Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 2:56 PM GMT on June 01, 2010
The hurricane season of 2010 is upon us. With unprecedented sea surface temperatures in the Atlantic, El Niño gone and possibly transitioning to La Niña, a massive oil slick in the Gulf of Mexico, a million earthquake refugees in Haiti at the mercy of a hurricane strike, and an ever-increasing number of people living on our coasts, the arrival of this year's hurricane season comes with an unusually ominous tone. NOAA is forecasting a very active and possibly hyperactive season, and Dr. Bill Gray has said he expects "a hell of a year." However, our ability to forecast hurricane activity months in advance is limited, and we don't yet know how the large scale weather patterns like the Bermuda High will set up during the peak part of hurricane season. In particular, I very much doubt that we are in for a repeat of the unprecedented violence of the Hurricane Season of 2005, with its 28 named storms, 15 hurricanes, and 7 intense hurricanes. While sea surface temperatures are currently warmer this year than in 2005, that year featured some very unusual atmospheric circulation patterns, with a very strong ridge of high pressure over the eastern U.S., record drought in the Amazon, and very low surface pressures over the Atlantic. A repeat of 2005's weather patterns is unlikely, though I am expecting we will get at least four major hurricanes this year. An average year sees just two major hurricanes.
Figure 1. Tracks of all June tropical storms and hurricanes in the Gulf of Mexico, 1995 - 2009. Allison was a subtropical storm (coded blue). Image credit: NOAA Coastal Services Center.
The latest long-range computer model guidance suggests there's no reason to suspect that the first two weeks of this year's hurricane season will bring any unusual activity. Climatologically, June is typically the quietest month of the Atlantic hurricane season. On average, we see only one named storm every two years in June. Only one major hurricane has made landfall in June--Category 4 Hurricane Audrey of 1957, which struck the Texas/Louisiana border area on June 27 of that year, killing 550. The highest number of named storms for the month is three, which occurred in 1936 and 1968. In the fifteen years since the current active hurricane period began in 1995, there have been eleven June named storms (if we include 2008's Tropical Storm Arthur, which really formed on May 31). Five tropical storms have formed in the first half of June in that 14-year period, giving a historical 36% chance of a first-half-of-June named storm. Five June storms in the past 14 years have passed close enough to the Deepwater Horizon oil spill location to have caused significant transport had there been an oil slick on the surface.
Sea Surface Temperatures
Sea Surface Temperatures (SSTs) are at record high levels over the tropical Atlantic between Africa and Central America this year (Figure 2). As I discussed in my May 15 post, the area between 10°N and 20°N, between the coast of Africa and Central America (20°W - 80°W), is called the Main Development Region (MDR) because virtually all African waves originate in this region. These African waves account for 85% of all Atlantic major hurricanes and 60% of all named storms. When SSTs in the MDR are much above average during hurricane season, a very active season typically results (if there is no El Niño event present.) SSTs in the Main Development Region (10°N to 20°N and 20°W to 80°W) were an eye-opening 1.46°C above average during April. This is the third straight record warm month, and the warmest anomaly measured for any month--by a remarkable 0.2°C. The previous record warmest anomalies for the Atlantic MDR were set in June 2005 and March 2010, at 1.26°C. The Arctic Oscillation (AO) and its close cousin, the North Atlantic Oscillation (NAO), are largely to blame for the record SSTs. The AO and NAO are climate patterns in the North Atlantic Ocean related to fluctuations in the difference of sea-level pressure between the Icelandic Low and the Azores-Bermuda High. If the difference in sea-level pressure between Iceland and the Azores is small (negative NAO), this creates a weak Azores-Bermuda High, which reduces the trade winds circulating around the High. During December - February, we had the most negative AO/NAO since records began in 1950, and this caused trade winds between Africa and the Lesser Antilles Islands in the hurricane Main Development Region to slow to 1 - 2 m/s (2.2 - 4.5 mph) below average. Slower trade winds mean less mixing of the surface waters with cooler waters down deep, plus less evaporational cooling of the surface water. As a result, the ocean heated up significantly, relative to normal, over the winter and Spring.
However, over the past two weeks, the AO/NAO has trended close to average, and trade winds over the tropical Atlantic have increased to near normal speeds as the Bermuda-Azores High has strengthened. SST anomalies have been falling in recent weeks, and will continue to fall in the coming two weeks, based on the latest forecast from the GFS model. While I expect that record SSTs will continue into mid-June, current trends suggest that by July, SST anomalies will be close to what they were in 2005. SST anomalies in the MDR could fall below the record 2005 levels by the peak part of hurricane season, August - October. Even so, SSTs in the Caribbean this year will be plenty warm to cause an abnormal number of major hurricanes. These warm SSTs may also cause extensive damage to the coral reefs, which suffered huge die-offs from the record SSTs of 2005.
