| Above: The strongest Atlantic hurricane of 2018, Hurricane Michael, as seen by the GOES-16 satellite at 10:45 am EDT October 10, 2018. At the time, Michael was nearing landfall in the Florida Panhandle as a Category 5 storm with 160 mph winds. Image credit: NOAA/RAMMB. |
Residents of Hurricane Alley can anticipate a near-normal Atlantic hurricane season in 2019, said NOAA’s Climate Prediction Center on Thursday. In their first seasonal forecast for 2019, NOAA predicted a 40% chance for a near-normal Atlantic hurricane season, a 30% chance for an above-normal season and a 30% chance for a below-normal season. NOAA gave a 70% likelihood of 9 - 15 named storms, 4 - 8 hurricanes, 2 - 4 major hurricanes (Category 3 or higher on the Saffir-Simpson Hurricane Wind Scale) and an Accumulated Cyclone Energy (ACE) 65% - 140% of the median. If we take the midpoint of these ranges, NOAA called for 12 named storms, 6 hurricanes and 3 major hurricanes. This is the same as the 1981-2010 seasonal averages of 12 named storms, 6 hurricanes and 3 major hurricanes.
NOAA cited three main factors influencing their Atlantic forecast:
1) The likely continuation of El Niño conditions over the tropical Pacific Ocean [ENSO refers to El Niño/ Southern Oscillation, which has three phases: El Niño, Neutral, and La Niña]. El Niño suppresses hurricane development in the Atlantic by increasing the amount of vertical wind shear and dry, stable air that tends to prevail over the Main Development Region (MDR) for hurricanes, which includes the tropical North Atlantic Ocean and Caribbean Sea between 9.5°N and 21.5°N latitude. The latest monthly NOAA/IRI probabilistic ENSO forecast, issued May 20, calls for approximately a 57% chance of El Niño conditions during the peak August-September-October period of the Atlantic hurricane season, and a 37% chance of neutral conditions. Several computer models, including a fair number of members of the NOAA CFSv2 ensemble model, call for the current weak El Niño to shift into neutral conditions by later this summer.
2) Competing with the suppressive effect of El Niño will be the expected warmer-than-average sea-surface temperatures (SSTs) in the MDR--warmer than what we saw during the 2018 hurricane season. SSTs are currently near average in the MDR, but nearly all climate models predict SSTs to be above average in the MDR during August, September, and October; most predict departures of +0.2°C to + 0.4°C during that peak portion of the hurricane season. These predictions are consistent with the warm phase of the AMO (Atlantic Multidecadal Oscillation), which has persisted since 1995 and has favored more active hurricane seasons. (Along with the risk of warmer-than-average SSTs in the MDR discussed in the NOAA outlook, temperatures have been consistently warmer than average in the subtropical Atlantic, which would lend support to tropical or subtropical cyclones developing or maintaining themselves north of the MDR.)
3) Also competing with El Niño this season is the expectation for weaker trade winds in the eastern portion of the MDR, along with an enhanced West African monsoon. These conditions favor more active hurricane seasons, and are also typical of the warm AMO phase.
NOAA issued these words of wisdom: “Hurricane-related disasters can occur whether the season is active or relatively quiet. It only takes one hurricane (or tropical storm) to cause a disaster. It is crucial that residents, businesses, and government agencies of coastal and near-coastal regions prepare for every hurricane season regardless of this, or any other, seasonal outlook.” It is good to remember that 2004, which featured a weak El Niño event similar in many respects to the 2019 El Niño event, was one of the most active on record for U.S. landfalling hurricanes: six hurricanes, including three major hurricanes (Charley, Ivan, and Jeanne), made U.S. landfalls that year.
Forecasts from other groups
The Barcelona Supercomputing Center and Colorado State University have a web page summarizing all of the major Atlantic hurricane season forecasts. Here are some of the major forecasts made since April:
TSR predicts a slightly below-average Atlantic hurricane season: 12 named storms
The April 5 forecast for the 2019 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) calls for a slightly below-average Atlantic hurricane season--about 20% below the long-term (1950-2018) norm and 30% below the recent 2009-2018 ten-year norm. TSR is predicting 12 named storms, 5 hurricanes, 2 intense hurricanes and an Accumulated Cyclone Energy (ACE) of 81 for the period May through December. The long-term averages for the past 69 years are 11 named storms, 6 hurricanes, 3 intense hurricanes and an ACE of 104. TSR rates their skill level as low for these April forecasts—just 2 - 7% higher than a "no-skill" forecast made using climatology. TSR predicts a 29% chance that U.S. landfalling ACE index will be above average, a 24% chance it will be near average, and a 47% chance it will be below average. They project that two named storms and one hurricane will hit the U.S. The averages from the 1950-2018 climatology are three named storms and one hurricane. They rate their skill at making these April forecasts for U.S. landfalls at 0% - 3% higher than a "no-skill" forecast made using climatology. In the Lesser Antilles Islands of the Caribbean, TSR projects one tropical storm and no hurricanes. Climatology is one tropical storm and less than 0.5 hurricanes. The next TSR forecast will be issued on May 30.
