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Tropical Depression Twenty Forecast Discussion Number 1
2021-09-29 16:36:42| National Hurricane Center (Atlantic)
Issued at 200 PM CVT Wed Sep 29 2021 000 WTNT45 KNHC 291436 TCDAT5 Tropical Depression Twenty Discussion Number 1 NWS National Hurricane Center Miami FL AL202021 200 PM CVT Wed Sep 29 2021 The area of low pressure located over the far eastern tropical Atlantic that NHC has been monitoring has now become a tropical depression, the twentieth tropical cyclone of the 2021 Atlantic hurricane season. Satellite images indicate that the system is producing a large area of showers and thunderstorms that have become fairly well organized around the center. An ASCAT-A pass from a few hours ago indicated that the system now has a well-defined circulation and that the radius of maximum wind is 30-40 n mi north of the center. The initial intensity is set at 30 kt based on the ASCAT data and the T2.0/30 kt Dvorak classifications from TAFB and SAB. The minimum pressure of 1007 mb is partially based on data from a ship report that passed near the depression. Based on satellite fixes, the depression appears to be moving west-northwestward at about 12 kt. The system is currently located on the south side of a deep-layer ridge, and that feature should keep the cyclone on a west-northwest track during the next couple of days. After that time, the models show a mid- to upper-level low developing over the central tropical Atlantic, which erodes the western portion of the ridge. This change in the steering flow should cause the system to turn northwestward on Friday and then northward toward the end of the forecast period. Although the models generally agree on the evolution of the large-scale pattern, there are notable differences in the details, which leads to a fair amount of spread concerning when and where the turn to the north occurs. The GFS is farthest east while the ECMWF shows the westernmost solution. The NHC track forecast lies between those models and is near the TVCA multi-model consensus. The depression is expected to remain in conducive environmental conditions for strengthening during the next 2 or 3 days. During that time period, the storm is forecast to move over 28-29 deg C waters while embedded within an airmass of high mid-level moisture and very low wind shear (less than 10 kt). Therefore, steady strengthening seems likely, and the NHC forecast brings the system to a tropical storm by tonight and a hurricane in a couple of days. Beyond a few days, however, the models show a pronounced increase in southwesterly shear and a notably drier atmosphere. These unfavorable conditions should end the opportunity for strengthening and promote a weakening trend. The NHC intensity forecast is a near a blend of the HCCA, IVCN, and IVDR consensus models. FORECAST POSITIONS AND MAX WINDS INIT 29/1500Z 8.3N 24.6W 30 KT 35 MPH 12H 30/0000Z 8.8N 25.8W 35 KT 40 MPH 24H 30/1200Z 9.5N 27.6W 45 KT 50 MPH 36H 01/0000Z 10.3N 29.9W 55 KT 65 MPH 48H 01/1200Z 11.2N 31.9W 65 KT 75 MPH 60H 02/0000Z 12.4N 33.7W 70 KT 80 MPH 72H 02/1200Z 14.0N 35.4W 70 KT 80 MPH 96H 03/1200Z 18.4N 38.8W 55 KT 65 MPH 120H 04/1200Z 24.3N 40.9W 45 KT 50 MPH $$ Forecaster Cangialosi
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