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Tropical Depression Thirteen Forecast Discussion Number 1
2020-08-20 04:52:34| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Wed Aug 19 2020 000 WTNT43 KNHC 200252 TCDAT3 Tropical Depression Thirteen Discussion Number 1 NWS National Hurricane Center Miami FL AL132020 1100 PM AST Wed Aug 19 2020 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 POSITIONS AND MAX WINDS INIT 20/0300Z 14.6N 47.9W 30 KT 35 MPH 12H 20/1200Z 16.1N 50.8W 30 KT 35 MPH 24H 21/0000Z 17.4N 54.6W 35 KT 40 MPH 36H 21/1200Z 18.4N 58.4W 40 KT 45 MPH 48H 22/0000Z 19.1N 62.1W 45 KT 50 MPH 60H 22/1200Z 19.9N 65.9W 50 KT 60 MPH 72H 23/0000Z 20.8N 69.7W 55 KT 65 MPH 96H 24/0000Z 22.9N 76.7W 60 KT 70 MPH 120H 25/0000Z 26.2N 82.2W 60 KT 70 MPH $$ Forecaster Cangialosi
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