| Above: The strongest Atlantic hurricane of 2017, Hurricane Irma, as seen by the MODIS instrument on September 5, 2017. At the time, Irma was at peak strength, a Category 5 storm with 180 mph winds. Irma was downgraded slightly in the post-season report by the National Hurricane Center, which judged that the peak sustained 1-minute winds of the storm were 180 mph, not 185 mph, as originally thought. Image credit: NASA. |
Residents of Hurricane Alley can anticipate a near-normal or above-normal Atlantic hurricane season in 2018, said NOAA’s Climate Prediction Center on Thursday. In their first seasonal forecast for 2018, NOAA predicted a 40% chance for a near-normal Atlantic hurricane season, a 35% chance for an above-normal season and a 25% chance for a below-normal season. NOAA gave a 70 percent likelihood of 10 – 16 named storms, 5 – 9 hurricanes, 1 – 4 major hurricanes (Category 3 or higher on the Saffir-Simpson Hurricane Wind Scale) and an Accumulated Cyclone Energy (ACE) 65% – 145% of the median. If we take the midpoint of these ranges, NOAA called for 13 named storms, 7 hurricanes and 2.5 major hurricanes. This is near the 1981-2010 seasonal averages of 12 named storms, 6 hurricanes and 3 major hurricanes. NOAA also predicted a near-normal or above-normal 2018 hurricane season in both the Eastern Pacific (for storms affecting Mexico) and the Central Pacific (for storms affecting Hawaii).
NOAA cited four main factors influencing their Atlantic forecast:
1) Either ENSO-neutral or weak El Niño conditions are expected 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 18, calls for approximately a 45% chance of El Niño conditions during the peak August-September-October period of the Atlantic hurricane season.
2) Near-average sea-surface temperatures (SSTs) are expected in the MDR–cooler than what we saw during the 2017 and 2016 hurricane seasons.
3) A continuation of the active hurricane cycle we’ve been in since 1995, from a positive phase of the Atlantic Multidecadal Oscillation (AMO).
NOAA issued these words of wisdom: “It only takes one storm hitting an area to cause a disaster, regardless of the overall season strength. Therefore, residents, businesses, and government agencies of coastal and near-coastal regions are urged to prepare every hurricane season regardless of this, or any other, seasonal outlook.”
A bigger-than-usual spread in prognoses for 2018
There is more disagreement than usual among the various outlooks issued since April as to how busy a hurricane season the Atlantic will see. One source of uncertainty is El Niño, which may or may not emerge later this year. In general, El Niño tends to suppress hurricane activity in the Atlantic. However, even if we do see an El Niño event, there are already signs that it may be a “Modoki El Niño“—the type where equatorial warming of the sea surface is focused more toward the central Pacific than the eastern Pacific. Modoki El Niño events are considered less likely to suppress Atlantic hurricanes.
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 near-average Atlantic hurricane season: 12 named storms
The April 5 forecast for the 2018 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) calls for calls for an Atlantic hurricane season with a close-to-average number of named storms, but an overall level of activity slightly below average–about 15% below the long-term (1950-2017) norm and the recent 2008-2017 ten-year norm. TSR is predicting 12 named storms, 6 hurricanes, 2 intense hurricanes and an Accumulated Cyclone Energy (ACE) of 84 for the period May through December. The long-term averages for the past 68 years are 11 named storms, 6 hurricanes, 3 intense hurricanes and an ACE of 103. 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 32% chance that U.S. landfalling ACE index will be above average, a 25% chance it will be near average, and a 43% chance it will be below average. They project that two named storms and one hurricane will hit the U.S. The averages from the 1950-2017 climatology are three named storms and one hurricane. They rate their skill at making these April forecasts for U.S. landfalls at 0% – 4% 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 above-average Atlantic hurricane season: 14 named storms
A slightly above-average Atlantic hurricane season is likely in 2018, said the hurricane forecasting team from Colorado State University (CSU) in their latest seasonal forecast issued April 5. Led by Dr. Phil Klotzbach, with coauthor Dr. Michael Bell, the CSU team is calling for an Atlantic hurricane season with 14 named storms, 7 hurricanes, 3 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 130. 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 calls for a 63% chance of a major hurricane hitting the U.S. in 2018 (long term average is 52%), with a 39% chance for the East Coast and Florida Peninsula (long term average is 31%), and a 38% chance for the Gulf Coast (long term average is 30%). The Caribbean is forecast to have a 52% chance of seeing at least one major hurricane (long term average is 42%). Their next forecast will be released May 31.
