Tropical Storm Grace Takes Aim At Leeward Islands

The seventh-named storm for the 2021 Atlantic Hurricane Season has formed east of the Lesser Antilles. After days of being monitored as Invest 95L, then Potential Tropical Cyclone Seven, then Tropical Depression Seven, the system organized sufficiently to be designated Tropical Storm Grace early Saturday morning.

Grace is forecast to begin affecting mainly the Leeward Islands and parts of the French Antilles later today, with the strongest winds and rains tracking across the Leewards tonight into early tomorrow. This tropical storm formed from Tropical Wave 33, which is also forecast to move across the region tonight.

Specifically for Trinidad and Tobago, while direct impacts are not expected, periphery impacts such as an atypical wind regime are expected from today (Saturday) through tomorrow (Sunday). There may also be the possibility of feeder bands (strong showers, thunderstorms, and gusty winds associated with Tropical Storm Grace) developing tonight into Sunday. There are no alerts, watches, or warnings in effect for T&T from the Trinidad and Tobago Meteorological Service at this time for T&T.

The Latest From The National Hurricane Center

Tropical Storm Grace's Forecast Track as of 8:00 AM Saturday 14th August 2021. (National Hurricane Center)
Tropical Storm Grace’s Forecast Track as of 8:00 AM Saturday 14th August 2021. (National Hurricane Center)

At 8:00 AM AST, the center of Tropical Storm Grace was located near latitude 15.9 North, longitude 56.6 West. Grace is moving toward the west near 35 KM/H. A motion toward the west with a gradual decrease in forward speed is expected during the next few days. On the forecast track, the center of the storm is forecast to move over the Leeward Islands tonight, over the Virgin Islands and Puerto Rico on Sunday, and then over the Dominican Republic on Monday.

Maximum sustained winds are near 65 KM/H with higher gusts. Some strengthening is forecast during the next couple of days. Weakening is expected early next week as the system interacts with the Greater Antilles.

Grace is a small tropical storm. Tropical-storm-force winds extend outward up to 35 kilometers from the center. The estimated minimum central pressure is 1007 millibars.

 Tropical Storm Grace's Forecast Track as of 8:00 AM Saturday 14th August 2021. (National Hurricane Center)
Tropical Storm Grace’s Forecast Track as of 8:00 AM Saturday 14th August 2021. (National Hurricane Center)

Tropical Storm Grace’s Watches & Warnings

Tropical Storm Grace's Tropical Storm Watches and Warnings as of 8:00 AM Saturday 14th August 2021. (National Hurricane Center)
Tropical Storm Grace’s Tropical Storm Watches and Warnings as of 8:00 AM Saturday 14th August 2021. (National Hurricane Center)

A Tropical Storm Warning is in effect for:

  • Antigua and Barbuda
  • Anguilla
  • St. Kitts and Nevis
  • Montserrat
  • Saba
  • Sint Eustatius
  • Sint Maarten
  • St. Martin
  • St. Barthelemy
  • British Virgin Islands
  • U.S. Virgin Islands
  • Puerto Rico, including Vieques and Culebra

A Tropical Storm Warning means that tropical storm conditions are expected somewhere within the warning area within 36 hours.

Interests in the Dominican Republic and Haiti should monitor the progress of this system. Additional watches and warnings will likely be required for this area later today.

For storm information specific to your area in the United States, including possible inland watches and warnings, please
monitor products issued by your local National Weather Service forecast office. For storm information specific to your area outside of the United States, please monitor products issued by your national meteorological service.

Tropical Storm Grace's Tropical Storm Watches and Warnings as of 8:00 AM Saturday 14th August 2021. (National Hurricane Center)
Tropical Storm Grace’s Tropical Storm Watches and Warnings as of 8:00 AM Saturday 14th August 2021. (National Hurricane Center)

Tropical Storm Grace: Hazards Affecting Land

Tropical Storm Grace's probabilities for tropical-storm-force winds as of 8:00 AM Saturday 14th August 2021. (National Hurricane Center)
Tropical Storm Grace’s probabilities for tropical-storm-force winds as of 8:00 AM Saturday 14th August 2021. (National Hurricane Center)

WIND: Tropical storm conditions are expected within the warning area over the Leeward Islands by tonight or early Sunday, and the Virgin Islands and Puerto Rico beginning on Sunday.

