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Tropical Depression Eighteen Forecast Discussion Number 2
2021-09-23 04:38:42| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Wed Sep 22 2021 000 WTNT43 KNHC 230238 TCDAT3 Tropical Depression Eighteen Discussion Number 2 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Wed Sep 22 2021 The depression's organization has not improved since earlier this afternoon. An analysis of the ambiguities from 0000 UTC ASCAT-B data suggests that the surface circulation is just barely closed, with the center still attached to the northern end of a trough. Peak wind retrievals from the scatterometer pass were 25-30 kt, so the initial intensity remains 30 kt. Based on the ASCAT data, the depression appears to be moving just north of due west, or 275/13 kt. Low- to mid-level ridging to the north is expected to maintain the cyclone on a westward heading for the next 36 hours, followed by a path toward the west-northwest from day 2 through day 5. The more notable part of the forecast is the forward speed. Global models are showing anomalously strong 500-mb ridging developing over the eastern and central Caribbean Sea in 2 to 3 days, which is likely to block the depression's forward progress toward the end of the forecast period. In fact, the current NHC forecast has the system moving west-northwestward at only 7 or 8 kt well east of the Lesser Antilles on days 3 through 5. All this means that it will probably be a few more days before we have a better idea if and how the system might potentially threaten areas farther west. For the 5-day forecast period, there is a normal amount of spread among the track guidance, and the updated NHC forecast is very close to a blend of the TVCA and HCCA consensus aids. This solution is not too different from the previous forecast. Overall, an environment of low shear, warm sea surface temperatures, and a moist, unstable atmosphere appears conducive for strengthening. However, there is some sort of signal being conveyed by the global models (e.g., the GFS, ECMWF, and UKMET) whereby the cyclone's surface circulation does not really tighten up for another 24 to 48 hours. Given the system's current structure, this scenario seems plausible. Therefore, the updated NHC intensity forecast shows a slower rate of strengthening during the first 48 hours, just a bit below the intensity consensus in deference to the global model solutions. After 48 hours, the official forecast converges on top of the previous NHC forecast and still brings the cyclone to hurricane, and then possibly major hurricane, strength by the end of the forecast period. FORECAST POSITIONS AND MAX WINDS INIT 23/0300Z 10.2N 35.0W 30 KT 35 MPH 12H 23/1200Z 10.4N 37.0W 30 KT 35 MPH 24H 24/0000Z 10.9N 39.6W 35 KT 40 MPH 36H 24/1200Z 11.4N 42.1W 45 KT 50 MPH 48H 25/0000Z 12.0N 44.3W 55 KT 65 MPH 60H 25/1200Z 12.6N 46.0W 70 KT 80 MPH 72H 26/0000Z 13.2N 47.5W 80 KT 90 MPH 96H 27/0000Z 14.5N 50.2W 90 KT 105 MPH 120H 28/0000Z 16.0N 53.3W 100 KT 115 MPH $$ Forecaster Berg
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