pardo/AFP/Getty Images).

Invest 95L in Bay of Campeche
Waterlogged Mexico has yet another tropical rain-making system to worry about--Invest 95L, over the Southwestern Gulf of Mexico's Bay of Campeche. Satellite loops show that 95L has a pronounced spin, but only a small area of heavy thunderstorms. The thunderstorms are causing some isolated heavy downpours along the coast of Mexico, as seen on Mexican radar. Wind shear is moderate, and ocean temperatures are warm, 29°C (84°F.) There is some dry air over the Bay of Campeche, which appears to be slowing down development. A hurricane hunter flight is scheduled to investigate 95L on Thursday afternoon, but I expect this flight will be cancelled, due to 95L's lack of heavy thunderstorms.

Invest 95L will likely stay trapped in the Bay of Campeche
Wind shear is expected to be low to moderate over the next five days. NHC gave the disturbance 2-day development odds of 70% and 5-day odds of 80% in their 8 am EDT Thursday Tropical Weather Outlook. Steering currents will be weak in the Bay of Campeche over the next five days, and most of the models predict that 95L will stay trapped there, moving slowly and erratically. However, the UKMET model and approximately 20% of the 20 individual forecasts from the GFS ensemble and European Center ensemble predict that 95L will turn north next week and hit the U.S. Gulf Coast. A cold front and associated upper-level trough of low pressure are expected to push southeastwards over the Gulf of Mexico on Saturday, and it is possible that the upper-level westerly winds associated with this weather system will reach far enough south to pull 95L out of the Bay of Campeche. It typically takes a stronger upper-level trough of low pressure to flush a tropical cyclone out of the Bay of Campeche, though, and 95L will most likely stay put. Regardless, tropical moisture from 95L will likely stream northeastwards along the cold front over much of the U.S. Gulf Coast on Saturday and Sunday, bringing heavy rains of 2 - 4".


Figure 3. MODIS satellite image of Typhoon Usagi, taken at approximately 02:30 UTC on September 19, 2013. At the time, Usagi was a Category 2 storm with 105 mph winds. Image credit: NASA.

Dangerous Typhoon Usagi headed for Taiwan and Hong Kong
In the Western Pacific, powerful and dangerous Typhoon Usagi has intensified to a Category 4 storm with 140 mph winds. Further intensification is likely over the next day, as Usagi is over very warm waters of 29 - 30°C with high heat content, and is under light wind shear of 5 - 10 knots. The official JTWC forecast brings Usagi to a Category 5 super typhoon by Friday, which would make Usagi Earth's first Category 5 storm of 2013. Usagi will pass very close to the southern tip of Taiwan at 06 UTC Saturday (2 am EDT), and interaction with land could potentially weaken the storm. However, Usagi is still expected to be a major Category 3 typhoon when it makes landfall near Hong Kong at approximately 00 UTC Sunday (8 pm EDT Saturday.) Satellite images show that Usagi is a large and very impressive typhoon with a prominent eye surrounded by a thick eyewall with intense thunderstorms that reach high into the atmosphere.

Friday's blog update will be later than usual, as I am taking the day off, and one of wunderground's San Francisco-based meteorologists will be filling in for me.

Jeff Masters

Hurricane

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Mexican Death Toll 57 From Manuel and Ingrid; 95L Likely to Bring More Rain

By: JeffMasters, 2:20 PM GMT on September 18, 2013

Tropical Storm Manuel has regenerated into a strong tropical storm just west of Mexico's Baja Peninsula this Wednesday morning, and the destructive storm promises to bring even more misery to a Mexican nation already reeling from the combined one-two punch of Manuel on its Pacific coast and Hurricane Ingrid on the Atlantic coast earlier this week. The two storms, which hit Mexico within 24 hours of each other on Monday and Tuesday, are now being blamed for the deaths of at least 57 people in Mexico. It was the first time since 1958 that two tropical storms or hurricanes had hit both of Mexico's coasts within 24 hours. Hardest hit was the Acapulco region on Mexico's Pacific coast, where the airport is barely functioning, many roads still flooded and blocked, and tens of thousands of tourists are scrambling to get home. Acapulco received 7.41" of rain from Manuel September 12 - 16, and much higher amounts of rain fell in the surrounding mountains. Newly regenerated Manuel promises to bring 5 - 10" of rain to the Mexican state of Sinaloa, which is likely to cause dangerous flash floods and mudslides.


Figure 1. A soldier wades through the water at the airport of Acapulco, after floods from Tropical Storm Manuel hit on September 17, 2013. Image credit: Pedro Pardo/AFP/Getty Images).


Figure 2. View of a street in Acapulco, Guerrero state, Mexico, after a landslide caused by heavy rains from Tropical Storm Manuel on September 15, 2013. Image credit: CLAUDIO VARGAS/AFP/Getty Images)


Figure 3. Storm sandwich: Tropical Storm Manuel and Hurricane Ingrid lay siege to Mexico on September 15, 2013. Tropical Storm Manuel came ashore on the Pacific coast near Manzanillo on the afternoon of September 15, and Hurricane Ingrid followed suit from the Atlantic on September 16. Image credit: NASA.

Invest 95L headed for Bay of Campeche
Waterlogged Mexico has yet another tropical rain-making system to worry about this week. An area of disturbed weather (Invest 95L) over the Yucatan Peninsula will emerge into the Southwestern Gulf of Mexico's Bay of Campeche by Wednesday night. Satellite loops show that 95L already has a pronounced spin and a respectable area of heavy thunderstorms. Wind shear is moderate, and ocean temperatures are warm, 29°C (84°F.) A hurricane hunter flight is scheduled to investigate 95L on Thursday afternoon.

Invest 95L will likely stay trapped in the Bay of Campeche
Wind shear is expected to be low to moderate over the next five days. There is some dry air over the Bay of Campeche, but I doubt this will be an impediment to development, given the low to moderate wind shear. NHC gave the disturbance 2-day development odds of 70% and 5-day odds of 80% in their 8 am EDT Wednesday Tropical Weather Outlook. Steering currents will be weak in the Bay of Campeche over the next five days, and most of the models predict that 95L will stay trapped there, moving slowly and erratically. However, the UKMET model and approximately 20% of the 20 individual forecasts from the GFS ensemble and European Center ensemble predict that 95L will turn north next week and hit the U.S. Gulf Coast. A cold front and associated upper-level trough of low pressure are expected to push southeastwards over the Gulf of Mexico on Saturday, and it is possible that the upper-level westerly winds associated with this weather system will reach far enough south to pull 95L out of the Bay of Campeche. It typically takes a stronger upper-level trough of low pressure to flush a tropical cyclone out of the Bay of Campeche, though, and 95L will most likely stay put. Regardless, tropical moisture from 95L will likely stream northeastwards along the cold front over much of the U.S. Gulf Coast on Saturday and Sunday, bringing heavy rains of 2 - 4". A non-tropical low pressure system is expected to form along this front over the Southeast U.S. on Sunday, spreading heavy rains up the East Coast early next week.

Typhoon Usagi headed for Taiwan
In the Western Pacific, Typhoon Usagi has intensified to a Category 1 storm with 75 mph winds. Further intensification is likely over the next few days, as Usagi is over very warm waters of 29 - 30°C with high heat content, and is under light wind shear of 5 - 10 knots. The official JTWC forecast brings Usagi to a Category 3 storm before encountering Taiwan, when interaction with land could potentially weaken the storm. Satellite images show Usagi has developed a prominent eye surrounded by intense thunderstorms that reach high into the atmosphere. The typhoon is expected to pass north of the Philippines Islands on Friday, and will threaten southern Taiwan on Saturday.

Jeff Masters

Hurricane

Updated: 6:04 PM GMT on September 18, 2013

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Sunshine Aids Colorado Evacuations; 38 Dead in Mexico From Manuel and Ingrid

By: JeffMasters, 2:42 PM GMT on September 17, 2013

After nine consecutive days with rain, skies have finally cleared over flood-ravaged Boulder, Colorado. Flooding from the past week's rains have killed at least seven, destroyed over 1,500 homes, damaged 18,000 homes, and caused close to $1 billion in damage, according to insurance broker Aon Benfield. Over 70 bridges have been damaged or destroyed, and hundreds of roads damaged, including major sections of U.S. Highways 34, 36 and 72. Clear skies are forecast for the remainder of the week, which will allow rescue helicopters to safely operate to evacuate the hundreds of people still trapped in mountain towns cut off by the rockslides, collapsed bridges, and destroyed roads. The rains that fell early Monday morning in Boulder officially put the city over its all-time annual precipitation record with three and a half months left in the year. Boulder has been deluged with 30.13" so far this year; the previous record was 29.93", set in 1995.


Figure 1. A raging waterfall destroys a bridge along Highway 34 east of Estes Park, Colorado, on September 15, 2013. (AP Photo/Colorado Heli-Ops, Dennis Pierce)


Figure 2. Damage to Highway 34 along the Big Thompson River, on the road to Estes Park, Colorado. Image credit: Colorado National Guard.


Figure 3. A field of parked cars and trucks sits partially submerged near Greeley, Colorado, Saturday, Sept. 14, 2013, as debris-filled rivers flooded into towns and farms miles from the Rockies (AP Photo/John Wark).

A 1-in-1,000 year flood and rainfall event
The Colorado Emergency Management Agency reported that “Some areas in Larimer County experienced a 100-year flood and other areas experienced a 1,000-year flood. It all depends on where the heaviest rain fell. Areas with more extensive damage experienced the 1,000 year flooding.” The U.S. Geological Survey office in Colorado called the flood of Boulder Creek in the city of Boulder as at least a 1-in-100-year event. In the towns of Lyons and Estes Park, officials separately described the current event in each area as a 1-in-500-year flood. According to Bob Henson of the National Center for Atmospheric Research in Boulder, 17.16" of rain fell in the past week in Boulder. Using about a century of precipitation records, NOAA has constructed a Precipitation Frequency Data Server, which estimates how often we might expect to see extreme rainfall events recur. For Boulder, a 5.87" rain event in one week has an average recurrence interval of once every 1,000 years. The city received almost triple that amount of rain over the past week--a truly extreme and rare weather event.

As extreme as the 2013 Colorado flood was, there are two flood events in Colorado history that compare. One was the June 1965 flood that hit the Colorado Front Range, causing $4 billion in damage (adjusted for inflation.) Colorado's deadliest flood on record was the Big Thompson Flood of 1976, which killed 145 people between Estes Park and Loveland. More than 12" of rain fell in just three hours, causing a flood rated at between a 500-year and 1,000-year event.


Figure 4. The rains that fell in a 24-hour period ending at 12 pm MDT September 13, 2013 over regions near Boulder, Colorado were the type of rains with a 0.2% chance of falling in a particular year, or once every 500 years (purple colors), according to MetStat, Inc. (http://www.metstat.com.) MetStat computed the recurrence interval statistics based on gauge-adjusted radar precipitation and frequency estimates from NOAA Atlas 14 Volume 8, published in 2013 (http://dipper.nws.noaa.gov/hdsc/pfds/.) MetStat does not supply their precipitation recurrence interval forecasts or premium analysis products for free, but anyone can monitor the real-time analysis (observed) at: http://metstat.com/solutions/extreme-precipitation-index-analysis/

Colorado's rains set to cause record flooding in Nebraska
Much of the water from the past week's record rains in Colorado have funneled into the South Platte River, which flows eastwards in Nebraska. A 10 - 11' high flood crest is headed downriver, and is setting all-time flood height records as it heads east. By Wednesday night, the crest will reach western Nebraska at Roscoe, where the flood waters may cover Interstate 80, unless sandbagging efforts to protect the highway are done. Interstate 80 is one of the two most heavily traveled transcontinental highways in the United States.


