Tropical Depression Four (TD 4) formed off the coast of South Carolina at 11 a.m. EDT Monday and has the potential to become Tropical Storm Danny before making landfall in South Carolina Monday evening. Early Monday afternoon, the outer rain bands of the depression were already bringing heavy rain showers to the coasts of South Carolina, Georgia, and extreme northeastern Florida. UPDATE: At 3:05 p.m. EDT Monday, TD 4 was upgraded to Tropical Storm Danny, based on data from Doppler radar and the Hurricane Hunters.

At 11 a.m. EDT Monday, TD 4 was located 110 miles east-southeast of Charleston, South Carolina, headed west-northwest at 16 mph. TD 4 had marginal conditions for development, with moderate wind shear of 10-20 knots and a dry atmosphere with a mid-level relative humidity near 50%. However, TD 4 was crossing over the Gulf Stream, which had warm sea surface temperatures near 28 degrees Celsius (82°F). These warm waters were giving TD 4 an energy boost. Satellite images showed TD 4 was compact and had a well-defined surface circulation, but strong upper-level winds out of the east were creating enough wind shear to keep all of TD 4’s heavy thunderstorms confined to its west side, near the coast of South Carolina and Georgia.

National Hurricane Center forecast for TD 4

Steering currents are expected to carry TD 4 into South Carolina by Monday night, so it does not have much time to develop into Tropical Storm Danny; it is unlikely that TD 4 will have sustained winds as strong as 50 mph at landfall. Not a very moist system, TD 4 is likely to bring rains of no more than 1-3 inches along the coast. Soils are relatively dry in this region, which will limit the potential for damaging flooding. An Air Force hurricane hunter aircraft is scheduled to investigate TD 4 Monday afternoon.

Figure 1. Radar image of TD 4 at 15:30Z (11:30 a.m. EDT) June 28, 2021. (Image credit: Mark Nissenbaum)

2021 is well ahead of climatology

If Danny does form today, its formation date of June 28 would be nearly two months before the usual August 23 arrival of the Atlantic’s fourth named storm, according to the National Hurricane Center. Only three Atlantic hurricane seasons have had four named storms develop earlier in the year: 2020, 2016, and 2012 (thanks go to Phil Klotzbach for this fact). The record-busy 2020 season had its fourth named storm on June 23 and fifth named storm on July 6; this was the earliest fifth storm on record in the Atlantic.

Figure 2. Visible GOES-16 satellite image of 95L at 15:30Z (11:30 a.m. EDT) Monday, June 28, 2021. (Image credit: Tropical Tidbits)

Tropical disturbance 95L in central tropical Atlantic remains disorganized

In the central tropical Atlantic, a tropical disturbance designated 95L was headed west at about 20 mph on Monday afternoon. 95L had marginal conditions for development, with sea surface temperatures near 26.5 degrees Celsius (80°F); 95L’s position close to the equator (about 12°N latitude) was also limiting its ability to use Earth’s spin to help get itself spinning.

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 60-65%. The dry air of the Saharan Air Layer was located immediately to the north and west of 95L, and this dry air was infiltrating the system. Wind shear was light, at 5-10 knots, but there was enough dry air surrounding 95L that even light wind shear was able to drive dry air into the system’s core, disrupting it.

Forecast for 95L

As 95L progresses west to west-northwest at about 20 mph through Tuesday, it will continue to struggle with marginal sea surface temperatures and dry air from the Saharan Air Layer, though light wind shear of 5-10 knots will be favorable for development, according to the 12Z Monday run of the SHIPS model. On Wednesday, when 95L will be nearing the Lesser Antilles Islands, sea surface temperatures 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, 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.

Two of the three best models for predicting tropical cyclone genesis – the European and GFS models – support some limited development of 95L, and a number of their ensemble members develop 95L into a tropical storm by late this week. In an 8 a.m. EDT Monday Tropical Weather Outlook, the National Hurricane Center gave 95L two-day and five-day formation odds of 20% and 40%, respectively.

Yet another tropical wave to watch

A new tropical wave that moved off the coast of Africa on Sunday has some limited model support for development later in the week, and it will need to be watched as it heads west at 15-20 mph. The wave was large and quite close to the equator, and these factors will make any development slow to occur.

This wave may arrive in the Lesser Antilles Islands as early as Friday, but the arrival could be later than that. The next name on the Atlantic list of storms after Danny is Elsa.

Figure 3. Visible GOES-17 satellite image of Hurricane Enrique at 14:50Z (10:50 a.m. EDT) Monday, June 28, 2021. (Image credit: NOAA/RAMMB)

Hurricane Enrique drenches Mexico

The first hurricane of the season of the Eastern Pacific, Enrique, formed on Saturday, June 26, which is the typical date for the season’s first hurricane. However, Enrique is the fifth named storm of the season, a threshold that typically isn’t reached until July 22.

At 11 a.m. EDT Monday, Enrique was a category 1 hurricane with 85 mph winds, moving north-northwest at 5 mph just off the southwestern Mexican coast. Enrique was predicted to bring 6-12 inches of rain, with isolated maximum amounts of 18 inches, to portions of southwestern Mexico, causing dangerous flash flooding and mudslides.

Enrique is predicted to turn more to the northwest on Monday afternoon, then potentially make landfall in Mexico’s Baja Peninsula on Wednesday morning. Dry air flowing from the interior of Mexico has infiltrated the core of the hurricane, and this dry air, combined with lower sea surface temperatures, should reduce Enrique to tropical storm strength by Tuesday morning.

A surge of moisture associated with Enrique is likely to push northwestward late in the week, bringing a bit of drought relief to northwestern Mexico, Arizona, and New Mexico.

Bob Henson contributed to this post.

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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...