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Tropical Depression Thirteen Forecast Discussion Number 2
2020-08-20 11:01:35| National Hurricane Center (Atlantic)
Issued at 500 AM AST Thu Aug 20 2020 000 WTNT43 KNHC 200901 TCDAT3 Tropical Depression Thirteen Discussion Number 2 NWS National Hurricane Center Miami FL AL132020 500 AM AST Thu Aug 20 2020 There has been little change in the organization of Tropical Depression Thirteen during the past several hours. An area of ragged but persistent convection continues near the estimated center position, and there is some outer banding in the southern semicircle. Dvorak-based satellite intensity estimates have changed little since the last advisory, so the initial intensity remains 30 kt. The initial motion is a somewhat uncertain 295/18. The subtropical ridge to the north of the cyclone is expected to build westward to the north of the Greater Antilles during the next several days, and this should steer the cyclone generally west-northwestward through the forecast period. The track guidance remains in fairly good agreement with this scenario, but there remains a spread between the GFS/UKMET on the south side of the guidance and the Canadian/HMON on the north side. Overall, the envelope has shifted a little to the south since the previous advisory and the new forecast track, which lies just to the north of the consensus models, is also nudged southward. The new track calls for the cyclone to pass near the Leeward Islands in 36-48 h, near or north of Puerto Rico and the Virgin Islands in 48-60 h, and then near or over Hispaniola and the southeastern Bahamas by 72 h. Overall, the environment looks generally favorable for strengthening, with the cyclone expected to encounter light- to moderate shear during forecast period. However, the guidance responds to this environment with a wide range of solutions. The HWRF/HMON forecast the cyclone to intensify into a major hurricane by 120 h. On the other hand, the GFS and ECMWF show the system degenerating into an open wave by 120 h. The UKMET and Canadian models are between these extremes. The weak GFS solution appears to be due to forecast dry air entrainment, which is a possibility as satellite imagery suggests Saharan dust not far from the cyclone. Between these factors and the possibility of land interaction, the low-confidence intensity forecast is changed little from the previous advisory. 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 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 20/0900Z 15.2N 49.8W 30 KT 35 MPH 12H 20/1800Z 16.4N 52.7W 30 KT 35 MPH 24H 21/0600Z 17.5N 56.4W 35 KT 40 MPH 36H 21/1800Z 18.4N 60.2W 40 KT 45 MPH 48H 22/0600Z 19.1N 63.9W 45 KT 50 MPH 60H 22/1800Z 19.8N 67.1W 50 KT 60 MPH 72H 23/0600Z 20.8N 71.0W 55 KT 65 MPH 96H 24/0600Z 23.0N 78.0W 60 KT 70 MPH 120H 25/0600Z 26.0N 83.0W 60 KT 70 MPH $$ Forecaster Beven
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