Figure 5. Observed and predicted flood heights on the South Platte River in Western Nebraska, where an all-time record flood is expected on Wednesday. Records at this gauge go back 30 years. Image credit: NOAA/

Links
Bob Henson of the National Center for Atmospheric Research in Boulder summarizes the great flood.

Wunderblogger Dr. Ricky Rood has a home in Boulder, and discusses his take on the flood.

Colorado’s ‘Biblical’ Flood in Line with Climate Trends by Andrew Freedman of Climate Central.

Wunderground weather historian Christopher C. Burt discusses how this year's flood compares to previous Colorado floods in his latest post.

A map of Boulder flood zones and detailed history of previous floods in the area may be found here.


Video 1. A slow-motion mudslide pours into Boulder Creek, Colorado on September 14, 2013.

Ingrid and Manuel kill 38 in Mexico; Invest 95L headed for Bay of Campeche
Flooding from the combined one-two punch of Hurricane Ingrid on the Atlantic coast and Tropical Storm Manuel on the Pacific coast is being blamed for the deaths of at least 38 people in Mexico, according to AP. The two storms hit Mexico nearly simultaneously on Monday, packing sustained winds of 65 - 70 mph and torrential rains. Hardest hit was the Acapulco region on Mexico's Pacific coast, where the airport is closed, many roads flooded and blocked, and much of the city without water or power.

The waterlogged Gulf Coast of Mexico has yet another tropical rain-making system to worry about this week. An area of disturbed weather (Invest 95L) over the Yucatan Peninsula will emerge into the Southwestern Gulf of Mexico's Bay of Campeche on Wednesday. Belize radar and satellite loops show that 95L already has a pronounced spin and a respectable area of heavy thunderstorms. Wind shear is low and is expected to stay low over the next five days. There is some dry air over the Bay of Campeche, but I doubt this will be an impediment to development, given the low wind shear. NHC gave the disturbance 2-day development odds of 30% and 5-day odds of 50% in their 8 am EDT Tuesday Tropical Weather Outlook. The disturbance is likely to stay trapped in the Bay of Campeche and take a slow west-northwest path towards the same region of coast affected by Hurricane Ingrid. On Saturday, a cold front is expected to push southeastwards over the Gulf of Mexico, and moisture from 95L will likely stream northeastwards along the cold front over much of the U.S. Gulf Coast, bringing heavy rains on Saturday and Sunday. A non-tropical low pressure system could form along this front and move northeastwards into the Florida Gulf Coast, bringing heavy rains to the Southeast U.S. on Sunday and Monday.

Jeff Masters

Flood Hurricane

Updated: 7:27 PM GMT on September 17, 2013

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Tropical Storms Ingrid and Manuel Hit Mexico, Killing 21

By: JeffMasters, 2:34 PM GMT on September 16, 2013

Tropical Storm Ingrid hit the Mexican coast about 200 miles south of the Texas border with top sustained winds of 65 mph at approximately 8 am EDT Monday morning. Ingrid weakened below hurricane strength just before landfall, and it appears that strong upper-level winds from the outflow of Tropical Storm Manuel to its west may have been responsible for keeping Ingrid weaker than expected for the past 24 hours. Satellite loops show that Ingrid is a relatively small storm, but the storm is bringing torrential rains to portions of Mexico a few hundred miles south of the Texas border. Flooding from the combined one-two punch of Ingrid on the Atlantic coast and Tropical Storm Manuel on Mexico's Pacific coast are already being blamed for the deaths of 21 people, according to AP. However, Ingrid is also bringing beneficial rains to areas of northern Mexico and South Texas that are in extreme drought.


Figure 1. Radar image from the Brownsville, Texas radar of Tropical Storm Ingrid at landfall, near 8 am EDT September 16, 2013.

Ingrid was the second hurricane of the 2013 Atlantic hurricane season, and Ingrid's peak intensity of 85 mph on Saturday tied it with Hurricane Humberto as the strongest hurricane of the 2013 season so far. Ingrid's intensification into a hurricane on September 14 came eighteen days later than the usual appearance of the Atlantic's second hurricane of the season, which is August 28. Ingrid is the third named storm to hit Mexico's Gulf of Mexico coast so far in 2013, which is a very high level of tropical activity for the region. Only 1933 (seven storms), 1936 (six storms), and 2005 (five storms) had more tropical storms or hurricanes make landfall on Mexico's Gulf of Mexico coast. Two other years have also had three such landfalls, 1944 and 1931.




Figure 2. Rainfall for the 24-hour period ending at 8 am EDT September 14, 2013 (top) and 8 am EDT September 15, 2013 (bottom). Rainfall amounts in excess of five inches (red colors) affected portions of Mexico both days. Image credit: Conagua.


Figure 3. Predicted rainfall for Tropical Storm Ingrid from the 06Z (2 am EDT) September 16, 2013 run of the HWRF model. Image credit: NOAA/GFDL.

Ingrid's impact on Texas
In South Texas, Ingrid brought a storm surge of one foot on Sunday afternoon and Monday morning to South Padre Island, where a coastal flood warning is in effect. Seas were eight feet high Tuesday morning at buoy 42020, located 58 miles southeast of Corpus Christi, and large waves are causing dangerous surf all along the South Texas coast. No flash flood watches are posted for South Texas ay present, but 2 - 5" of rain may cause some isolated flooding problems. Radar-estimated rainfall from the Brownsville radar shows that some areas north of the city have received 3 - 4" of rain, and coastal and mountain areas of Mexico 100 - 200 miles south of the border have received 5 - 10".

Elsewhere in the Atlantic
The remains of Hurricane Humberto are regenerating back into a tropical storm, but newly re-formed Humberto will stay far out to sea and will not be a threat to any land areas.

An area of disturbed weather over the Western Caribbean will move over Mexico's Yucatan Peninsula and into the Southwestern Gulf of Mexico's Bay of Campeche by late this week. NHC gave the disturbance 5-day development odds of 20% in their 8 am EDT Monday Tropical Weather Outlook. Moisture from the disturbance is likely to stream northeastwards across the Yucatan Peninsula, Western Cuba, Florida, and the Western Bahamas during the period 7 - 14 days from now, bringing heavy rainfall.

The tropical Atlantic will be dominated by dry air this week, and the models are not showing any development from new African tropical waves during the coming week. With the African monsoon now beginning to wane, and the Madden Julian Oscillation (MJO) expected to be in a phase that will bring sinking air to the tropical Atlantic during the remainder of September, the Cape Verde hurricane season is likely over; I give only a 30% chance that we will see a tropical storm develop between the Lesser Antilles Islands and coast of Africa during the remainder of hurricane season. However, we will likely get several more tropical storms forming in the Caribbean, Gulf of Mexico, or waters near the Bahamas during the remainder of the season.

Jeff Masters

Hurricane

Updated: 3:04 PM GMT on September 16, 2013

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Hurricane Ingrid and Tropical Storm Manuel Drench Mexico, Killing 5

By: JeffMasters, 4:09 PM GMT on September 15, 2013

Dangerous Hurricane Ingrid weakened Sunday morning, and is barely a hurricane, but the storm's heavy rains remain a major threat to Eastern Mexico. Sunday morning wind data from the Hurricane Hunters showed the highest surface winds in Ingrid were just below hurricane force, and it appears that upper-level outflow from Tropical Storm Manuel to its west may have increased wind shear over Ingrid, causing weakening. Ingrid is still embedded in a very moist environment with high ocean temperatures, making re-intensification likely if the wind shear drops, which appears likely, as Manual weakens after making landfall on Sunday afternoon. Satellite loops show that Ingrid is a relatively small storm, and has changed little in size today. The storm's heaviest rains were offshore Sunday morning, as seen on Mexican radar. Flooding from Hurricane Ingrid has already killed two people in Mexico, and Tropical Storm Manuel's floods have killed three people, according to CNN.

Ingrid is the second hurricane of the 2013 Atlantic hurricane season, and the hurricane's peak intensity of 85 mph winds on Saturday tied it with Hurricane Humberto as the strongest hurricane of the 2013 season so far. Ingrid's intensification into a hurricane on September 14 came eighteen days later than the usual appearance of the Atlantic's second hurricane of the season, which is August 28.


Figure 1. Rainfall for the 24-hour period ending at 8 am EDT September 14, 2013. Rainfall amounts in excess of five inches (red colors) affected portions of Mexico. Image credit: Conagua.

Forecast for Ingrid
All of the models predict that a ridge of high pressure building in to Ingrid's north will force the storm nearly due west into the coast of Mexico on Monday. The soils along the Mexican Gulf Coast in the state of Veracruz where Ingrid will be dumping its heaviest rains are already saturated from the rains of Tropical Depression Eight and Tropical Storm Fernand, and the expected 10 - 15 inches of rain, with isolated amounts of up to 25", will cause extremely dangerous flash flooding and mudslides. On the other side of Mexico, Tropical Storm Manuel will be making landfall on Sunday afternoon, and will bring similar prodigious amounts of rainfall.


Figure 2. Ingrid's rainfall amounts may rival those of Hurricane Alex, which struck Mexico north of Tampico as a Category 2 hurricane with 110 mph winds in 2010. Extremely heavy rains of up to 35" fell mainly on the Mexican side of the Rio Grande/Rio Bravo, particularly near Monterrey, where rainfall amounts were historic. The hurricane left 51 people dead or missing in Eastern Mexico, and damage was $1.8 billion. Flooding along the Rio Grande/Rio Bravo was the worst seen since 1960. Image credit: NOAA/HPC.

Ingrid's impact on Texas
In South Texas, Ingrid has already brought a storm surge of one foot to South Padre Island, where a coastal flood warning has been posted. Large swells up to seven feet high are causing dangerous surf, and the South Texas coast will receive a severe battering from waves expected to reach twelve feet high by Monday. No flash flood watches are posted for South Texas as present, but 2 - 4" of rain may cause some isolated flooding problems. Radar-estimated rainfall from the Brownsville radar shows that some areas north of the city have received 3" of rain, and coastal areas of Mexico 100 miles south of the border have received 5 - 10".

Jeff Masters

Hurricane

Updated: 12:57 PM GMT on September 16, 2013

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Tropical Storms Ingrid and Manuel: Extreme Rainfall Threats for Mexico

By: JeffMasters, 3:38 PM GMT on September 14, 2013

Extremely dangerous Tropical Storm Ingrid is near hurricane strength as it heads northwards over the Southwestern Gulf of Mexico's Bay of Campeche. Ingrid is embedded in a very moist environment, making it the most dangerous Atlantic Tropical Cyclone of 2013 thus far, due to its rainfall potential. The storm is already bringing sporadic heavy rains to the Eastern Mexican coast near Tampico, but the storm's heaviest rains remain offshore, as seen on Mexican radar. Satellite loops show that Ingrid is steadily growing in size, and the storm's heavy thunderstorms have cold cloud tops that reach high into the atmosphere. The Hurricane Hunters found 70 mph surface winds in Ingrid Saturday morning. Moderate wind shear of 15 - 20 knots is interfering with development, but ocean temperatures are a very warm 29 - 29.5°C (84 - 85°F).


