|Above: Visible satellite image of an area of disturbed weather over the Bahamas as seen at 11:12 am EDT May 2, 2018. Lightning activity from the GOES-16 lightning mapper is shown in color. Image credit: NOAA/RAMMB.|
A cold front that pushed through the Bahamas last weekend has stalled out over the islands, and was generating a large area of showers and thunderstorms on Wednesday afternoon. Sea surface temperatures underneath this disturbance were near 26.5°C (80°F), about 2°F above average, which was just warm enough to support formation of a tropical depression. However, wind shear was a prohibitively high 70 knots, which will keep any development from happening through Thursday.
On Friday and Saturday, an upper-level trough of low pressure will move over the Bahamas, causing the wind shear to fall, and thus increasing the chances of tropical cyclone formation. Recent runs of two of our top models for predicting tropical cyclone genesis, the GFS and European, have been showing the disturbance developing into a weak area of surface low pressure over the Bahamas on Friday, then moving westward over South Florida on Saturday. About 10% of the 50 members of the 0Z Wednesday run of the European model ensemble showed this low becoming a tropical or subtropical depression by early next week, according to a custom CFAN analysis WU subscribes to, and about 50% of the 20 members of the 0Z Wednesday run of the GFS model ensemble did so. On Wednesday morning, the National Hurricane Center (NHC) was not highlighting this disturbance as an “Invest”—an area of interest capable of becoming a tropical cyclone—probably in part because of the unfavorable climatology for this time of year (see below). However, I give the disturbance a 10% chance of development over the next five days. The first name on the list of Atlantic storms for 2018 is Alberto.
Atlantic named storms by mid-May: an uncommon occurrence
It’s uncommon to get a tropical or subtropical depression in the Atlantic by May 15. According to the NOAA Historical Hurricane Tracks website, this has happened 11 times since satellites began monitoring the ocean in the 1970s, with 7 of these depressions going on to become named storms--about a 1-in-7-year event. However, we have had Atlantic named storms form by May 15 the past three years in a row--an event with a 0.3% chance of occurrence based on statistics from the past 48 years:
May 8, 2015: Tropical Storm Ana formed off the coast of South Carolina, and made landfall there on May 10 with 45 mph winds. Ana was the earliest landfalling U.S. tropical cyclone on record, and caused one drowning death and minimal property damage.
January 12, 2016: Hurricane Alex became a tropical storm in the waters south of the Azores. After peaking as a Category 1 hurricane with 75 mph winds, Alex hit the Azores as a tropical storm with 65 mph winds on January 15. Alex caused minimal damage and no deaths.
April 20, 2017: Tropical Storm Arlene formed in the central Atlantic. After reaching top winds of 45 mph and spending 30 hours as a tropical storm, Arlene died. Arlene did not affect any land areas.
Bob Henson will be back with a post on today’s severe weather threat later this afternoon.