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Tropical Depression Eleven Forecast Discussion Number 1
2020-08-11 22:32:51| National Hurricane Center (Atlantic)
Issued at 500 PM AST Tue Aug 11 2020 000 WTNT41 KNHC 112032 TCDAT1 Tropical Depression Eleven Discussion Number 1 NWS National Hurricane Center Miami FL AL112020 500 PM AST Tue Aug 11 2020 Visible satellite imagery and ASCAT-C data from earlier in the day have shown that the area of low pressure NHC has been monitoring over the tropical Atlantic has developed a less-elongated circulation with a well-defined center. For the most part, deep convection has persisted with the system since about this time yesterday, save a brief period of warming cloud tops this morning. The low now meets the criteria of a tropical cyclone, and advisories have been initiated on Tropical Depression Eleven with 30-kt winds, in line with the latest Dvorak Current Intensity numbers from TAFB and SAB. The depression is moving westward, or 280/14 kt, to the south of a large mid-tropospheric high centered over the central Atlantic. This pattern is expected to evolve rather quickly, with a break developing in the ridge over the central Atlantic by 48 hours. This change should allow the depression to begin making more poleward progress, moving west-northwestward from 36 hours until the end of the forecast period. The track models are in good agreement on this scenario, as well as the system's forward speed, and bring the center of the cyclone near or just to the north of the northern Leeward Islands in 4-5 days. This first NHC forecast lies just to the north of the multi-model consensus cluster through day 3, out of respect for the northern-lying ECMWF model, and then is close to HCCA on days 4 and 5. Conventional satellite imagery and Saharan Air Layer analyses suggest that the center of the depression is being shielded from much drier air to its north and west. However, as has been the case for a few days, at least 15 kt of easterly shear has been pushing deep convection to the western side of the circulation. This shear is expected to decrease over the next day or two, which should allow for gradual strengthening to begin by 36 hours, and a peak in the cyclone's intensity should occur in about 3 days. For this period, the NHC intensity forecast is a little above HCCA and the IVCN intensity consensus. After that time, westerly or southwesterly shear is forecast to develop and increase to 20-30 kt by days 4 and 5, which is likely to induce significant weakening. In fact, it's notable that the conditions become hostile enough that the global models are showing the system opening up into a trough near the northern Leeward Islands by day 5, which is a plausible alternate scenario. FORECAST POSITIONS AND MAX WINDS INIT 11/2100Z 11.7N 40.0W 30 KT 35 MPH 12H 12/0600Z 12.0N 42.1W 30 KT 35 MPH 24H 12/1800Z 12.4N 44.5W 30 KT 35 MPH 36H 13/0600Z 13.0N 46.9W 35 KT 40 MPH 48H 13/1800Z 13.9N 49.4W 40 KT 45 MPH 60H 14/0600Z 15.1N 51.8W 45 KT 50 MPH 72H 14/1800Z 16.4N 54.3W 45 KT 50 MPH 96H 15/1800Z 18.5N 59.5W 40 KT 45 MPH 120H 16/1800Z 21.0N 64.0W 35 KT 40 MPH $$ Forecaster Berg
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