Figure 1. Percent chance of receiving more than 8" of rain during a five day period, from the Saturday 2 am EDT run of the experimental GFDL ensemble model for Tropical Storm Ingrid. More than 8" of rain are predicted for large swaths of Mexico, due, in part, to the effects of Tropical Storm Manuel in the Pacific. Image credit: NOAA/GFDL.

Forecast for Ingrid
All of the models predict that Ingrid's current northerly motion will be short-lived, as a ridge of high pressure builds in to its north and forces the storm nearly due west into Mexico on Monday. The soils along the Mexican Gulf Coast in the state of Veracruz where Ingrid will be dumping some of its heaviest rains are already saturated from the rains of Tropical Depression Eight and Tropical Storm Fernand,. Additional heavy rains affected the region early this week, leading to flash flooding and multiple landslides, including one massive landslide in the town of Manzanatitla on Monday that killed eight people. It won't take much rain to generate a catastrophic flood disaster, and 10 - 15 inches of rain, with isolated amounts of up to 25", are expected.


Figure 2. Twin scourges assault Mexico: Hurricane Ingrid in the Atlantic and Tropical Storm Manuel in the Pacific, at 3:47 pm CDT September 14, 2013.

At the same time that Ingrid is making landfall, Tropical Storm Manuel will be bringing similar rainfall amounts to the other side of Mexico. This morning's 2 am EDT run of the experimental GFDL ensemble model predicted that a large area of Mexico is at high risk of 8+ inches of rain due to the combined effects of Ingrid and Manuel. The greatest danger is on the Pacific side in Oaxaca State, where the combined effects of the circulations of the two storms will pull a flow of very moist air upwards over the mountains, creating torrential rains. The Mexican Weather Service has already received a report of about 8 inches of rain in eastern Mexican state of Oaxaca due to Manuel's rains. The massive rains that will fall in Mexico from this one-two punch of extremely wet tropical storms will cause an extremely dangerous and expensive flood disaster in Mexico.

Jeff Masters

Hurricane

Updated: 8:54 PM GMT on September 14, 2013

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Boulder's 1-in-100 Year Flood Diminishing; Ingrid a Dangerous Flood Threat for Mexico

By: JeffMasters, 3:43 PM GMT on September 13, 2013

Colorado's epic deluge is finally winding down, as a trough of low pressure moves across the state and pushes out the moist, tropical airmass that has brought record-breaking rainfall amounts and flooding. Devastating flash floods swept though numerous canyons along the Front Range of Colorado's Rocky Mountains Wednesday night and Thursday morning, washing out roads, collapsing houses, and killing at least three people. The flood that swept down Boulder Creek into Boulder, Colorado was a 1-in-100 year event, said the U.S. Geological Survey. A flash flood watch continues through noon Friday in Boulder. According to the National Weather Service, Boulder's total 3-day rainfall as of Thursday night was 12.30". Based on data from the NWS Precipitation Frequency Data Server, this was a greater than 1-in-1000 year rainfall event. The city's previous record rainfall for any month, going back to 1897, was 9.59", set in May 1995. Some other rainfall totals through Thursday night include 14.60" at Eldorado Springs, 11.88" at Aurora, and 9.08" at Colorado Springs. These are the sort of rains one expects on the coast in a tropical storm, not in the interior of North America! The rains were due to a strong, slow-moving upper level low pressure system to the west of Colorado that got trapped to the south of an unusually strong ridge of high pressure over Western Canada. This is the same sort of odd atmospheric flow pattern that led to the most expensive flood disaster in Canadian history, the $5.3 billion Calgary flood of mid-June this summer. The upper-level low responsible for this week's Colorado flood drove a southeasterly flow of extremely moist tropical air from Mexico that pushed up against the mountains and was lifted over a stationary front draped over the mountains. As the air flowed uphill and over the front, it expanded and cooled, forcing the moisture in it to fall as rain. Balloon soundings from Denver this morning continued to show levels of September moisture among the highest on record for the station, as measured by the total Precipitable Water (PW), which is how much water would fall at the ground if the entire amount of water vapor through the depth of the atmosphere was condensed. Four of the top eight all-time September highs for Precipitable Water since records began in 1948 have been recorded over the past two days:

1.33" 12Z September 12, 2013
1.31" 00Z September 12, 2013
1.24" 12Z September 13, 2013
1.23" 12Z September 10, 1980
1.22" 00Z September 2, 1997
1.21" 00Z September 7, 2002
1.20" 00Z September 13, 2013

Wunderground weather historian Christopher C. Burt discusses how this year's flood compares to previous Colorado floods in his latest post.

A map of Boulder flood zones and detailed history of previous floods in the area may be found here.


Figure 1. A torrent of water rushes alongside a swamped house following flash flooding near Left Hand Canyon, south of Lyons, Colo., Sept 12, 2013. (AP Photo/Brennan Linsley)



Figure 2. Observed rainfall for the Colorado's Front Range from the September 11 - 13 rain event. Rainfall amounts greater than 10" (pink colors) were indicated near Boulder. Image credit: NWS Denver.

Tropical Storm Ingrid a Dangerous Rainfall Threat for Mexico
Tropical Storm Ingrid, the ninth named storm of the 2013 Atlantic hurricane season, has formed in the Southwestern Gulf of Mexico's Bay of Campeche. Ingrid is the most dangerous Atlantic tropical cyclone of 2013 thus far, due to its rainfall potential. Ingrid is embedded in an exceptionally moist environment, and is already bringing heavy rains to the Mexican coast in Veracruz state, as seen on Mexican radar. Satellite loops show that Ingrid is not well-organized, and has only a modest area of heavy thunderstorms. However, the Friday morning hurricane hunter mission found 45 mph winds, prompting NHC to upgrade Ingrid. Moderate wind shear of 15 - 20 knots is interfering with development, but ocean temperatures are a very warm 29 - 29.5°C (84 - 85°F).


Figure 3. Percent chance of receiving more than 16" of rain during a five day period, from the Friday 2 am EDT run of the experimental GFDL ensemble model for Tropical Storm Ingrid. More than 16" of rain are predicted for the Oaxaca and Tampico areas of Mexico. Image credit: NOAA/GFDL.

Forecast for Ingrid
The soils along the Mexican Gulf Coast are already saturated from the rains of Tropical Depression Eight and Tropical Storm Fernand, and it won't take much additional rain to generate dangerous flash floods and mudslides. An added danger is the presence of tropical disturbance 90E in the Eastern Pacific Ocean, on the other side of Mexico. If Ingrid intensifies sufficiently, it could draw in the moisture from 90E across Southern Mexico, resulting in torrential rains on both the Pacific and Atlantic sides of Southern Mexico. Ninety-E represents a threat to develop into a tropical depression in its own right; in their 8 am EDT Friday Tropical Weather Outlook, NHC gave 2-day odds of development of 70% for the disturbance, and predicted a north to northwest motion of the storm towards the coast. This morning's 2 am EDT run of the experimental GFDL ensemble model predicted that a some areas of Mexico are at high risk of 16+ inches of rain due to the combined effects of Ingrid and 90E. The greatest danger is on the Pacific side in Oaxaca state, where the combined effects of the circulations of Ingrid and 90E will pull a flow of very moist air upwards over the mountains, creating torrential rains. All of the models predict a west-northwest to northwest track for Ingrid into Mexico, but heavy rains of 2 - 4" may also affect extreme South Texas by early next week.

Today is the 25th anniversary of the intensification of Hurricane Gilbert into the strongest hurricane ever observed in the Atlantic (at the time.) I was on the hurricane hunter flight into Gilbert that day, and will be posted an account of the mission later today.

Jeff Masters

Flood Hurricane

Updated: 7:29 PM GMT on September 13, 2013

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Flash Floods Kill 3 in Colorado; Dangerous 93L Developing in Gulf of Mexico

By: JeffMasters, 3:06 PM GMT on September 12, 2013

Devastating flash floods swept though numerous canyons along the Front Range of Colorado's Rocky Mountains Wednesday night and Thursday morning, washing out roads, collapsing houses, and killing at least two people. The floods were triggered by widespread torrential rains of 4 - 6" that fell in less than twelve hours, thanks to a flow of extremely moist air from the southeast that pushed up against the mountains. These sort of upslope rain events are so-named because as the air flows uphill, it expands and cools, forcing the moisture in it to fall as rain. Balloon soundings from Denver last night and this morning recorded the highest levels of September moisture on record for the station. The total precipitable water (PW), which is how much water would fall at the ground if the entire amount of water vapor through the depth of the atmosphere was condensed, was 1.33" at 12Z (8 am EDT) on September 12, and 1.31" at 00Z September 12. The previous September record was 1.23", set on September 10, 1980. Balloon soundings began in 1948. Wednesday night's rainfall was heaviest near Boulder, Colorado, where a flash flood watch continues through Thursday evening. Though rainfall amounts today are not expected to be as great as on Wednesday, the soils are saturated, and additional flash flooding will occur today as an upper-level low centered over the southern Great Basin continues to pull a moist southeasterly surface flow of air across Eastern Colorado. A map of Boulder flood zones and detailed history of previous floods in the area may be found here.


Figure 1. Flooding in Boulder, Colorado on Wednesday evening, September 11, 2013. Photo posted by brandish on Instagram @photogjake.


Figure 2. Radar-estimated rainfall for the Colorado's Front Range from the September 11 - 12 flash flood event. Rainfall amounts of 6 - 8" (dark rad colors) were indicated near Boulder (circle with a "+" symbol), and confirmed by rain gauge measurements.


Video 1. Flooding in Boulder, Colorado at 36th and Colorado Street on Wednesday night, September 11, 2013.

Dangerous 93L developing in the Gulf of Mexico
A low pressure system (Invest 93L) over the Southwestern Gulf of Mexico's Bay of Campeche is generating heavy rains over the Yucatan Peninsula and adjacent waters as it moves west-northwest at about 5 mph. Radar loops from Mexico show that 93L has a pronounced rotation and is developing low-level spiral bands, and there is a strong possibility that the Air Force hurricane hunter plane scheduled to investigate 93L Thursday afternoon will find that a tropical depression has formed. Satellite loops show only a modest area of heavy thunderstorms, but these are steadily growing in intensity and areal coverage. The atmosphere is unusually moist, ocean temperatures are a very warm 29 - 29.5°C (84 - 85°F), and wind shear is a moderate 10 - 15 knots. These favorable conditions for development are expected to continue through Monday, according to the latest SHIPS model forecast. The computer models predict a landfall location along the Mexican coast between Veracruz and a location a few hundred miles south of the Texas/Mexico border by early next week. The storm is expected to maintain a forward speed of about 5 mph during the coming five days, and this slow motion will potentially allow 93L enough time to intensify into a hurricane before landfall. The high levels of moisture and slow motion of 93L make it a very dangerous rainfall threat, and I expect rainfall amounts of 5 - 10 inches will affect portions of the Mexican coast between Veracruz and Brownsville, Texas over the next five days. An added danger is the presence of tropical disturbance 90E in the Eastern Pacific Ocean, on the other side of Mexico. If 93L intensifies sufficiently, it could draw in the moisture from 90E across Southern Mexico, resulting in torrential rains on both the Pacific and Atlantic sides of Southern Mexico. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave 2-day odds of development of 60% and 5-day odds of 80%.


Figure 3. Latest satellite image of Tropical Depression Ten.