Typically, June storms only form over the Gulf of Mexico, Western Caribbean, and Gulf Stream waters just offshore Florida, where water temperatures are warmest. SSTs are 28 - 30°C in these regions, which is about 0.5 - 1.5°C above average for this time of year. June storms typically form when a cold front moves off the U.S. coast and stalls out, with the old frontal boundary serving as a focal point for development of a tropical disturbance. African tropical waves, which serve as the instigators of about 85% of all major hurricanes, are usually too far south in June to trigger tropical storm formation. Every so often, a tropical wave coming off the coast of Africa moves far enough north to act as a seed for a June tropical storm. This was the case for Arthur of 2008 (which also had major help from the spinning remnants of the Eastern Pacific's Tropical Storm Alma). Another way to get Atlantic June storms is for a disturbed weather area in the Eastern Pacific Intertropical Convergence Zone (ITCZ) to push north into the Western Caribbean and spawn a storm there. This was the case for Tropical Storm Alberto of 2006 (which may have also had help from an African wave). SSTs are too cold in June to allow storms to develop between the coast of Africa and the Lesser Antilles Islands--there has only been once such development in the historical record--Ana of 1979.
Figure 2. Sea Surface Temperature (SST) departure from average for May 31, 2010. SSTs averaged more that 1°C above average over the entire tropical Atlantic and Gulf of Mexico. Note the large region of below average SSTs along the Equatorial Pacific off the coast of South America, signaling the possible start of an La Niña episode. Image credit: NOAA/NESDIS.
Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart. The jet stream's band of strong high-altitude winds is the main source of wind shear in June over the Atlantic hurricane breeding grounds, since the jet is very active and located quite far south this time of year.
The jet stream over the past few weeks has been locked into a pattern where a southern branch (the subtropical jet stream) brings high wind shear over the Caribbean, and a northern branch (the polar jet stream) brings high wind shear offshore of New England. This leaves a "hole" of low shear between the two branches off the coast of North Carolina, which is where Invest 90L formed.
The jet stream is forecast to maintain this two-branch pattern over the coming ten days (Figure 3.) This means that the waters offshore of North Carolina is the most likely place for a tropical storm to form during this period, though the southwestern Caribbean will at times have shear low enough to allow tropical storm formation. The Gulf of Mexico is forecast to have wind shear too high to support a tropical storm during the first half of June. None of our reliable forecast models call for tropical storm formation over the coming 7 days, though the NOGAPS model indicates the possibility of a tropical disturbance forming off the coast of Nicaragua on Friday.
Figure 3. Wind shear forecast from the 00Z GMT June 1, 2010 run of the GFS model for June 7. Currently, the polar jet stream is bringing high wind shear to the waters offshore New England, and the subtropical jet is bringing high wind shear to the northern Caribbean. This leaves the waters off the coast of North Carolina and southern Caribbean under low shear, making these areas the most favored region for tropical storm formation over the next 7 - 10 days. Wind speeds are given in m/s; multiply by two to get a rough conversion to knots. Thus, the red regions of low shear range from 0 - 16 knots.
Dry air and African dust
It's too early to concern ourselves with dry air and dust coming off the coast of Africa, since these dust outbreaks don't make it all the way to the June tropical cyclone breeding grounds in the Western Caribbean and the Gulf of Mexico. Developing storms do have to contend with dry air from Canada moving off the U.S. coast; this was a key reason why our first "Invest" of the year, 90L off the coast of South Carolina, never became a subtropical storm.
Dust expert Professor Amato Evan of the University of Virginia has posted his forecast for African dust for the 2010 hurricane season. Dr. Evan is predicting that due to plentiful rains during last year's rainy season over the Sahel region of Africa, and near average amounts of African dust observed in May 2010 and during the 2009 hurricane season, we can expect near average or moderately below average levels of dust over the tropical Atlantic during the 2010 hurricane season.
The forecast steering current pattern over the next two weeks is a typical one for June, with an active jet stream bringing many troughs of low pressure off the East Coast of the U.S. These troughs will be frequent enough and strong enough to recurve any tropical storms or hurricanes that might penetrate north of the Caribbean Sea. Steering current patterns are predictable only about 3 - 5 days in the future, although we can make very general forecasts about the pattern as much as two weeks in advance. There is no telling what might happen during the peak months of August, September, and October--we might be in for a repeat of the favorable 2009 steering current pattern that recurved every storm out to sea--or the unfavorable 2008 pattern, that steered Ike and Gustav into the Gulf of Mexico.
