Start of Hurricane Season Brings Reduced Activity Forecasts From CSU, TSR

May 31, 2018, 5:05 PM EDT

Above: The 2018 Atlantic hurricane season has already had an unusual storm—Subtropical Storm Alberto. After making landfall in the Florida Panhandle as a subtropical storm with 45 mph winds on May 28, Alberto became a tropical depression which held together for three more days during its trek northwards into the Great Lakes. Alberto was finally declared post-tropical in northern Michigan on Thursday morning, May 31. So, Michigan has experienced a tropical depression this year before any of the coastal U.S. states have! The image above is of Tropical Depression Alberto over Indiana on Wednesday afternoon, May 30, 2018. Alberto was looking impressively well-organized thanks to low vertical wind shear and a moist atmosphere. Image credit: NOAA.

It’s June 1, and the Atlantic hurricane season is officially here. We’ve already had one named storm that jumped the gun this season—Subtropical Storm Alberto, which made landfall in the Florida Panhandle with 45 mph winds on May 28. However, a pronounced cooling of the waters in the tropical Atlantic over the past two months has prompted Colorado State University (CSU) and Tropical Storm Risk, Inc. (TSR) to lower their forecast numbers for the 2018 Atlantic hurricane season in outlooks released this week.

CSU predicts a near-average Atlantic hurricane season: 14 named storms

The 2018 Atlantic hurricane season is expected to be near-average, said the hurricane forecasting team from Colorado State University (CSU) in their latest seasonal forecast issued May 31. Led by Dr. Phil Klotzbach, with coauthor Dr. Michael Bell, the CSU team is calling for an Atlantic hurricane season with 14 named storms (including Alberto), 6 hurricanes, 2 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 88. This is a step down from their April 5 forecast, which called for 14 named storms, 7 hurricanes, 3 intense hurricanes, and an 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 51% chance of a major hurricane hitting the U.S. in 2018 (long term average is 52%), with a 30% chance for the East Coast and Florida Peninsula (long term average is 31%), and a 29% chance for the Gulf Coast (long term average is 30%). The Caribbean is forecast to have a 41% chance of seeing at least one major hurricane (long term average is 42%). Their next forecast will be released July 2.

Four years with similar pre-season April and May atmospheric and oceanic conditions since 1950 were selected as “analog” years that the 2018 hurricane season may resemble. These years were characterized by neutral to weak El Niño conditions and cooler than average tropical Atlantic SSTs during August - October:

1986 (6 named storms, 4 hurricanes, and 0 intense hurricanes)
2001 (15 named storms, 9 hurricanes, and 4 intense hurricanes)
2012 (19 named storms, 10 hurricanes, and 2 intense hurricanes)
2014 (8 named storms, 6 hurricanes, and 2 intense hurricanes)

The average activity for these years was 12 named storms, 7.3 hurricanes, 2.0 major hurricanes, and an ACE of 86—close to the long-term average. The most notable storms during these years were Category 4 Hurricane Michelle of 2001 and Category 3 Hurricane Sandy of 2012.

Departure of SST from average
Figure 1. May 30, 2018 sea surface temperatures in the tropical Atlantic (10-20°N, 60-20°W, black box) were about 1°C below average--colder than any year since 1994. This coolness has led to reduced forecasts of hurricane activity from CSU and TSR. Image credit: Levi Cowan, tropicaltidbits.com.
Departure of SLP from average
Figure 2. Departure of sea level pressure from average (in millibars) during April and May, 2018. Thanks to a positive phase of the North Atlantic Oscillation (NAO), the Azores-Bermuda High had a central pressure up to 5 mb stronger than average, while the Icelandic Low had a central pressure more than 5 mb below average. This difference in pressure drove stronger surface trade winds (black arrows) over the tropical Atlantic, cooling the ocean surface through increased evaporation and increased mixing. Image credit: NOAA/ESRL and CSU.

TSR predicts a quiet Atlantic hurricane season: 9 named storms

The May 30 forecast for the 2018 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) calls for a quiet Atlantic hurricane season with an overall level of activity 50% or more below the long-term (1950-2017) norm and the recent 2008-2017 ten-year norm. TSR predicts 9 named storms, 4 hurricanes, 1 intense hurricane and an Accumulated Cyclone Energy (ACE) of 43 for the period May through December. This is a major step down from their April 5 forecast, which called for 12 named storms, 6 hurricanes, 2 intense hurricanes and an ACE of 84. The long-term averages for the past 68 years are 11 named storms, 6 hurricanes, 3 intense hurricanes and an ACE of 103. They give a 69% likelihood that hurricane activity in 2018 will be in the lowest one-third of years since 1950. TSR rates their skill level as modest for these May forecasts--16 - 31% higher than a "no-skill" forecast made using climatology.

TSR predicts a 7% chance that U.S. landfalling ACE index will be above average, a 24% chance it will be near average, and a 69% chance it will be below average. They project that two named storms, including one hurricane, will hit the U.S. The averages from the 1950-2017 climatology are three named storms, including one hurricane. They rate their skill at making these May forecasts for U.S. landfalls just 3% - 5% higher than a "no-skill" forecast made using climatology, though. 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 July 5.

NAO forecast
Figure 3. In their latest May 29 forecast of the North Atlantic Oscillation (NAO), The Weather Company called for a mostly positive phase of this cycle during much of June, which would continue to drive strong trade winds over the tropical Atlantic, keeping the waters there cooler than average. Reliable predictions of the NAO are difficult to make more than about two weeks in advance. Their May 18 forecast calls for a near-average Atlantic hurricane season with 12 named storms, 5 hurricanes, and 2 major hurricanes.

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. In our May 24 post, we summarized some of the other seasonal hurricane forecasts made by other entities, including NOAA (a near-normal or above-normal Atlantic hurricane season with 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).

When will Beryl come?

The next name on the Atlantic list of storms for 2018 is Beryl, but Friday morning’s 5-Day Tropical Weather Outlook from NHC highlighted no areas of concern for the next five days.  None of the 0Z Friday runs of our top three models for forecasting tropical cyclone genesis—the European, UKMET and GFS models—called for a tropical depression to develop in the coming week. The GFS model in recent runs has been predicting what may be a Central American Gyre (CAG)--similar to the one that spawned Subtropical Storm Alberto--to form over Central America as early as June 6, though. About 30% of the 20 members of the 0Z Friday GFS model ensemble predicted that a tropical depression would spin up out of this CAG in the Western Caribbean sometime during the period June 6 – 12, according to a custom forecast product supplied to WU by cfanclimate.com. Back in mid-May, the GFS model did correctly forecast the genesis of Alberto from a CAG seven days in advance, but only after issuing about a week’s worth of false alarm forecasts. So, until the European and/or UKMET model echo the GFS forecast, we should not pay undue attention to its long-range musings. Philippe Papin has an excellent series of tweets discussing Central American Gyres.

Have a great weekend, everyone!

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Dr. Jeff Masters

Dr. Jeff Masters co-founded Weather Underground in 1995, and flew with the NOAA Hurricane Hunters from 1986-1990.

jeff.masters@weather.com

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