Humberto peaks in intensity
Hurricane Humberto is headed northwards over the far Eastern Atlantic, and has likely peaked in strength, with 85 mph winds. Humberto is listed in NHC's preliminary Best-Track data repository as having achieved hurricane strength at 8 am EDT on September 11, which would tie it with Hurricane Gustav of 2002 for latest appearance of the season's first hurricane since 1941. Satellite loops show that Humberto is well-organized hurricane with a distinct eye, but the storm has moved over waters cooler than 26°C, and wind shear has risen to 20 knots, which will likely cause weakening over the next few days. Humberto is not expected to be a threat to any land areas.

Gabrielle heads towards Canada
Tropical Storm Gabrielle is headed northwards to Canada after bringing 40 mph winds and just over an inch of rain to Bermuda on Tuesday night. Satellite loops show that wind shear has ripped up Gabrielle, leaving the storm with almost no heavy thunderstorms, and exposing the low level circulation to view. Wind shear is expected to rise to an extremely high 45 - 70 knots on Friday as Gabrielle encounters a trough of low pressure, which will likely destroy the storm. Gabrielle's remnants will likely bring heavy rain to the Canadian Maritime Provinces on Friday and Saturday.

Jeff Masters

Flood Hurricane

Updated: 1:14 AM GMT on September 13, 2013

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Humberto Becomes the Atlantic's First Hurricane of 2013

By: JeffMasters, 2:33 PM GMT on September 11, 2013

The Atlantic's first hurricane of 2013 finally arrived this Wednesday morning, as Hurricane Humberto intensified into a minimum-strength Category 1 hurricane in the far Eastern Atlantic. Humberto is listed in NHC's preliminary Best-Track data repository as having achieved hurricane strength at 8 am EDT on September 11, which would tie it with Hurricane Gustav of 2002 for the latest appearance of the season's first hurricane, since 1941. Humberto is also Earth's first tropical cyclone to reach Category 1 strength in three weeks--the last was Typhoon Trami, which hit China on August 21 as a Category 1 storm with 85 mph winds. To go three weeks during late August and early September without a Category 1 or stronger tropical cyclone is a very unusual event. Satellite loops show that Humberto is a small but well-organized hurricane with a prominent eye, located just west of the Cape Verde Islands. Humberto has until Thursday night to continue the intensification process, at which time high wind shear and cooler waters will likely cause weakening. Humberto is not expected to be a threat to any land areas.


Figure 1. First eye of 2013: MODIS satellite image of Hurricane Humberto, taken at 8:30 am EDT on September 11, 2013. At the time, Humberto was a Category 1 hurricane with 75 mph winds. Humberto was declared the Atlantic's first hurricane of 2013 in the 5 am EDT advisory on September 11. Image credit: NASA.

Gabrielle hits Bermuda
Tropical Storm Gabrielle blew past Bermuda last night, bringing sustained winds of 40 mph, gusting to 53 mph, along with just over an inch of rain to the Bermuda Airport. Satellite loops show that high wind shear of 20 - 25 knots has ripped apart Gabrielle, leaving the storm with almost no heavy thunderstorms, exposing the low level circulation to view. Wind shear is expected to remain high for the next two days, which will likely keep Gabrielle from strengthening. On Friday and Saturday, moisture from Gabrielle or its remnants will likely bring heavy rain to the Canadian Maritime Provinces.


Figure 2. Radar image of Tropical Storm Gabrielle at 9:03 pm AST September 10, 2013. At the time, Gabrielle had top winds of 60 mph, and strong upper level winds out of the west had pushed Gabrielle's heaviest thunderstorms to the east side of the center of circulation. The island is under the white "+" at the center of the image. Image credit: Bermuda Weather Service.

93L a threat to develop in the Gulf of Mexico
A trough of low pressure over Mexico's Yucatan Peninsula (Invest 93L) is generating heavy rains over the peninsula and adjacent waters as it moves west-northwest at 5 - 10 mph. Satellite loops and radar loops from Mexico and Belize show that 93L has a decent amount of spin and a modest area of heavy thunderstorms. After 93L emerges over the Southwestern Gulf of Mexico's Bay of Campeche on Thursday, the disturbance has a high chance of developing into a tropical depression as it crosses the Bay of Campeche. The atmosphere will be unusually moist, ocean temperatures will be a very warm 29°C (84°F), and wind shear will be a moderate 10 - 15 knots, according to the latest SHIPS model forecast. The computer models predict a landfall location along the Mexican coast between Veracruz and a location a few hundred miles south of the Texas/Mexico border by early next week, and it appears that 93L's heaviest rains will stay south of Texas. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave 2-day odds of development of 40% and 5-day odds of 70%. Due to its slow motion, 93L will have more time to intensify than the other storms we've seen in the Bay of Campeche this year--Tropical Storm Barry, Tropical Storm Fernand, and Tropical Depression Eight--and 93L will likely pack heavier rains and higher winds at landfall than these three previous storms.

Jeff Masters

Hurricane

Updated: 3:49 PM GMT on September 11, 2013

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Hurricane Season is Half Over; Will it Remain Quiet?

By: JeffMasters, 2:18 PM GMT on September 10, 2013

September 10 marks the traditional halfway point of the Atlantic hurricane season, and the first half of the hurricane season of 2013 is making its mark in the history books as one of the least active such periods on record. Going back to before when the Hurricane Hunters first began flying in 1944, there has been only one hurricane season that made it past the half-way point without a hurricane forming: the El Niño year of 2002, when Hurricane Gustav formed at 8 am EDT on September 11. Tropical Storm Humberto is looking poised to become a hurricane later today, and 2013 will likely end up ranking in 2nd place for latest formation of the season's first hurricane, going back to 1941. Here are the Atlantic hurricane seasons since 1941 in which the first hurricane did not form until after September 7:

2002: September 11, Hurricane Gustav
2013: September 10+ (Nothing yet)
1984: September 10, Hurricane Diana
2001: September 8, Hurricane Erin

Prior to 1944, there were four hurricane seasons that had the first hurricane form after September 15:

1914: No hurricanes in the Atlantic
1907: No hurricanes in the Atlantic
1905: October 8
1941: September 16

According to NHC, August 10 is the average date the first Atlantic hurricane arrives, and the season's third hurricane usually develops by September 9. So, assuming Humberto makes it to hurricane status, we are two hurricanes behind the average season pace. An average season brings six hurricanes, two of them being intense hurricanes.

Remarkably low Accumulated Cyclone Energy (ACE) so far in 2013
The first half of 2013's hurricane season had one of the lowest Accumulated Cyclone Energy (ACE) totals on record for the Atlantic. ACE is calculated as the square of the wind speed every 6 hours for every named storm with at least 40 mph sustained winds (scaled by a factor of 10,000 for usability.) Since the damage potential of a hurricane is proportional to the square or cube of the maximum wind speed, ACE is not only a measure of tropical cyclone activity, but also a measure of the damage potential. During the 20-year period 1981 - 2010, the Atlantic averaged 104 ACE units, and the 20-year average ACE through September 9 was 50. Through September 9 of 2013, we've managed just 9.6 ACE units, about 20% of average. Since the current active hurricane period we are in began in 1995, no other year had a lower ACE by this point in the year. Since reliable satellite-based ACE records began in 1966, there have been seven years with comparable levels of ACE by this point in the season. Four of these years were El Niño years, when we expect hurricane activity to be low due to high wind shear. The other years had August ocean temperatures in the hurricane Main Development Region (MDR, from 10 - 20°N, 20 - 70°W) that were cooler than in 2013, though the ocean temperatures in 1988 were only 0.1°C cooler than in 2013. This year's combination of no El Niño, warm MDR SSTs, and an exceptionally low first half of the season ACE is thus an event unparalleled in the historical record, going back to 1966.


Figure 1. Accumulated Cyclone Energy (ACE) totals for the first half of the Atlantic hurricane season, through September 9, 2013, were among the lowest on record, since reliable satellite data began in 1966. Other years with low first-half-of-the-season ACE occurred during El Niño years, or when August sea surface temperatures (SSTs) were cooler.

Forecast for the next two weeks: below average activity
The main reason for the quiet first half of the season has been the large amount of dry, stable air over the Atlantic. The primary source of this dry air has been the Sahara desert of Africa. However, dry air from Northeast Brazil may also have contributed, argues wunderblogger Lee Grenci. That region of the country experienced a record $8.3 billion drought in 2013--the most expensive natural disaster in Brazil's history. Even with all the dry air we've seen over the Atlantic in 2013, it is really remarkable that activity has been so low when all of the other factors--lack of an El Niño, wind shear near climatological averages, an active African Monsoon spitting out plenty of tropical waves, and above average ocean temperatures--have favored development. Instability increased over the tropical Atlantic over the last few days of August and the first week of September, thanks to the influence of the Madden Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days. Instability was also boosted by a Convectively Coupled Kelvin Wave (CCKW) that brought rising air to the Atlantic. This increase in instability helped the formation of Gabrielle, Humberto, and Tropical Depression Eight, and may help boost the odds of a potential tropical storm forming this weekend over the Southwestern Gulf of Mexico's Bay of Campeche. The influence of the MJO is fading over the Atlantic, though. Beginning next week, we will be entering a phase of the MJO where it will likely bring more stable, sinking air to the Tropical Atlantic. This suppressed phase of the MJO could last through the first week of October. The models are also pointing to another outbreak of dry air from the Saharan Air Layer (SAL) coming off the coast of Africa this weekend, which will keep the Tropical Atlantic dryer than usual next week. The steering pattern over the next two weeks features a strong trough of low pressure over the U.S. East Coast, giving high odds that any hurricane that manages to form and approach the U.S. will recurve out to sea, without affecting any land areas. So keep your fingers crossed--we're doing unusually well for this point in the hurricane season, with no landfalling hurricanes, and it looks like we have above average chances of keeping it that way deep into September.


Figure 2. Vertical instability over the tropical Atlantic in 2013 (blue line) compared to average (black line.) The instability is plotted in °C, as a difference in temperature from near the surface to the upper atmosphere. Thunderstorms grow much more readily when vertical instability is high. Instability was been much lower than average during most of August, primarily due to outbreaks of dry air from Saharan Air Layer (SAL). Low instability reduces the potential for tropical storm formation. During the last few days of August and the first week of September, an MJO event boosted instability. The MJO or Madden Julian Oscillation is a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days. When the area of increased thunderstorms associated with the MJO is located in the Western Hemisphere, it brings rising air over the Tropical Atlantic, increasing instability. Image credit: NOAA/NESDIS/CIRA.

Be prepared for a destructive hurricane in 2013
Residents of Hurricane Alley shouldn't assume the rest of the season will end with a whimper, though. All it takes is one bad hurricane to ruin an otherwise quiet hurricane season. Recall that last year's worst storm--Hurricane Sandy--didn't occur until the third week of October. Another thing to consider: the season with the greatest similarity to what we've seen during the first half of the 2013 season was 1988. That year, we also had unusual quietness before September 10, no El Niño, and above average ocean temperatures in the MDR. But the most powerful Atlantic hurricane ever recorded up to that time ripped through the Caribbean, Hurricane Gilbert, as well as two other major hurricanes. I flew into Hurricane Gilbert at the height of its fury 25 years ago, and plan to recount the story of that amazing flight this Friday.

Two simultaneous named storms in the Atlantic
The dearth of Accumulated Cyclone Energy in the Atlantic will get made up some in the next few days, as we have two simultaneous named storms in the Atlantic for the first time this year. Tropical Storm Gabrielle reformed this morning, about 160 miles south of Bermuda. Satellite loops show a respectable area of heavy thunderstorms have developed near the core of the storm, despite the presence of high wind shear of 20 - 25 knots. Wind shear is expected to remain high for the next three days, which will likely keep Gabrielle below hurricane strength. The storm is expected to bring 40 mph winds and heavy rain to Bermuda Tuesday night through Wednesday morning. On Friday and Saturday, moisture from Gabrielle or its remnants will likely bring heavy rain to the Canadian Maritime Provinces.