Wind shear over the main breeding grounds for June tropical cyclones, the Gulf of Mexico and Western Caribbean, is expected to be high enough over the next two weeks to give us an average chance of a June named storm. I give a 30% chance of a named storm between now and June 15, and a 60% chance for the entire month of June. There is approximately a 30% chance of a June storm passing close enough to the Deepwater Horizon oil spill to cause significant transport of the oil. See my post, What a hurricane would do the Deepwater Horizon oil spill, for more information on this.
Agatha the 6th deadliest Eastern Pacific storm on record
Central America's Tropical Storm Agatha is now the 6th deadliest Eastern Pacific tropical cyclones on record. Agatha was a tropical storm for just 12 hours, making landfall Saturday on the Pacific coast of Guatemala as a 45 mph tropical storm. However, the storm brought huge amounts of rain--as much as 36 inches--to the high mountains of Guatemala. So far, flooding and landslides have killed at least 123 people in Guatemala, with 59 others missing. The storm also killed 9 in neighboring El Salvador, and 14 in Honduras.
Figure 4. Journey to the center of the Earth: a massive sinkhole 200 feet (60 meters) deep opened up in the capital, Guatemala City, after heavy rains from Tropical Storm Agatha. How are they going to fix this hole? Wow! It doesn't even look real.
Guatemala's worst flooding disaster in recent history was due to Hurricane Stan of 2005, which killed 1,513. The deadliest Eastern Pacific tropical cyclone on record for Guatemala was Hurricane Paul of 1982, which made landfall in Guatemala as a tropical depression. Flooding from Paul's rains killed 620 people in Guatemala.
Oil spill update
Light onshore winds out of the south to southwest are expected to blow over the northern Gulf of Mexico all week, resulting increased threats of oil to the Alabama and Mississippi barrier islands, according to the latest trajectory forecasts from NOAA. These persistent southwesterly winds will likely bring oil very close to the Florida Panhandle by Saturday.
Oil spill resources
My post, What a hurricane would do the Deepwater Horizon oil spill
My post Wednesday with answers to some of the common questions I get about the spill
My post on the Southwest Florida "Forbidden Zone" where surface oil will rarely go
My post on what oil might do to a hurricane
Gulf Oil Blog from the UGA Department of Marine Sciences
NOAA trajectory forecasts
Deepwater Horizon Unified Command web site
Oil Spill Academic Task Force
University of South Florida Ocean Circulation Group oil spill forecasts
ROFFS Deepwater Horizon page
Surface current forecasts from NOAA's HYCOM model
Synthetic Aperture Radar (SAR) imagery from the University of Miami
Join the "Hurricane Haven" with Dr. Jeff Masters: a new Internet radio show
Today, I'll be experimenting with a live 1-hour Internet radio show called "Hurricane Haven." The show will be aired at 4pm EDT on Tuesdays during hurricane season. Listeners will be able to call in and ask questions. Some topics I'll cover on the first show:
1) What's going on in the tropics right now
2) Preview of the coming hurricane season
3) How a hurricane might affect the oil spill
4) How the oil spill might affect a hurricane
5) New advancements in hurricane science presented at this month's AMS Conference on Hurricanes and Tropical Meteorology
6) Haiti's vulnerability to a hurricane this season
I hope you can tune in to the broadcast, which will be at http://www.wunderground.com/wxradio/wubroadcast.h tml. If not, the show will be recorded and stored as a podcast.
Portlight receives a major grant to fund U.S. disaster relief work
The Christopher and Dana Reeve Foundation has announced today that it is awarding a Quality of Life Grant in the amount of $21,500 to Portlight Strategies, Inc. The grant will fund a ready-to-deploy container specifically outfitted to serve the immediate needs of people with disabilities in the aftermath of hurricanes and other domestic natural disasters. To read more about this award, check out the Portlight blog. Congratulations, Portlight team!
Portlight continues its Haiti response
Ready or not, the rainy season is here for Haiti. Portlight has done a tremendous amount to help the Haitians get ready for the upcoming hurricane season, as detailed in the Haitian Relief Recap blog post made last week. Please visit the Portlight.org web site or the Portlight blog to learn more and to donate to Portlight's efforts in Haiti.
Figure 5. A portion of the 30,000 pounds of rice donated to Haitian earthquake victims by Portlight earlier this month, shipped via the schooner Halie and Mathew.
I'll be back Wednesday afternoon with an analysis of the new Colorado State University hurricane forecast issued by Phil Klotzbach and Bill Gray, due out on June 2.
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