RAINFALL: The system is expected to produce the following rainfall amounts later today into Monday:

  • Over the northern Leeward Islands and Virgin Islands: 3 to 6 inches. This rainfall may produce scattered areas of flash and urban flooding.
  • Over Puerto Rico: 3 to 6 inches. Heavy rainfall could lead to flash, urban, and small stream flooding as well as potential
    mudslides.
  • Heavy rainfall from this system could impact Florida late next week.
 Tropical Storm Grace's probabilities for hurricane-force winds as of 8:00 AM Saturday 14th August 2021. (National Hurricane Center)
Tropical Storm Grace’s probabilities for hurricane-force winds as of 8:00 AM Saturday 14th August 2021. (National Hurricane Center)

This system is of no direct threat to Trinidad and Tobago.

Tropical Storm Grace’s Forecast Discussion

Tropical Storm Grace (Weathernerds.org)
Tropical Storm Grace (Weathernerds.org)

The cyclone has become a little better organized overnight, with an area of concentrated convection persisting over the center for the past several hours. The system remains compact, with the convective canopy about 100 n miles in diameter, while earlier ASCAT data revealed winds greater than 25 kt extended only about 30 n mi from its center, with peak winds of 27 kt. Despite the weaker maximum winds sampled earlier this evening, the increase in the organization of such a small system has likely produced a notable increase in the surface winds near the center. In addition, the latest Dvorak intensity estimates from TAFB, SAB, and the UW-CIMSS ADT and SATCON suggest that the intensity is now somewhere between 30 and 45 kt. Based on a conservative blend of these values, it is estimated that Tropical Depression Seven has strengthened into 35-kt Tropical Storm Grace.

Grace continues to move quickly westward, with an initial motion of 280/19 kt. A strong mid-level ridge to the north of the storm should continue to steer it westward for the next few days. Beyond that time, the forecast models begin to diverge in their track solutions, as they vary in the strength of the ridge. Overall, the models have trended toward a stronger, more persistent ridge to the north of the cyclone for the latter half of the forecast period, and as such the model solutions are generally showing a track that lies a bit south of the previous runs for those time periods. The official NHC forecast track is little changed through 72 h and lies near the middle of the tightly clustered track guidance. Beyond 72 h, the forecast track has been shifted slightly to the south, but still lies to the north of the consensus model tracks.

The earlier bout of wind shear that had entrained surrounding dry air into Grace appears to have abated, at least in the short term. And, global models indicate that the shear will remain low for about the next 24-36 h while the cyclone moves over increasing SSTs. Therefore, it is reasonable to assume that strengthening should occur over the next day or so. Thereafter, the intensity forecast becomes somewhat complicated as the system is forecast to interact with an upper-level trough while possibly crossing the Greater Antilles at the same time. Slight weakening is indicated in the NHC intensity forecast as the system crosses the northern portion of the Dominican Republic early next week. Thereafter, moderate to strong northerly shear is forecast to impact Grace as the system moves into the flow on the western side of an upper-level trough over the western Atlantic. This shear should prevent any further strengthening through the end of the forecast period.

The intensity forecast is a little lower than a blend of the NOAA HCCA and IVCN consensus, as there is likely a high bias from the HWRF in these solutions. The latter portion of the NHC intensity forecast is of lower-than-normal confidence due to the potential for a longer amount of time over the rugged terrain of the Greater Antilles than indicated, and also due to the fragile nature of the small cyclone and the possible effects of the stronger shear later in the forecast period.

Key Messages:

  1. Tropical storm conditions are expected in portions of the Leeward Islands by tonight or early Sunday, and the U.S. Virgin Islands and Puerto Rico on Sunday. Tropical storm conditions are also possible over the British Virgin Islands on Sunday. The risk of strong winds will then spread westward to the Dominican Republic Sunday night and Monday.
  2. Heavy rainfall could lead to flash and urban flooding over the Leeward and Virgin Islands. Across Puerto Rico, heavy rainfall may lead to flash, urban and small stream flooding, along with the potential for mudslides.
  3. There is a risk of wind and rainfall impacts across Haiti, the Turks and Caicos Islands, the southeastern Bahamas, Cuba, and Florida next week, and interests in those areas should monitor the progress of this system.

Forecast discussion by NHC Forecaster Latto.

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