Tropical Storm Humberto is churning across the far Eastern Atlantic, and is expected to turn northwards and intensify into a hurricane later today. Satellite loops show that Humberto is a small but well-organized storm that is staying just west of the Cape Verde Islands. Humberto is not expected to be a threat to any land areas.

The models are bullish on developing a tropical depression in the Southwestern Gulf of Mexico's Bay of Campeche by Saturday. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave 2-day odds of development of 10% and 5-day odds of 60%. Any storm developing the Gulf would likely track west-northwest into the Mexican coast a few hundred miles south of the Texas border, and could bring heavy rains as far north as Corpus Christi early next week.

Jeff Masters

Hurricane

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Tropical Storm Humberto Drenching the Cape Verde Islands

By: JeffMasters, 2:11 PM GMT on September 09, 2013

Tropical Storm Humberto, the eighth named storm of the 2013 Atlantic hurricane season, is here. Humberto has formed unusually far to the east, between the coast of Africa and the Cape Verde Islands. Tropical Storm Warnings are flying in the southern Cape Verde Islands, and Humberto's rain bands have already arrived in capital city of Praia, where 1.46" of rain has fallen, with sustained winds as high as 26 mph. Humberto's west to west-northwest motion at 12 mph will keep the storm just south and west of the islands through Tuesday, but this path will be close enough to bring potentially dangerous rainfall amounts of 3 - 6 inches to the southern islands. Satellite loops show that Humberto is well-organized with plenty of spin and a growing amount of heavy thunderstorms. The models are bullish of developing Humberto into a hurricane just west of the Cape Verde Islands by Wednesday. If Humberto reaches hurricane strength before 8 am EDT on Wednesday, 2013 will avoid setting the record for the latest formation date of the Atlantic's first hurricane, dating back the beginning of the aircraft reconnaissance era in 1944. Humberto is expected to take a sharp northwards turn later this week, which will carry the storm into a region of ocean where no land areas would likely be at risk from a strike, with the possible exception of the Azores Islands.


Figure 1. Morning satellite image of Tropical Storm Humberto, taken at 7:45 am EDT September 9, 2013. Image credit: NOAA.

Cape Verde Islands Hurricane History
The Atlantic's most terrifying and destructive hurricanes typically start as tropical waves that move off the coast of Africa and pass near the Cape Verde Islands. This class of storms is referred to as "Cape Verde hurricanes", in reference to their origin. Despite the fact that the Atlantic's most feared type of hurricanes are named after the Cape Verde Islands, these islands rarely receive significant impacts from one of their namesake storms. This is because tropical waves coming off the coast of Africa have very little time to organize into tropical storms before arriving at the Cape Verde Islands, which lie just 350 miles west of the African coast. According to EM-DAT, the International Disaster Database, there have been only two deadly tropical cyclones in Cape Verde history. The deadliest was Tropical Storm Fran of 1984, which brushed the southwestern islands on September 16 as a tropical storm with 50 mph winds. Fran brought sustained winds of 35 mph and torrential rains to the islands. The rains triggered flash flooding that killed 29 - 31 people and caused damages of almost $3 million (1984 dollars.) The other deadly named storm was Tropical Storm Beryl of 1982, which passed about 30 miles south of the southwestern islands on August 29, with 45 mph winds. The storm's heavy rains killed three people on Brava Island, injured 122, and caused $3 million in damage.

The most recent named storm to affect the islands was Hurricane Julia of 2010, which was the easternmost Category 4 hurricane ever recorded in the Atlantic. Julia passed about 50 miles south of Sao Filipe in the southern Cape Verde Islands as a tropical storm with 45 mph winds, bringing wind gusts of 30 mph to the islands and some minor flooding.


Figure 2. Track of Tropical Storm Fran of 1984, which brushed the southwestern Cape Verde Islands on September 16 as a tropical storm with 50 mph winds. Torrential rains from Fran killed 29 - 31 people in the Cape Verde Islands, making it the deadliest storm in their history.

Ex-Gabrielle
The remnants of Gabrielle (now being called Invest 92L) are generating heavy thunderstorms about 500 miles south-southwest of Bermuda, as seen on satellite loops. High wind shear of 25 knots is inhibiting development, and wind shear is expected to stay high for the next three days. The disturbance is headed to the north-northeast to northeast at 10 mph, and is expected to pass several hundred miles east of Bermuda Tuesday through Wednesday. Since strong westerly winds are keeping most of ex-Gabrielle's heavy thunderstorms displaced to the east, the bulk of this activity should miss Bermuda. NHC put ex-Gabrielle's 2-day and 5-day odds of development at 20% in their 8 am EDT Monday Tropical Weather Outlook.

Jeff Masters

Hurricane

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A Rare Tropical Cyclone-Free September 8th for Earth

By: JeffMasters, 4:31 PM GMT on September 08, 2013

It's a rare tropical cyclone-free September 8th on Earth, without a tropical depression, tropical storm, hurricane, or typhoon in the Atlantic, Pacific, or Indian Oceans. Considering that the first two weeks of September are the peak of the tropical cyclone season in the Atlantic, Eastern Pacific, and Northwest Pacific, today's lack of activity is quite unusual. It looks like the Atlantic will be the ocean basin to break the tropical cyclone-free spell, as a strong tropical wave (Invest 91L) emerged from the coast of Africa on Saturday, and is now moving west-northwest at 10 mph towards the Cape Verde Islands. Satellite loops show that 91L is well-organized with plenty of spin and a growing amount of heavy thunderstorms. All of the models develop this wave into a tropical depression just west of the Cape Verde Islands by Wednesday, and 91L has the potential to intensify into a hurricane by late in the week. The storm is expected to track to the northwest into a region of ocean where the Azores Islands would likely be the only land area at risk from a strike. In their 8 am EDT Sunday Tropical Weather Outlook, NHC put the 2-day odds of development at 70%, and the 5-day odds at 90%.


Figure 1. Could this be the beginnings of the Atlantic's first hurricane of 2013? MODIS satellite image of tropical wave 91L off the coast of Africa, taken at approximately 8 am EDT September 8, 2013. All of the models develop this wave into a tropical depression by Wednesday, and some of the models show it growing to hurricane strength by the end of the week. Image credit: NASA.

Ex-Gabrielle
The remnants of Gabrielle (now being called Invest 92L) are generating heavy thunderstorms about 400 miles north of the Eastern Dominican Republic, as seen on satellite loops. High wind shear of 20 knots is inhibiting development, and wind shear is expected to stay high for the next five days. NHC put ex-Gabrielle's 2-day odds of development at 10% and 5-day odds at 30% in their 8 am EDT Sunday Tropical Weather Outlook. The disturbance is headed to the northeast at 10 mph, and is expected to pass several hundred miles east of Bermuda on Tuesday.

Jeff Masters

Hurricane

Updated: 1:53 PM GMT on September 09, 2013

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Tropical Depression Eight Hits Tampico, Mexico

By: JeffMasters, 3:14 PM GMT on September 07, 2013

Blink, and you missed it: the latest minor blip on the almost inconsequential Atlantic hurricane season of 2013 was Tropical Depression Eight, which formed at 2 pm EDT Friday just offshore of Tampico, Mexico. The depression made landfall less than three hours later, and had top winds estimated at 35 mph. The depression has already dissipated, but brought heavy rains that were expected to accumulate to 3 - 5" along its path. Tampico, Mexico recorded 2.60" of rain from the storm.


Figure 1. MODIS satellite image of the Tropical Depression Eight, taken at 1:30 pm EDT on September 6, 2013. TD 8 was making landfall near Tampico, Mexico, with top winds of 35 mph. Image credit: NASA.

Ex-Gabrielle
The remnants of Gabrielle are generating heavy thunderstorms a few hundred miles north of Puerto Rico and the Eastern Dominican Republic, as seen on satellite loops. High wind shear of 20 - 25 knots is inhibiting development, and wind shear is expected to stay high for the next five days. NHC put ex-Gabrielle's 2-day odds of development at 20% and 5-day odds at 40% in their 8 am EDT Saturday Tropical Weather Outlook. The disturbance is headed to the north at 10 - 15 mph, and is expected to turn to the northeast and pass several hundred miles east of Bermuda on Tuesday. For such a meager storm, ex-Gabrielle will be getting a lot of attention today. NOAA is planning on flying all three of their hurricane research aircraft into the storm, and NASA will be sending up one of their two remotely piloted Global Hawks. An Air Force hurricane hunter flight is also scheduled for Saturday afternoon. With the tropical Atlantic not looking like it will generate anything more interesting to study for at least the next week, the hurricane researchers have to make do with what little they have.


Figure 2. Could this be the beginnings of the Atlantic's first hurricane of 2013? MODIS satellite image of a tropical wave emerging from the coast of Africa, taken at approximately 8 am EDT September 7, 2013. All of the models develop this wave into a tropical depression by Tuesday, and some of the models show it growing to hurricane strength by mid-week. Image credit: NASA.

New African tropical wave: is it destined to be the Atlantic's first hurricane of 2013?
A strong tropical wave is emerging from the coast of Africa this weekend, and all of the models develop this wave into a tropical depression just west of the Cape Verde Islands by Tuesday. Some of the models have been eager to intensify the storm into a hurricane by mid-week. The storm is expected to track to the northwest into a region of ocean where the Azores Islands would likely be the only land area at risk from a strike. In their 8 am EDT Saturday Tropical Weather Outlook, NHC put the 5-day odds of development at 80%.

Why such a quiet Atlantic hurricane season?
Unusual dryness over the Atlantic has been the main reason for this year's lack of hurricanes, it appears. Wunderblogger Lee Grenci's latest post, "The Lack of Atlantic Hurricanes: The Saga of Low Relative Humidity Continues", looks at how dry the Tropical Atlantic has been this hurricane season. Part of the unusual dryness, he maintains, is due to dry air coming off the coast of Brazil, which is in severe to extreme drought, according to the global drought monitor. Aon Benfield puts the cost of the Brazilian drought at $8.3 billion so far this year, making it Brazil's most expensive natural disaster in its history.

Have a great weekend, everyone!

Jeff Masters

Hurricane

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Gabrielle Dissipates; the Atlantic Quiets Down

By: JeffMasters, 2:49 PM GMT on September 06, 2013

After a brief 12-hour stint as a tropical storm, Gabrielle was torn apart Thursday night by a strong tropical disturbance to its east, and interaction with the rough terrain of Puerto Rico and the Dominican Republic. The remnants of Gabrielle and the tropical disturbance to its east are generating heavy thunderstorms over Puerto Rico, the Eastern Dominican Republic, and the Virgin Islands, as seen on satellite loops. The tropical disturbance that helped pull Gabrielle apart is a threat to become a tropical depression on its own--NHC put its 5-day odds of development at 30% in their 8 am EDT Friday Tropical Weather Outlook. The disturbance is headed to the northwest at 10 - 15 mph, and may merge with Gabrielle's remnants early next week. Heavy thunderstorms from Gabrielle dumped 5.37" of rain on St. Thomas in the Virgin Islands and Southeast Puerto Rico, but the storm did not generate any sustained winds of tropical storm force (39 mph) at any land stations.