The Weather Company predicts a near-average Atlantic hurricane season: 12 named storms
The May 18 forecast from The Weather Company called for a near-average Atlantic hurricane season with 12 named storms, 5 hurricanes, and 2 major hurricanes. The Weather Company forecast noted these key factors influencing their forecast:
1) A pattern of cooler-than-average water temperatures has developed in the eastern Atlantic and in the central northern Atlantic. They compared sea-surface temperatures in these regions in April for inactive vs. active hurricane seasons and found that the current pattern more closely represents inactive hurricane seasons.
2) A possible transition towards El Niño later this summer.
3) Strong trade winds in the tropical Atlantic due to a positive phase of the North Atlantic Oscillation (NAO) are stirring up the waters there and keeping SSTs cool. The positive NAO is expected to last through the spring.
4) The decades-long period of increased hurricane activity that began in 1995 might be over.
Penn State predicts a below-average Atlantic hurricane season: 10 named storms
Here’s a forecast worth paying attention to: the April 27 forecast made using a statistical model by Penn State’s Michael Mann, Sonya Miller, and alumnus Michael Kozar called for a below-active season with 10.2 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 2018 the late-April +0.3°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 develop in the fall, and the NAO would be near average. If no El Niño develops, their model predicted slightly higher activity: 11.1 named storms, plus or minus 3.3 named storms.
The PSU team has been making Atlantic hurricane season forecasts since 2007, and these predictions have done well, except for in 2012, when an expected El Niño did not materialize:
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
NCSU predicts a very active Atlantic hurricane season: 14 -18 named storms
The April 18 forecast from North Carolina State University (NCSU), called for a significantly above average Atlantic hurricane season with 14 – 18 named storms, 7 – 11 hurricanes and 3 – 5 major hurricanes. They used a statistical model encompassing more than 100 years of past Atlantic hurricane activity to make their forecasts.
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| Figure 1. The 12Z Thursday run of the European model predicted that 90L would make landfall on Monday afternoon near the Alabama/Mississippi border, and drop 7-day precipitation amounts of 8” near its landfall location. A large area of 3 – 5” is predicted to fall across the Southeast U.S. |
Update on 90L
The 12Z Thursday suite of model runs are in, and the European and GFS models continue to give support for 90L developing into Tropical Storm Alberto or Subtropical Storm Alberto by Saturday. However, the 12Z Thursday run of the UKMET model did not predict development until Tuesday. Given that 90L continues to look moderately well-organized on satellite loops in the face of a hostile 30 knots of wind shear, the best bet is to predict development into a depression on Friday evening or Saturday, when wind shear is forecast to drop significantly. In a special 2 pm EDT Thursday Tropical Weather Outlook, The National Hurricane Center boosted 90L’s 2-day and 5-day odds of development to 70% and 90%, respectively. An Air Force hurricane hunter aircraft is scheduled to investigate 90L on Friday afternoon.
The latest 12Z Thursday runs of our top intensity models (the HWRF, DSHIPS, LGEM and HMON models) predicted that 90L could get considerably stronger than previously thought, with peak winds ranging from 40 mph (LGEM model) to 80 mph (HWRF model) early next week. Until 90L actually forms a closed center of circulation and becomes a depression, we should be wary of these intensity forecasts. Heavy rain from 90L is still likely to be its number one threat, though we may also need to be concerned with storm surge damage and wind damage if the storm does manage to over-achieve.