CSU predicts a slightly below-average Atlantic hurricane season: 13 named storms
A slightly below-average Atlantic hurricane season is likely in 2019, said the hurricane forecasting team from Colorado State University (CSU) in their latest seasonal forecast issued April 4. Led by Dr. Phil Klotzbach, with coauthors Dr. Michael Bell and Jhordanne Jones, the CSU team called for an Atlantic hurricane season with 13 named storms, 5 hurricanes, 2 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 80. The long-term averages for the period 1981 - 2010 were 12 named storms, 6.5 hurricanes, 2 intense hurricanes, and an ACE of 92. The CSU outlook also predicted the odds of a major hurricane hitting the U.S. were about 48% (the long-term average is 52%). They gave a 28% chance for a major hurricane to hit the East Coast or Florida Peninsula (the long-term average is 31%), and a 28% chance for the Gulf Coast (the long-term average is 30%). The outlook included a 39% chance of seeing at least one major hurricane in the Caribbean basin (the long-term average is 42%). The next CSU forecast will come out on June 4.
The Weather Company predicts a slightly above-average Atlantic hurricane season: 14 named storms
The May 6 forecast from The Weather Company called for a slightly above-average Atlantic hurricane season with 14 named storms, 7 hurricanes, and 3 major hurricanes. The Weather Company forecast noted these key factors influencing their forecast:
1) "El Niño conditions are expected to persist or, at worst, slowly weaken over the next six months, which should act to help suppress activity a bit," said Dr. Todd Crawford, chief meteorologist at The Weather Company.
2) Sea-surface temperatures in the Atlantic are currently very similar to those observed during the last three years, which were all active hurricane seasons. There is a large pool of warmer-than-normal sea-surface temperatures off the southeastern U.S. coast, which may again help to favor 'homegrown' systems that develop closer to the U.S., in a similar manner to 2018.
Penn State predicts a below-average Atlantic hurricane season: 10 named storms
One of the most successful seasonal hurricane forecasts in recent years has been made using a statistical model by Penn State's Michael Mann, Sonya Miller, and alumnus Michael Kozar. Their April 27 forecast called for a below-average Atlantic hurricane season with 10.1 named storms (expected range: 7 to 13). Their prediction was made using statistics of how past hurricane seasons have behaved in response to sea surface temperatures (SSTs), the El Niño/La Niña oscillation, the North Atlantic Oscillation (NAO), and other factors. The statistical model assumed that in 2019 the early-May +0.35°C departure of temperature from average in the Main Development Region (MDR) for hurricanes in the tropical Atlantic would persist throughout hurricane season, a weak El Niño would persist through the fall, and climatological mean conditions for the North Atlantic Oscillation (NAO) would exist in the Fall/Winter of 2019-2020.
The PSU team has been making Atlantic hurricane season forecasts since 2007, and these predictions have done well, except in 2012 and 2018:
2007 prediction: 15 named storms, Actual: 15
2009 prediction: 12 named storms, Actual: 9
2010 prediction: 23 named storms, Actual: 19
2011 prediction: 16 named storms, Actual: 19
2012 prediction: 11 named storms, Actual: 19
2013 prediction: 16 named storms, Actual: 14
2014 prediction: 9 named storms, Actual: 8
2015 prediction: 7 named storms, Actual: 11
2016 prediction: 19 named storms, Actual: 15
2017 prediction: 15 named storms, Actual: 17
2018 prediction: 10 named storms, Actual: 15
NCSU predicts a near-average Atlantic hurricane season: 14.5 named storms
The April 16 forecast from North Carolina State University (NCSU) called for a near-average Atlantic hurricane season with 13 - 16 named storms, 5 - 7 hurricanes, and 2 – 3 major hurricanes. They used a statistical model encompassing more than 100 years of past Atlantic hurricane activity to make their forecasts.
A long-range threat in the Atlantic next week?
Recent long-range runs of the GFS model and its ensemble members have been predicting that a broad area of low pressure will form over Central America 5 – 10 days from now. Such a “Central American Gyre” (CAG) can spawn a tropical cyclone, though it usually requires many days for this to occur. Hurricane Michael of October 2018 had its origins in a long-lived CAG.
Some of the model solutions predict that a tropical storm will spin up from a CAG late next week and potentially move north to threaten the U.S. This process will be made more likely to occur by a phase of the Madden-Julian Oscillation (MJO) that is expected to favor upward moving air and low pressure over the region. However, as described in detail in an excellent series of tweets on Thursday morning by Levi Cowan, we shouldn’t believe the GFS model until the European model, which has a better track record handling such systems, comes on board. The strength of a ridge of high pressure over the Southeast U.S. next week will ultimately determine if the CAG and any potential tropical cyclone it may spawn can move northwards and affect the U.S., and it is too early to judge the odds of that occurring.
NOAA predicts an above-average hurricane season in the central and eastern Pacific
NOAA also predicted a 70% chance of an above-normal 2019 hurricane season in both the Eastern Pacific (for storms affecting Mexico) and the Central Pacific (for storms affecting Hawaii). The eastern Pacific outlook calls for a 70% probability of 15 to 22 named storms, of which 8 to 13 are expected to become hurricanes, including 4 to 8 major hurricanes. If we take the midpoint of these ranges, NOAA is calling for 18.5 named storms, 10.5 hurricanes, and 6 major hurricanes, well above the 1981-2010 averages of 15 named storms, 8 hurricanes, and 4 major hurricanes.
The central Pacific outlook calls for a 70% probability of 5 to 8 tropical cyclones, which includes tropical depressions, tropical storms, and hurricanes. A near-normal season there has four to five tropical cyclones, and an above-normal season has six or more tropical cyclones. El Niño conditions typically lead to more active hurricane seasons in both the eastern and central Pacific, due to warmer-than-average ocean temperatures.