Figure 1. Total rainfall from Gabrielle, as compiled by NOAA.

Gabrielle's place in history
Gabrielle's formation date of September 5 (UTC time) comes eleven days earlier than the usual formation date of the season's 7th storm, September 16. However, by this point in the season, we should already have had two hurricanes, one being an intense hurricane, and 2013 has not yet had a hurricane. Gabrielle is the 7th consecutive named storm in the Atlantic that has not reached hurricane strength. Only one season since record keeping began in 1851 has had a longer string of consecutive storms that did not reach hurricane strength--2011, when the season began with eight such storms. It is possible such an event occurred before the advent of reliable satellite data in 1966, when we were first able to identify weak tropical storms that stayed out to sea. Several seasons have had six consecutive tropical storms without a hurricane, most recently in 2002. The air over the Tropical Atlantic has been more stable and drier than usual (and was so in 2011), making it difficult for storms to attain hurricane strength. Wunderblogger Lee Grenci's latest post, "The Lack of Atlantic Hurricanes: The Saga of Low Relative Humidity Continues", looks at how dry the Tropical Atlantic has been this hurricane season. Part of the unusual dryness, he maintains, is due to dry air coming off the coast of Brazil, which is in severe to extreme drought, according to the global drought monitor. Aon Benfield puts the cost of the Brazilian drought at $8.3 billion so far this year, making it Brazil's most expensive natural disaster in its history.

Elsewhere in the Atlantic
A tropical wave over the Western Gulf of Mexico's Bay of Campeche (Invest 99L) will move ashore on the Mexican coast near Tampico by early afternoon. Satellite images show that 99L has a moderate area of heavy thunderstorms that are slowly increasing in size and intensity, and the storm may have a closed circulation, as evidenced by northwest winds observed at Tampico, Mexico at 8 am CDT on Friday. In their 8 am EDT Friday Tropical Weather Outlook, NHC put the 5-day and 2-day odds of development at 20%.

A tropical wave in the Eastern Atlantic about 600 miles west of the Cape Verde Islands (Invest 98L) is headed west-northwest at about 10 mph. Satellite images show that 98L has a decent amount of spin, but a very limited amount of heavy thunderstorms. The system has encountered an area of dry, stable air, and in their 8 am EDT Friday Tropical Weather Outlook, NHC put the 5-day and 2-day odds of development at 0%.

A strong tropical wave is predicted to emerge from the coast of Africa on Saturday, and all of the models develop this wave into a tropical depression just west of the Cape Verde Islands by Tuesday. The storm is expected to track to the northwest into a region of ocean where very few tropical cyclones ever make the long crossing of the Atlantic Ocean to threaten North America. In their 8 am EDT Friday Tropical Weather Outlook, NHC put the 5-day odds of development at 50%.

Jeff Masters

Hurricane

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Gabrielle Getting Ripped Apart, Now a Tropical Depression

By: JeffMasters, 3:05 PM GMT on September 05, 2013

After a brief 12-hour life as a tropical storm, Gabrielle has weakened to a tropical depression as it moves northwest at 9 mph. Wind shear, dry air, and interaction with the rough terrain of Puerto Rico and a strong tropical disturbance to its northeast have significantly disrupted Gabrielle. The surface center of circulation is near the Mona Passage between Puerto Rico and the Dominican Republic, and is displaced about 150 miles to the west-southwest of the main area of heavy thunderstorms over the Virgin Islands, as seen on satellite loops. A strong tropical disturbance located about 300 miles to the northeast of Gabrielle pulled the mid-level circulation of Gabrielle into it, leaving the surface circulation behind. This tropical disturbance that helped pull Gabrielle apart is a threat to become a tropical depression on its own--NHC gave it a 20% chance of development in their 8 am EDT Thursday Tropical Weather Outlook. The disturbance is headed to the northwest into an area with high wind shear of 20 - 30 knots, so development is unlikely.

Heavy thunderstorms from Gabrielle dumped over two inches of rain over portions of the Virgin Islands and Southeast Puerto Rico as of 10 am AST Thursday. Heavy rains will continue today across Puerto Rico and the Virgin Islands, as seen on radar out of San Juan, Puerto Rico. Gabrielle did not generate any sustained winds of tropical storm force (39 mph) at any land stations.


Figure 1. Morning satellite image of Gabrielle showing the surface center displaced 150 miles to the west-southwest of the main heavy thunderstorms over the Virgin Islands.

Forecast for Gabrielle
Wind shear is expected to steadily increase as Gabrielle heads northwest over the next few days. The storm will also be encountering the high mountains of Puerto Rico and Hispaniola, which should act to further disrupt the circulation. The tropical disturbance to the northeast of Gabrielle will also act to pull it apart, and it appears likely that Gabrielle will dissipate by Friday morning, unless the surface center can reform underneath the heaviest thunderstorms 150 miles to the east-northeast of the surface center. Gabrielle or its remnants should begin heading to the north over the weekend, and may pass close to Bermuda next week.


Figure 2. Radar-estimated precipitation from Gabrielle as of 10:32 am AST September 5, 2013.

Elsewhere in the Atlantic
A tropical wave over the extreme Southwestern Gulf of Mexico's Bay of Campeche (Invest 99L) will move ashore on the Mexican coast near Tampico on Friday. Satellite images show that 99L has a moderate area of heavy thunderstorms that are slowly increasing in size and intensity. Wind shear is moderate and water temperatures are a warm 30°C, so 99L will likely show steady development until landfall on Friday. In their 8 am EDT Thursday Tropical Weather Outlook, NHC put the 5-day and 2-day odds of development at 30%. A hurricane hunter aircraft is scheduled to check out the disturbance Thursday afternoon.

A tropical wave in the Eastern Atlantic about 450 miles west-southwest of the Cape Verde Islands (Invest 98L) is headed west-northwest at about 10 mph. Satellite images show that 98L has a decent amount of spin, but a very limited amount of heavy thunderstorms. Wind shear is moderate and water temperatures are a warm 28°C, so 98L may show some slow development today and Friday. By Saturday, 98L will encounter drier and more stable air, and be over waters barely warm enough to support a tropical cyclone--26.5°C. In their 8 am EDT Thursday Tropical Weather Outlook, NHC put the 5-day and 2-day odds of development at 10%.

A strong tropical wave is predicted to emerge from the coast of Africa on Saturday, and the GFS model develops this wave into a tropical depression near the Cape Verde Islands by Monday. The storm is expected to track to the northwest into a region of ocean where very few tropical cyclones ever make the long crossing of the Atlantic Ocean to threaten North America. Both the GFS and European models predict that this system will develop into a hurricane next week. In their 8 am EDT Thursday Tropical Weather Outlook, NHC put the 5-day odds of development at 30%.

Jeff Masters

Hurricane

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Tropical Depression Seven Forms Near Puerto Rico

By: JeffMasters, 12:37 AM GMT on September 05, 2013

Tropical Storm Warnings are flying in Puerto Rico, as Tropical Depression Seven steadily organizes and moves north-northwest at 2 mph towards the south coast of the island. TD 7 had been moving to the northwest most of Wednesday, and this new NNW motion may be due to the influence of a strong tropical disturbance located about 300 miles to the east-northeast of TD 7. This disturbance to the ENE of TD 7 has greater areal extent and more intense thunderstorms than TD 7, and is a threat to become a tropical depression on its own--NHC gave it a 10% chance of development in their 8 pm EDT Tropical Weather Outlook. If this happens, the disturbance may be able to steal moisture from TD 7 and keep it weaker than it would otherwise become. It is more probable that TD 7 will absorb a portion of the disturbance, stealing some of its moisture and spin, and becoming larger. Heavy thunderstorms from TD 7 have dumped over an inch of rain in many locations in the Virgin Islands and Northern Puerto Rico as of 8 pm AST Wednesday, but TD 7 has contracted, keeping its heaviest rains focused offshore near the center of circulation. These heavy rains will move ashore Thursday morning, bringing three-day rain totals of 5 - 10" to areas along the south coast. Satellite loops show that TD 7 is developing some respectable heavy thunderstorms with very cold cloud tops near its center, and there are good upper-level outflow channels to the north and south. Radar out of San Juan, Puerto Rico shows good low-level spiral bands on the storm's southeast side, and TD 7 is not far from becoming Tropical Storm Gabrielle. Upper level winds are favorable for development, with wind shear a low 5 - 10 knots.


Figure 1. True-color MODIS satellite image of TD 7 in its formative stages, taken at 1:30 pm EDT on September 4, 2013. Image credit: NASA.

I'll have a full update in the morning.

Jeff Masters

Hurricane

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97L Dumping Heavy Rains on Puerto Rico, Virgin Islands

By: JeffMasters, 3:31 PM GMT on September 04, 2013

A tropical wave over the Northeast Caribbean (Invest 97L) is slowly growing more organized as it moves west-northwest at 10 mph towards Puerto Rico and the Dominican Republic. Winds gusts of 36 mph have been recorded this morning at buoy 41052 south of St. John in the Virgin Islands, and heavy thunderstorms from 97L have dumped over an inch of rain in many locations in the Virgin Islands and Northern Puerto Rico as of 11 am AST Wednesday. Puerto Rico doesn't need the rain, as they have had one of their wettest years on record; San Juan is nearly two feet (23.49") above average in rainfall for the year. Satellite loops show that 97L has a moderate amount of spin and heavy thunderstorms, with a respectable upper-level outflow channel to the north. There is no sign of a well-organized surface circulation. Long-range radar out of San Juan, Puerto Rico shows some modest spin to 97L's echoes, but no well-organized low-level spiral bands. Upper level winds are favorable for development, with wind shear a low 5 - 10 knots. There is some dry air from the Saharan Air Layer (SAL) that is interfering with development, but this is becoming less of an issue as heavy thunderstorms from 97L continue to moisten the storm's environment.


Figure 1. True-color MODIS satellite image of 97L taken at 1:30 pm EDT on September 4, 2013. Image credit: NASA.

Forecast for 97L
Wind shear is expected to be low through Friday night, and ocean temperatures will be warm, 29°C. One wild card is how 97L will interact with a strong tropical wave about 500 miles to its east, just east of the Lesser Antilles Islands. This new tropical wave may compete for moisture, slowing development of 97L, and could also modify the track of 97L. The models take 97L to the west-northwest, bringing the center over the eastern tip of the Dominican Republic on Thursday morning. This track will allow the high mountains of Puerto Rico and Hispaniola to disrupt the circulation of 97L, forcing the storm to regroup on Friday over the Southeast Bahamas and Turks and Caicos Islands. The best chance for development of 97L would appear to be on Saturday, after the storm has had time to recover from its encounter with Hispaniola. The UKMET model predicts that 97L will become a tropical depression just north of the Southeast Bahamas and Turks and Caicos Islands on Saturday, but the GFS and European models show little development over the next three days. There will be a strong trough of low pressure off the U.S. East Coast this weekend, and the models predict that this trough will be strong enough to turn 97L to the north and northeast by Sunday, keeping 97L well offshore from the U.S. East Coast, but with a possible threat to Bermuda next week. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC boosted the 5-day odds of formation of 97L to 60%, and 2-day odds to 40%. The Hurricane Hunters are on call to investigate 97L Wednesday afternoon. The Hurricane and Severe Storm Sentinel Mission, a 5-year field project utilizing two remotely crewed Global Hawk aircraft to overfly tropical storms and hurricanes, will have one of their aircraft investigate 97L today. You can follow the progress of the aircraft using NASA's live-plane tracker map (Global Hawk AV6 is tail number NASA872, and Global Hawk AV1 is tail number NASA871.)

