A tropical disturbance off the Southeast U.S. coast, designated 96L by the National Hurricane Center (NHC), has the potential to become a tropical depression or perhaps Tropical Storm Danny before making landfall over South Carolina or Georgia on Monday evening.

UPDATE: Tropical Storm Danny has formed. Read the latest in this post.

On Sunday afternoon, 96L was headed west at about 15 mph. The system had marginal conditions for development, with sea surface temperatures (SSTs) near 27 degrees Celsius (81°F) and a dry atmosphere with a mid-level relative humidity of 50%. However, wind shear was a light 5-10 knots, which favored development.

Satellite images showed 96L was compact and had a surface circulation, but it was struggling to maintain its heavy thunderstorm activity because of dry air, as seen in the Tweet above from NHC hurricane forecaster Philippe Papin.

Figure 1. Predicted wind speed (colors) and sea level pressure (black lines) for 96L at 8 p.m. EDT Monday, June 28 (0Z June 29), from the 12Z Sunday, June 27, run of the HWRF model. The model predicted 96L would make landfall near the South Carolina/Georgia border as a tropical depression with 35 mph winds and a central pressure of 1014 mb. (Image credit: Tropical Tidbits)

NHC forecast for 96L

As 96L moves west-northwestward at about 15 mph on Monday, it will cross over the warm waters of the Gulf Stream, which may give the system enough of an energy boost to allow formation of a short-lived tropical depression or tropical storm. Steering currents are expected to carry the system into South Carolina or Georgia by Monday night. Not a very moist system, 96L, if it remains below tropical storm strength, is likely to bring rains of 1-3 inches along the coast.

The three best models for predicting tropical cyclone genesis – the European, GFS, and UKMET models – all support some limited development of 96L. In a 2 p.m. EDT Sunday Tropical Weather Outlook, NHC gave 96L two-day and five-day development odds of 50%. The next name on the Atlantic list of storms is Danny. An Air Force hurricane hunter aircraft is scheduled to investigate 96L on Monday afternoon.

Figure 2. Visible GOES-16 satellite image of 95L, 96L, and a new tropical wave off the coast of Africa at 15:30Z (11:30 a.m. EDT) Sunday, June 27, 2021. (Image credit: NOAA/RAMMB)

Watching tropical disturbance 95L in central tropical Atlantic

In the central tropical Atlantic, a tropical disturbance designated 95L was headed west at about 15 mph on Sunday afternoon. 95L had marginal conditions for development, with sea surface temperatures near 26.5 degrees Celsius (80°F) and moderate wind shear of 10 knots. The wave’s development was being retarded by its close proximity to the equator (about 11°N latitude): Earth’s spin helps storms gain more spin the farther they are from the equator.

Satellite images  showed 95L to be compact, with a moderate amount of spin, but it was struggling to maintain its heavy thunderstorm activity. The atmosphere surrounding 95L was not especially moist, with a mid-level relative humidity of 65%. The dry air of the Saharan Air Layer (SAL) was located immediately to the north and west of 95L, and this dry air was infiltrating the system.

Forecast for 95L

As 95L progresses west to west-northwest at 15-20 mph through Tuesday, it will continue to struggle with marginal SSTs and dry air from the Saharan Air Layer, though light wind shear of 5-10 knots will be favorable for development, according to the 18Z Sunday run of the SHIPS model. On Wednesday, when 95L will be nearing the Lesser Antilles Islands, SSTs will warm to 27.5 degrees Celsius (82°F), making it more likely 95L could wall off the dry air and form a strong inner core region of heavy thunderstorms resistant to the dry air. The system’s more northerly position, farther from the equator, will aid this process. Also favoring intensification will be a large-scale region of ascending air over the Atlantic, caused by passage of an atmospheric disturbance called a Convectively Coupled Kelvin Wave (CCKW), as explained in a tweet by Michael Ventrice.

Steering currents will likely carry 95L into the Lesser Antilles Islands by Wednesday night, bringing gusty winds and heavy rains. Many of these islands, with a much drier than average May, could use the rain.

The three best models for predicting tropical cyclone genesis – the European, GFS, and UKMET models – support some limited development of 95L. In a 2 p.m. EDT Sunday Tropical Weather Outlook, NHC gave 95L two-day and five-day odds of formation of 20% and 30%, respectively.

Yet another tropical wave to watch

A new tropical wave that moved off the coast of Africa on Sunday has some model support for development later in the week, and it will need to be watched as it heads west at 15-20 mph. This wave is expected to arrive in the Lesser Antilles Islands on Friday.

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Jeff Masters

Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...