Tropical Storm Watches Issued For Leewards Due To Tropical Depression Thirteen

Tropical Depression Thirteen (previously INVEST 98L) has changed little in structure Thursday morning in the Central Atlantic. It is still forecast to become a tropical storm by late Thursday.

If this system gets named, it would be Laura. The current record for the earliest ‘L’ Atlantic named storm is Luis on August 29, 1995.

This tropical cyclone is not forecast to directly impact Trinidad, Tobago and the Windward Islands. However, strong low-level convergence following this system, as well as its influence on our wind regime and moisture will bring inclement weather to the area Friday into Saturday.

At 11:00 AM AST, the center of Tropical Depression Thirteen was located near latitude 16.0 North, longitude 52.0 West. The depression is moving toward the west-northwest near 21 MPH (33 KM/H) and this motion is expected to continue for the next few days. On the forecast track, the depression is expected to move near or north of the northern Leeward Islands by late Friday and near or north of the Virgin Islands and Puerto Rico on Saturday.

Maximum sustained winds are near 35 MPH (55 KM/H) with higher gusts. Gradual strengthening is forecast, and the depression is expected to become a tropical storm later today. The estimated minimum central pressure is 1008 millibars.

Tropical Depression Thirteen Forecast Track as of 11:00 AM AST Thursday 20th August 2020. (National Hurricane Center)
Tropical Depression Thirteen Forecast Track as of 11:00 AM AST Thursday 20th August 2020. (National Hurricane Center)

Watches & Warnings

Tropical Depression ThirteenWatches and Warnings as of 11:00 AM AST Thursday 20th August 2020. (National Hurricane Center)
Tropical Depression Thirteen Watches and Warnings as of 11:00 AM AST Thursday 20th August 2020. (National Hurricane Center)

A Tropical Storm Watch is in effect for:

  • Saba and St. Eustatius
  • St. Maarten
  • Antigua, Barbuda, St. Kitts, Nevis, and Anguilla

A Tropical Storm Watch means that tropical storm conditions are possible within the watch area, generally within 48 hours.

Interests elsewhere in the northern Leeward Islands, the Virgin Islands, and Puerto Rico should monitor the progress of this system, as additional tropical storm watches or warnings will be required for portions of those areas later today.

For storm information specific to your area, please monitor products issued by your national meteorological service.

Hazards Affecting Land

Tropical Depression Thirteen probabilities for tropical-storm-force winds as of 11:00 AM AST Thursday 20th August 2020, as well as the most likely time of arrival of tropical-storm-force winds. (National Hurricane Center)
Tropical Depression Thirteen probabilities for tropical-storm-force winds as of 11:00 AM AST Thursday 20th August 2020, as well as the most likely time of arrival of tropical-storm-force winds. (National Hurricane Center)

RAINFALL: The depression is expected to produce 1 to 3 inches of rain with isolated maximum totals of 5 inches over the northern Leeward Islands, and maximum totals of 3 to 6 inches over Puerto Rico and the Virgin Islands through Sunday.

WIND: Tropical storm conditions are possible within the watch area by late Friday.

This system is of no direct threat to the Windward Islands, including Trinidad and Tobago.

Tropical Depression Thirteen Forecast Discussion

Tropical Depression Thirteen East of the Lesser Antilles (Weathernerds)
Tropical Depression Thirteen East of the Lesser Antilles (Weathernerds)

The organization of the depression has not changed much overnight or this morning. An area of convection has persisted near the estimated center, with some banding noted over the northwestern portion of the circulation. An earlier SSMIS overpass was very helpful in locating the center of what appears to be a small circulation. A very recent ASCAT overpass has also revealed a small circulation that is weak on the southeastern side, but with
winds near tropical storm strength to the north of the center. The ASCAT data along with subjective Dvorak classification from TAFB and SAB support maintaining the 30-kt initial intensity.

The depression continues to move briskly west-northwestward or 290/18 kt. The track forecast philosophy has not changed from before. A subtropical ridge over the central Atlantic is forecast to build westward and strengthen over the next several days. This pattern is expected to keep the cyclone on a west-northwestward heading throughout the forecast period. The dynamical models continue to agree on this overall scenario, but there some differences in both forward speed and how close it gets to the Greater Antilles. In general, the models that indicate a stronger cyclone favor a more northern track, while those which depicted a weaker system are along the southern and faster side of the envelope. The latest consensus aids are little north of the previous track, and the new NHC forecast lies between the HFIP corrected consensus and the TVCA multi-model consensus. This is slightly north of the previous advisory, and not far from the GFS ensemble mean.

The environment consisting of light to moderate vertical wind shear is expected to allow for gradual strengthening over the next few days, and the NHC forecast calls for the system to become a tropical storm later today or tonight. The upper-level wind pattern is expected to remain favorable in the latter portion of the forecast period, and if there is minimal land interaction, a faster rate of strengthening is possible at that time. The NHC intensity forecast now shows the system becoming a hurricane by 96 hours, but it is a little lower than the consensus aids at days 4 and 5 due to uncertainty in how much the system will interact with the Greater Antilles.

Key Messages:

  1. Tropical storm conditions are possible across portions of the northern Leeward Islands by Friday night, and Tropical Storm Watches have been issued for some of these islands. Heavy rainfall is likely across this area beginning late Friday.
  2. There is a risk of tropical storm conditions in the Virgin Islands and Puerto Rico Friday night and Saturday and Tropical Storm Watches could be required for these islands later today. Interests there should closely monitor the progress of this system.
  3. The details of the long-range track and intensity forecasts are more uncertain than usual since the system could move over portions of the Greater Antilles this weekend. However, this system could bring some storm surge, rainfall and wind impacts to portions of Hispaniola, Cuba, the Bahamas, and Florida this weekend and early next week. Interests there should monitor this system’s progress and updates to the forecast over the next few days.

Forecast Discussion by Forecaster Brown from the NHC.

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