Regardless of whether or not 97L becomes a tropical depression today, the major danger from this slow-moving storm will be heavy rains. Three to six inches of rain are predicted to fall over Puerto Rico and the Virgin Islands by Thursday morning, with 3-day rainfall totals of 5 - 10 inches expected along the south and southeast shores of Puerto Rico. These rains are capable of causing dangerous flash flooding and mudslides, and a Flash Flood Watch has been posted. Similar rainfall amounts will fall in the eastern Dominican Republic, and heavy rains of 3 - 6 " are also likely to affect the Southeast Bahamas and Turks and Caicos Islands later in the week. Since 97L is relatively small, Haiti may see lower rainfall amounts of 2 - 4 inches.

Elsewhere in the Atlantic
A tropical wave over Mexico's Yucatan Peninsula and adjacent waters of the Gulf of Mexico is causing scattered disorganized heavy thunderstorms. This activity will move over the extreme Southern Gulf of Mexico's Bay of Campeche on Wednesday and Thursday, then moving ashore on the Mexican coast between Veracruz and Tampico on Friday. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC put the 5-day and 2-day odds of development at 20%.

A strong tropical wave is predicted to emerge from the coast of Africa on Saturday, and the GFS model develops this wave into a tropical depression near the Cape Verdes Islands by Monday. The storm is expected to track to the northwest into a region of ocean where very few tropical cyclones ever make the long crossing of the Atlantic Ocean to threaten North America.

Jeff Masters

Hurricane

Updated: 8:09 PM GMT on September 04, 2013

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Could Climate Change Reduce the Frequency of Tracks Like Hurricane Sandy's?

By: JeffMasters, 10:14 PM GMT on September 03, 2013

We're used to seeing hurricane-battered beaches and flooded cities in Florida, North Carolina, and the Gulf Coast. But to see these images from the Jersey Shore and New York City in the wake of Hurricane Sandy was a shocking experience. New Jersey rarely gets hit by hurricanes, because it lies in a portion of the coast that doesn't stick out much, and is too far north. Hurricanes generally move from east to west in the tropics, where the prevailing trade winds blow that direction. But the prevailing wind direction reverses at mid-latitudes, due to the spin of the Earth, and the flow becomes predominately west-to-east. Hurricanes that penetrate to approximately Northern Florida's latitude typically get caught up in these westerly winds and are whisked northeastwards, out to sea. However, the jet stream, that powerful band of upper-atmosphere west-to-east flowing air, has many dips and bulges. These troughs of low pressure and ridges of high pressure allow winds at mid-latitudes to flow more in a south-to-north direction. Every so often, a trough in the jet stream bends back on itself when encountering a ridge of high pressure stuck in place ahead of it--a so-called "blocking high". According to NOAA's Climate Prediction Center, a "blocking high" near the longitude of Greenland (50°W) only occurs about 2% of the time in the fall, and can cause winds that flow from southeast to northwest over the Northeast U.S. It is this sort of unusual flow that sucked Sandy into New Jersey and allowed the hurricane to take the most perpendicular track into this section of coast of any tropical cyclone in the historical record (Hall and Sobel, 2013.) Using historical climate data, these scientists estimated that the return period of a Category 1 or stronger storm hitting New Jersey at such an odd angle was 1-in-700-years.


Figure 1. Tracks of all tropical storms and hurricanes to hit Southern New Jersey, 1851 - 2012. Hurricane Sandy had a track unprecedented in the historical record. Image created by TWC's Stu Ostro using data from NOAA/CSC.


Figure 2. Hurricane Sandy at 10:10 am EDT October 28, 2012. Image credit: NASA/GSFC.

Was climate change responsible for Sandy's unusual track?
Either Sandy was an extremely rare event, or else climate change has shifted the odds of such a track to make it more likely. A paper published on Monday in the Proceedings of the National Academy of Sciences by Elizabeth Barnes of Colorado State and co-authors, "Model projections of atmospheric steering of Sandy-like superstorms", argues that our best climate models project we should see a decrease in the type of steering patterns that brought Sandy to the coast at such an unusual angle. Of the 22 models used to formulate the 2013 IPCC report, 17 predict a decrease in the type of "blocking highs" responsible for Sandy's unusual track. Nineteen out of 22 of these models also predict that the jet stream will shift farther to the north, particularly in the fall. The authors argue that this jet stream shift will bring about a decrease in easterly winds south of Greenland, resulting in fewer Sandy-like storms hitting the Northeast U.S. However, Dr. Jennifer Francis, who has authored several studies linking Arctic sea ice loss to unusual jet stream patterns, noted in an email to me that "One of the strongest pieces of evidence for the study’s main conclusion is that easterly winds are projected to decrease in a large zone north of Newfoundland. The location of the strongest decreases, however, is north of the location of the block during Sandy, exactly in the region where stronger west winds would occur when blocks like this existed. This suggests that the pattern may actually cause an increase in unusually high pressures in the same location of the Sandy blocking high."


Figure 3. Jet stream winds at a pressure of 300 mb on October 29, 2012, as Hurricane Sandy approached the coast of New Jersey. Note that the wind direction over New Jersey (black arrows) was from the southeast, due to a negatively tilted trough of low pressure over the Eastern U.S. caused by a strong blocking ridge of high pressure over Greenland. Image credit: NOAA/ESRL.

Commentary
While the 2013 IPCC models used in the study are the best that we have, the uncertainties are very high in the sort of projected atmospheric changes the authors are analyzing. In the current climate, the models underestimate the frequency of the type of blocking high pressure systems that led to Sandy's unusual track. Thus, we should look with suspicion upon their predictions for a decrease in blocking highs in the future--something that Barnes et al. acknowledge in their paper. In addition, Arctic sea ice loss is occurring much faster than these models predicted; in 2012, September sea ice loss was more than 1 standard deviation below where these models predicted it should be. Thus, these models may be underestimating the influence of sea ice loss on hurricane steering flow in the North Atlantic. As I discussed in an April post, "Arctic sea ice loss tied to unusual jet stream patterns", three studies published in the past year have found that the jet stream has been getting stuck in unusually strong blocking patterns in recent years. These studies found that the recent record decline in Arctic sea ice could be responsible, since this heats up the pole, altering the Equator-to-pole temperature difference, forcing the jet stream to slow down, meander, and get stuck in large loops. The 2012 Arctic sea ice melt season was extreme, with sea ice extent hitting a record low. The 1-in-700 year Hurricane Sandy track could have had its odds boosted by the 2012 record sea ice loss, one can argue, based on this research. This research, however, is disputed by Dr. Barnes in a separate study just published in Geophysical Research Letters, "Revisiting the evidence linking Arctic Amplification to extreme weather in midlatitudes."

The Atlantic hurricane season has been getting longer in recent decades, in association with increasing ocean temperatures. A longer season gives the opportunity for more strong hurricanes to penetrate to the Northeast U.S. in late fall. Warmer ocean waters may also lead to an increase in strong hurricanes farther to the north, since cool ocean temperatures are a key reason why we see so few strong hurricanes affecting the Northeast. These influences would potentially offset any decrease in Sandy-like storms caused by fewer blocking highs forming in a future climate. Much more research is needed before we can be confident how climate change may or may not affect the tracks and frequency of future storms like Hurricane Sandy. One thing that is almost a sure thing: as global warming continues to cause sea levels to rise, the impacts of these storms will be worse as storm surge flooding penetrates farther inland.


Figure 4. Extent of Arctic sea ice predicted by the mean of 20 climate models used to formulate the 2013 IPCC report (thick red line); the pink area denotes plus or minus one standard deviation from the mean. The actual levels of sea ice (thick black line) fell below one standard deviation from what the model were predicting in 2012. The older predictions from the set of models used to formulate the 2007 IPCC report are shown in blue. Image credit: Stroeve et al. 2012, "Trends in Arctic sea ice extent from CMIP5, CMIP3 and observations", Geophysical Research Letters, DOI: 10.1029/2012GL052676.

Europe expected to see a large increase in Hurricane Sandy-like hybrid storms?
While the new study by Barnes et al. gives some hope that global warming might lead to fewer Sandy-like storms hitting the Northeast U.S., dangerous part-hurricane, part extratropical hybrid storms like Hurricane Sandy are expected to be an increasing threat for Western Europe by the end of the century due to global warming, said a team of scientists led by Reindert J. Haarsma of the Royal Netherlands Meteorological Institute. In a paper called "More hurricanes to hit Western Europe due to global warming", published in April 2013 in Geophysical Research Letters, the researchers describe the results from runs of a high-resolution (25 km grid spacing) climate model based on the European Centre for Medium Range Weather Forecasts (ECMWF) numerical weather prediction model. The model predicts that the breeding ground for Atlantic hurricanes will shift approximately 700 miles eastwards as the oceans warm this century. Hurricanes which form farther to the east can spend more time over warm tropical waters before turning north and northeast towards Europe, increasing the odds that these storms will have hurricane-force winds upon arrival in Europe. The model showed that wind shear will change little in the region over the coming decades, resulting in a large increase in storms with hurricane-force winds affecting Western Europe. Most of the these storms will not be hurricanes upon arrival in Europe, but will be former hurricanes that have transitioned to extratropical storms with hurricane-force winds. As we saw with Hurricane Sandy, these hybrid storms can be extremely dangerous. Summed over Norway, the North Sea, and the Gulf of Biscay, the model found that the number of hurricane-force storms in August - October increased by over a factor six, increasing from an average of two such storms in the current climate to thirteen per year by 2100. Almost all of these future Western European hurricane-force storms were predicted to originate as hurricanes or tropical storms in the tropics. The researchers conclude that "tropical cyclones will increase the probability of present-day extreme events over the North Sea and the Gulf of Biscay with a factor of 5 and 25 respectively, with far reaching consequences especially for coastal safety."

References
Barnes, E.A, L.M. Polvani, and A.H. Sobel, 2013, "Model projections of atmospheric steering of Sandy-like superstorms", PNAS September 3, 2013, doi: 10.1073/pnas.1308732110

Haarsma et al., 2013, More hurricanes to hit Western Europe due to global warming, Geophysical Research Letters, DOI: 10.1002/grl.50360

Hall, T.M., and A.H. Sobel, 2013, "The impact angle of Hurricane Sandy’s New Jersey landfall," Geophysical Research Letters 40:23122315.

"Europe expected to see a large increase in Hurricane Sandy-like hybrid storms", my April 2013 blog post.

"Why did Hurricane Sandy take such an unusual track into New Jersey?" my October 2012 blog post.

Arctic Warming May Not Be Altering Jet Stream: Study
by Andrew Freedman of climatecentral.org, analyzing Dr. Barnes' paper disputing the link between Arctic sea ice loss and changes in the jet stream.

Quick update on 97L
The tropical wave that moved through the Lesser Antilles Islands on Tuesday morning (Invest 97L) is showing increasing signs of organization. It seemed like an atmospheric switch got thrown early this afternoon, and the storm began spinning up. Satellite loops show that 97L has developed some respectable low-level spiral bands, and we can see upper-level outflow channels opening to both the south and the north. There is no sign of a well-organized surface circulation, though, and dry air is still hampering the storm, as evidenced by surface-based outflow boundaries coming out of some of 97L's thunderstorms on its northwest flank. Upper level winds are favorable for development, with wind shear a low 5 - 10 knots, and an upper-level anticyclone overhead. In their 2pm EDT Tuesday Tropical Weather Outlook, NHC bumped up the 2-day odds of development to 30%. I put these odds at 50%, and Puerto Rico and the Eastern Dominican Republic can expect 3 - 6 inches of rain from 97L on Wednesday and Thursday. I'll be back Wednesday morning with an update.

