Tropical Depression Four formed in the Central Atlantic on Wednesday evening, but is struggling against dry air and is unlikely to survive long enough to threaten any land areas. At 11 am EDT Thursday, TD 4 was located about midway between the Cabo Verde Islands and the Lesser Antilles Islands, and was headed west-northwest at a speedy 21 mph.
Satellite images on Thursday morning showed that the circulation of TD 4 was weakening. The storm had a decreasing amount of thunderstorm activity, and the cloud tops of these thunderstorms were warming, showing that they did not extend as high in the atmosphere due to weaker updrafts supporting them. This weakening trend was likely due to dry air from the Saharan Air Layer (SAL), which had wrapped into the circulation center from the east. Wind shear was favorable for development, a light 5 - 10 knots, and sea surface temperatures (SSTs) were marginal for development, near 26.5°C (80°F)--about 1°F above the seasonal norm.
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Figure 1: The Saharan Air Layer (SAL) analysis from 8 am EDT Thursday, July 6, 2017, showed a large area of dry Saharan air to the east of TD 4. This dry air was being entrained into the center of the storm. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division. |
Unfavorable conditions for development Friday and beyond
Dry air will be TD 4’s main impediment to development through Friday, as diagnosed by the 12Z Thursday run of the SHIPS model--relative humidity at mid-levels of the atmosphere will fall from 60% Thursday morning to 50% on Friday morning, which will make it difficult for the storm to develop. However, with wind shear expected to stay low through Friday morning, there is a brief opportunity for TD 4 to get its act together and become Tropical Storm Don. By Friday night, though, strong upper level winds out of the west will begin impacting the storm, bringing high wind shear of 15 – 25 knots through the weekend. The combination of dry air and high wind shear should be enough to destroy TD 4 by Monday. Only 4% of the 50 members of the 0Z Thursday European ensemble forecast--and none of the 20 members of the 0Z Thursday GFS ensemble forecast--predicted that TD 4 would still be alive on Monday evening. The models showed a west-northwest track for the storm, bringing TD 4’s remnants about 200 - 500 miles north of the Lesser Antilles Islands by Sunday. Given the dry air surrounding TD 4 and the meager amount of heavy thunderstorms associated with the storm, I’m not expecting any dangerous flooding rains in the Lesser Antilles Islands from TD 4.
The next threat
The next threat in the Atlantic to watch may well be a tropical wave expected to come off the coast of Africa on Saturday. About 10% of the members of the GFS and European model ensemble forecasts are predicting that this wave will develop into a tropical depression in the Central Atlantic as early as Monday. The wave is predicted to take a mostly westward track, and potentially arrive in the Lesser Antilles Islands during the weekend of July 15 – 16.
CSU, TSR raise odds for an active Atlantic hurricane season
Odds have risen for an active Atlantic hurricane season, according to the forecast updates issued by two leading seasonal prediction groups this week. In their July 5 outlook, Dr. Phil Klotzbach and Dr. Michael Bell (Colorado State University) called for 15 named storms, 8 hurricanes, 3 major hurricanes and an accumulated cyclone energy (ACE) of 132. These numbers include the three tropical storms already observed this year: Arlene, Bret and Cindy. The CSU team boosted the numbers from its June 1 outlook, which called for 14 named storms, 6 hurricanes, 2 major hurricanes, and an ACE of 100. Their April 14 outlook called for 11 named storms, 4 hurricanes, 2 major hurricanes and an ACE of 75.
The July 4 forecast for the 2017 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) also showed increased numbers: 17 named storms, 7 hurricanes, 3 intense hurricanes and an Accumulated Cyclone Energy (ACE) of 116. Their May 26 forecast predicted 14 named storms, 6 hurricanes, 3 intense hurricanes and an Accumulated Cyclone Energy (ACE) of 98. The long-term averages for the past 65 years are 11 named storms, 6 hurricanes, 3 intense hurricanes and an ACE of 101.
The two main reasons for the upgraded forecast given by both groups:
—a reduced likelihood of a significant El Niño this year
—warming of the tropical Atlantic relative to the seasonal norm
