Tropical Depression Thirteen Forms In The Central Atlantic

Tropical Depression Thirteen (previously INVEST 98L) has formed in the Central Atlantic, 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 PM AST, the center of Tropical Depression Thirteen was located near latitude 14.6 North, longitude 47.9 West. The depression is moving toward the west-northwest near 20 MPH(31 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 by late Thursday. The estimated minimum central pressure is 1008 millibars.

Tropical Depression Thirteen Forecast Track as of 11:00 PM AST Wednesday 19th August 2020. (National Hurricane Center)
Tropical Depression Thirteen Forecast Track as of 11:00 PM AST Wednesday 19th August 2020. (National Hurricane Center)

Watches & Warnings

Tropical Depression ThirteenWatches and Warnings as of 11:00 PM AST Wednesday 19th August 2020. (National Hurricane Center)

A Tropical Storm Watch is in effect for:

  • Saba and St. Eustatius

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 tropical storm watches could be required for those areas on Thursday.

Tropical storm watches may be issued for parts of the Leewards at 5:00 AM Thursday, being upgraded to a warning by Thursday night.

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 PM AST Wednesday 19th 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 PM AST Wednesday 19th 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 through Friday night over the northern Leeward Islands and Puerto Rico.

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)

The low-pressure system that NHC has been monitoring over the central tropical Atlantic has now developed a well-defined center of circulation and maintained enough organized deep convection to be classified as a tropical depression, the thirteenth cyclone of the 2020 Atlantic hurricane season. The depression already has some banding features on its north and west sides as evident in geostationary satellite and microwave images. The initial intensity is set at 30 kt based on ASCAT data from around 0100 UTC and a T2.0/30 kt Dvorak classifications from TAFB.

The depression has been moving fairly quickly to the west-northwest, with the latest initial motion estimated to be 295/17 kt. It should be noted that the initial motion is somewhat uncertain given that the system has only recently formed. A subtropical ridge currently over the central Atlantic is expected to build westward during the next several days and should be the primary steering feature for the depression through the forecast period. This pattern should keep the depression on a fairly quick west-northwest track during the next several days, taking the cyclone near the northern
Leeward Islands by Friday night and near the Greater Antilles and southeastern Bahamas this weekend. The models are in fairly good agreement, but there is some north-south spread with the GFS being on the southern side of the guidance envelope and the ECMWF on the northern end. The NHC track forecast lies down the middle of the guidance suite.

The environmental conditions appear generally favorable for the depression to strengthen, with the wind shear expected to remain relatively low while the system moves over warm SSTs and remains in a moist airmass. These conditions should promote gradual strengthening and it seems quite likely that the cyclone will be a tropical storm when it moves near or north of the northern Leeward Islands in a couple of days. The bigger question is how much interaction will there be with the Greater Antilles. If the depression moves on the south side of the guidance envelope, further strengthening would be limited due to land interaction. Conversely, if the system gains more latitude and moves north of these highly topographic islands, it could have the opportunity for more significant intensification. The NHC intensity forecast, which is of low confidence, is roughly near the middle of the guidance envelope.

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 tomorrow. 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 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 Cangialosi from the NHC.

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