Jeff Masters

Hurricane Climate Change

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Lesser Antilles Disturbance 97L Still Disorganized

By: JeffMasters, 2:34 PM GMT on September 03, 2013

A tropical wave over the Lesser Antilles Islands (Invest 97L) is moving west-northwest at 10 mph, and is bringing sporadic heavy rain showers to the islands. Top sustained winds observed in the islands as of 10 am AST Tuesday were mostly below 15 mph. Heavy thunderstorm activity has been low since Monday, and is spread out, as seen on satellite loops. The thunderstorms are poorly organized, and there is no sign of a well-organized surface circulation. Martinique radar shows little evidence of rotation to the echoes, and no low-level spiral bands forming. Upper level winds are favorable for development, with wind shear a low 5 - 10 knots, and an upper-level anticyclone overhead. An area of dry air from the Saharan Air Layer (SAL) surrounds 97L and is interfering with development, though the the 12Z Tuesday balloon sounding from Guadeloupe near the center of 97L showed moist air through the entire depth of the atmosphere.


Figure 1. Latest satellite image of Invest 97L.

Forecast for 97L
Wind shear is expected to be low through Friday, and ocean temperatures will be warm, 29°C. The disturbance has not been able to generate enough thunderstorms to moisten the surrounding atmosphere much, and dry air may continue to slow down development over the next few days. The models take 97L to the west-northwest, bringing it near or over Puerto Rico or Hispaniola on Wednesday night. A track over the high mountains of Hispaniola would disrupt the circulation of 97L, forcing the storm to regroup on Thursday over the Southeast Bahamas and Turks and Caicos Islands. The best chance for development of 97L would appear to be on Friday, after the storm has finished its encounter with Hispaniola. This morning's 00Z run of the experimental GFDL ensemble--which produces 10 simulations with slightly varying initial conditions to create a plume of potential storm tracks--foresees that once 97L organizes into a tropical depression, it might take only two days for it to intensify into a hurricane to the north of Hispaniola. The wave will bring heavy rain showers to the Lesser Antilles islands on Tuesday and Wednesday, and this activity will likely spread to Puerto Rico, the Virgin Islands, the Dominican Republic, and Haiti on Wednesday and Thursday. These rains will be capable of causing life-threatening flash flooding and mudslides in mountainous regions of Puerto Rico and Hispaniola. Of our three reliable models for predicting genesis, the UKMET, GFS, and European models, only the UKMET model develops the disturbance, predicting it will become a tropical depression over the Central Bahamas on Saturday. There will be a strong trough of low pressure along the U.S. East Coast this weekend, which will be capable of turning 97L to the north before the storm can hit the U.S. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC left the 5-day odds of formation of 97L at 50%, and 2-day odds at 20%. The Hurricane Hunters are on call to investigate 97L Wednesday afternoon. The flight scheduled for today will likely be cancelled. The Hurricane and Severe Storm Sentinel Mission, a 5-year field project utilizing two remotely crewed Global Hawk aircraft to overfly tropical storms and hurricanes, will have one of their aircraft investigate 97L today, and again on Wednesday. You can follow the progress of the aircraft using NASA's live-plane tracker map (Global Hawk AV6 is tail number NASA872, and Global Hawk AV1 is tail number NASA871.)

98L off the coast of Africa and a Yucatan Peninsula tropical wave
A tropical wave that emerged from the coast of Africa on Monday (Invest 98L) is expected to track northwest over the Cape Verde Islands. This course will take 98L into a region of ocean where historically, very few tropical cyclones have made the long journey across the Atlantic to affect North America. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC put the 5-day odds of development at 20%.

A tropical wave over Mexico's Yucatan Peninsula has a respectable area of heavy thunderstorms associated with it that is bringing heavy rains. This activity will push over the extreme Southern Gulf of Mexico's Bay of Campeche on Wednesday. The wave will have two days over water to develop before moving ashore on the Mexican coast between Veracruz and Tampico on Friday. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC put the 5-day odds of development at 30%, and 2-day odds at 20%.

Jeff Masters

Hurricane

Updated: 6:23 PM GMT on September 03, 2013

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Lesser Antilles Disturbance 97L Disorganized

By: JeffMasters, 2:14 PM GMT on September 02, 2013

A tropical wave over the Lesser Antilles Islands (Invest 97L) is moving west to west-northwest at 10 mph, and is bringing sporadic heavy rain showers to the islands. Top sustained winds observed in the islands as of 10 am AST Monday were mostly below 15 mph. Heavy thunderstorm activity has decreased in organization since Sunday, and is spread out over a larger area, as seen on satellite loops. The thunderstorms are poorly organized, and there is no sign of a well-organized surface circulation. Martinique radar shows little evidence of rotation to the echoes, and no low-level spiral bands forming. Upper level winds are favorable for development, with wind shear a low 5 - 10 knots, and an upper-level anticyclone overhead. An area of dry air from the Saharan Air Layer (SAL) surrounds 97L and is interfering with development, though the atmosphere has moistened since Sunday. The 12Z Monday balloon sounding from Barbados in the southern Lesser Antilles showed less than 10 knots of wind shear and a moist atmosphere. Wind shear was in the moderate range and the atmosphere was drier in the northern Lesser Antilles at Guadeloupe.


Figure 1. Latest satellite image of Invest 97L.

Forecast for 97L
Wind shear is expected to be low for the next five days, and ocean temperatures will be warm, 28 - 29°C. The disturbance is steadily moistening the atmosphere and is moving into a moister environment, so dry air will be less of an impediment to development as the week progresses. A key factor keeping the disturbance from developing over the next two days is the fact that 97L is quite large, and is stretched out from east to west over a wide expanse. Large, elongated systems like 97L usually take several days to consolidate and spin up. Another factor that will likely retard development is the presence of strong surface trade winds over the Eastern Caribbean ahead of 97L, south of the Dominican Republic. These strong trade winds are a common feature of the Eastern Caribbean, and make the region something of a hurricane graveyard. As the surface wind flow to the west of 97L accelerates into this wind max, air will be sucked from aloft downward towards the surface, creating sinking air, interfering with the formation of thunderstorm updrafts.

The models have shifted markedly in their projected path for 97L, and now take the system more to the west-northwest, bringing it near or over Hispaniola on Wednesday. A track over the high mountains of that island would disrupt the circulation of 97L, forcing the storm to regroup on Thursday over the Southeast Bahamas and Turks and Caicos Islands. The best chance for development of 97L would appear to be on on Thursday, after it has finished its encounter with Hispaniola. This morning's 06Z run of the experimental GFDL ensemble--which produces 10 simulations with slightly varying initial conditions to create a plume of potential storm tracks--foresees that once 97L organizes into a tropical depression, it might take only two days for it to intensify into a hurricane to the north of Hispaniola. However, a number of storm tracks from the 00Z Monday GFS and ECMWF ensembles foresee a more westerly track for 97L over Eastern Cuba, which would further disrupt the storm. The wave will bring heavy rain showers to the Lesser Antilles islands on Monday and Tuesday, and this activity will likely spread to Puerto Rico, the Virgin Islands, the Dominican Republic, Haiti, and Eastern Cuba on Tuesday and Wednesday. These rains will be capable of causing life-threatening flash flooding and mudslides in mountainous regions in Puerto Rico, Hispaniola, and Cuba. Of our three reliable models for predicting genesis, the UKMET, GFS, and European models, only the UKMET model develops the disturbance, predicting it will become a tropical depression over the Southeast Bahamas on Thursday. There will be a strong trough of low pressure along the U.S. East Coast this weekend, which will be capable of turning 97L to the north before the storm can hit the U.S. In their 8 am EDT Monday Tropical Weather Outlook, NHC left the 5-day odds of formation of 97L at 50%, and dropped the 2-day odds to 20%. The Hurricane Hunters are on call to investigate 97L on Tuesday.

Have a great Labor Day, everyone!

Jeff Masters

Hurricane

Updated: 2:38 PM GMT on September 02, 2013

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Lesser Antilles Disturbance 97L a Threat to Develop

By: JeffMasters, 2:46 PM GMT on September 01, 2013

A tropical wave over the Lesser Antilles Islands (Invest 97L) is moving westward at 15 mph, and is bringing heavy rains and gusty winds to the islands. Top sustained winds in the islands as of 10 am AST Sunday were 26 mph at Monserrat. Heavy thunderstorm activity has sharply increased since Saturday, as seen on satellite loops. A large circulation is evident, with some westerly winds blowing to the south of the disturbance. However, the thunderstorms are poorly organized, and there is no sign of a well-organized surface circulation. Martinique radar shows some rotation to the echoes, though well-organized spiral bands are not evident, and do not appear to be forming. Wind shear has dropped to a moderate 10 - 20 knots over the system. An area of dry air from the Saharan Air Layer (SAL) surrounds 97L and is interfering with development.


Figure 1. Latest satellite image of Invest 97L.

Forecast for 97L
Wind shear is expected to be in the low to moderate range for the next five days, and ocean temperatures will be warm, 28 - 29°C. The disturbance is steadily moistening the atmosphere and is moving into a moister environment, so dry air will be less of an impediment to development as the week progresses. The main factor keeping the disturbance from developing over the next two days would appear to be the fact that 97L is quite large, and is stretched out from east to west over a wide expanse. Large, elongated systems like 97L usually take several days to consolidate and spin up. Another factor that will likely retard development is the presence of strong surface trade winds over the Eastern Caribbean ahead of 97L, south of the Dominican Republic. These strong trade winds are a common feature of the Eastern Caribbean, and make the region something of a hurricane graveyard. As the surface wind flow to the west of 97L accelerates into this wind max, air will be sucked from aloft downward towards the surface, creating sinking air, interfering with the formation of thunderstorm updrafts. The best chance for development of 97L would appear to be on Wednesday or Thursday, when the disturbance reaches the Central Caribbean. The wave will likely spread heavy rains to Puerto Rico, the Virgin Islands, and the Dominican Republic by Tuesday, and to Haiti by Wednesday. These rains will be capable of causing life-threatening flash flooding and mudslides in mountainous regions in Puerto Rico and Hispaniola. Of our three reliable models for predicting genesis, the UKMET, GFS, and European models, only the UKMET model develops the disturbance, predicting it will become a tropical depression south of Haiti on Wednesday, and near Jamaica on Thursday. In their 8 am EDT Sunday Tropical Weather Outlook, NHC boosted the 5-day odds of formation of 97L to 30%, and the 2-day odds to 20%.


Video 1. You can see why landslides triggered by heavy rains from a tropical cyclone are among the most dangerous hazards of these storms, thanks to a dashboard cam that caught this extraordinary rock slide in Northeast Taiwan on August 31, 2013, after heavy rains from a cold front drenched the island with up to 200 mm (7.87") of rain in 24 hours. The rains fell upon soils already saturated by Thursday's torrential rains from Tropical Storm Kong-Rey, which dumped up to 482 mm (19") of rain on Taiwan, killing three people. The driver of the car caught in the rock slide survived with minor injuries. Thanks go to wunderground member Robert Speta for bringing this video to my attention. A separate video showing the damage to the car and the course of the rock slide is here.

Jeff Masters

Hurricane

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About

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.