The islands of the Caribbean may experience their first named storm of 2021 on Friday, when a developing disturbance in the central Atlantic passes though the Lesser Antilles. At 8 a.m. EDT Wednesday, a tropical wave midway between Africa and the Lesser Antilles, designated 97L by the National Hurricane Center, was headed west-northwest at about 20 mph. The system was positioned at a very low latitude, near 9°. This position close to the equator will slow development of 97L, as will the relatively large size of the system and its rapid forward speed of 20 mph. However, 97L otherwise had favorable conditions for development, with sea surface temperatures near 27.5 degrees Celsius (82°F), light-to-moderate wind shear of 5-15 knots, and a very moist atmosphere with a mid-level relative humidity of 75%.

The system looked very impressive on satellite images, with a large area of heavy thunderstorms steadily growing more organized. However, satellite wind measurements from the ASCAT instrument Wednesday morning (Figure 1) revealed that 97L did not have a well-defined surface circulation: The center of calm winds was elongated along an area more than 100 miles across.

Figure 1. Satellite-observed surface winds from the ASCAT sensor (flags color-coded by wind speed, in knots) at 12:48Z (8:48 a.m. EDT) Wednesday, June 30, for tropical disturbance 97L. Winds up to 30 knots (35 mph, red flags) were observed on the north and east sides of the center. An infrared satellite image (black and white colors) from 12:40Z is overlaid. (Image credit: Navy Research Lab, Monterey, annotations from a Tweet by Jake Carstens)

Forecast for 97L

As 97L progresses west-northwest at about 20 mph Wednesday through Saturday, it will stay well south of the dry air from the Saharan Air Layer, and potentially benefit from a moister atmosphere left behind by the passage of tropical disturbance 95L in front of it. Conditions will be favorable for development, according to the 12Z Wednesday run of the SHIPS model, with mostly moderate wind shear of 10-20 knots and steadily rising sea surface temperatures. In addition, 97L’s development will be favored by a large-scale region of ascending air over the Atlantic, caused by the expected passage Thursday and Friday of an atmospheric disturbance called a Convectively Coupled Kelvin Wave, as explained in a Tweet by Eric Webb.

Steering currents likely will carry 97L into the Lesser Antilles Islands by Friday night, bringing gusty winds and heavy rains. Once 97L enters the eastern Caribbean on Saturday, it will encounter an area where the surface trade winds speed up. This speed-up creates a bit of a vacuum at the surface that forces air from aloft to sink, creating a more stable atmosphere hostile to tropical storms. This situation may weaken 97L. By Saturday night, 97L may encounter the high mountains of Hispaniola, which would potentially disrupt the system. This system could be a long-range threat to the U.S. by Tuesday, but it remains uncertain whether the Gulf Coast or East Coast would be most at risk.

Figure 2. Track forecasts out to seven days for 97L from the 6Z (2 a.m. EDT) Wednesday, June 30, run of the GFS ensemble model (GEFS). The black line is the mean of the 31 ensemble members; individual ensemble member forecasts are the thin lines, color-coded by the central pressure they predict for 97L. Most of the members predicted that 97L would become a tropical storm and move west-northwestward through the Caribbean. (Image credit: Tropical Tidbits)

The European and GFS models, and a number of their ensemble members, develop 97L into a tropical storm by late this week. The operational runs of the 6Z Wednesday GFS and HWRF models predicted that 97L would be at category 1 hurricane strength by Friday afternoon, when it is predicted to move through the central Lesser Antilles Islands. The 12Z Wednesday run of the GFS model predicted the same. I give a 20% chance that 97L will be a hurricane on Friday afternoon, a 50% chance it will be a tropical storm, and a 30% chance it will not be a named storm. In an 8 a.m. EDT Wednesday Tropical Weather Outlook, NHC gave 97L two-day and five-day odds of formation of 60% and 80%, respectively. The first hurricane hunter mission into 97L is scheduled for Friday morning.

The next name on the Atlantic list of storms is Elsa, a name that is debuting this year after Tropical Storm Erika in 2015 caused enough damage in Dominica to make it one of just two tropical storms on record to have its name retired (the other was Tropical Storm Allison in 2001). Any nation impacted by a severe hurricane or tropical storm can urge the World Meteorological Organization to retire the name of that storm.

Figure 3. Radar image of the Caribbean at 11:30 a.m. EDT June 30, showing heavy rains from 95L in the Lesser Antilles Islands. (Image credit: barbadosweather.org)

Disturbance 95L bringing heavy rains to Lesser Antilles

A tropical wave designated 95L was headed west to west-northwest at about 25 mph on Wednesday afternoon, and was bringing heavy rain showers and gusty winds to much of the Lesser Antilles. These rains are a concern on the island of St. Vincent, where ash from the recent eruption of the Soufrière volcano could potentially mobilize into dangerous mud flows. 95L had marginal conditions for development, with its fast forward speed, moderate wind shear of 10-20 mph, and a relatively dry atmosphere with a mid-level relative humidity of 60% all making development difficult. These hostile conditions are expected to continue for the remainder of the week. In an 8 a.m. EDT Wednesday Tropical Weather Outlook, NHC gave 95L two-day and five-day odds of formation of just 10%.

Figure 4. MODIS satellite image of Subtropical Storm Raoni off the coast of South America on Wednesday afternoon, June 29, 2021. (Image credit: NASA Worldview)

Subtropical Storm Raoni forms in the South Atlantic

The South Atlantic Ocean, where tropical and subtropical cyclones are rare due to the relatively cool waters, high wind shear, and lack of tropical waves to act as storm seeds, saw the formation of an uncommonly strong subtropical storm on Tuesday, June 29. The system, named Subtropical Storm Raoni by the Brazilian Navy, started as an extratropical cyclone over Uruguay on June 28, where it brought heavy rains and wind gusts up to 63 mph. After moving offshore over waters of 17-18° Celsius (63-64°F), the storm began to acquire tropical characteristics, and become a subtropical storm with winds of 50 mph and a central pressure of 986 mb at 12Z (8 a.m. EDT) June 29, according to the Brazilian Navy. Later in the day, Raoni had top winds of 60 mph; satellite scatterometer data found winds as high as 50 knots (57.5 mph) near the center of Raoni.

Hurricane Enrique kills 2 in Mexico

The first hurricane of the Eastern Pacific hurricane season, Enrique, brought torrential rains and damaging flooding and mudslides to the coast of southwestern Mexico on Monday, after passing just offshore as a category 1 hurricane with 85 mph winds. According to floodlist.com, Enrique killed two people in Mexico, damaged hundreds of homes, and washed out roads and bridges.

At 11 a.m. EDT Tuesday, Enrique was a much-weakened tropical storm with 40 mph winds, moving north-northwest at 8 mph towards Mexico’s Baja Peninsula. Enrique is expected to make landfall there on Wednesday morning, bringing 2-4 inches of rain, with isolated maximum amounts of 6 inches. Enrique is also expected to bring 3-6 inches of rain across Sinola, western Durango and southern Chihuahua in western Mexico. Dry air flowing from the interior of Mexico will continue to weaken Enrique, and the storm will likely be downgraded to a tropical depression by Wednesday.

A surge of moisture associated with Enrique is likely to push northwestward Wednesday through Friday, 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

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...