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Tropical Depression TWELVE Forecast Discussion Number 4
2016-09-15 10:32:58| National Hurricane Center (Atlantic)
Issued at 500 AM AST THU SEP 15 2016 000 WTNT42 KNHC 150832 TCDAT2 TROPICAL DEPRESSION TWELVE DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL AL122016 500 AM AST THU SEP 15 2016 AMSR2 microwave data from a few hours ago indicated that the depression's center was about a degree west of the deep convection due to 20 kt of westerly shear. Since the cyclone's structure has not improved, and subjective Dvorak estimates have not changed from six hours ago, the initial intensity remains 30 kt. The westerly shear is expected to increase during the next 24 hours, and with a relatively drier air mass surrounding the cyclone, any strengthening during the next couple of days now appears less likely. After 48 hours, the shear is forecast to relax to some degree, but the global models disagree on exactly how much. Nonetheless, if the depression can survive the next couple of days, the environment should improve enough to allow for some strengthening by the end of the forecast period. The new NHC intensity forecast now holds the cyclone as a 30-kt depression for the next 3 days, followed by slight strengthening on days 4 and 5. This forecast is in best agreement with the statistical-dynamical SHIPS and LGEM models. The microwave data indicate that the depression has been moving a little faster toward the west at 275/14 kt. The Bermuda-Azores high is expected to steer the depression generally westward across the tropical Atlantic during the entire 5-day forecast period. A south-of-due-west motion is even possible on days 2 and 3, especially if the cyclone remains weak. With the exception of the GFDL, which shows more strengthening and is an outlier way to the north of the other models, the guidance envelope is relatively tight. To account for the apparent north bias of the GFDL, the updated NHC track forecast is a bit south of the TVCN multi-model consensus. The new forecast is also a little faster than and south of the previous forecast, especially on days 4 and 5. FORECAST POSITIONS AND MAX WINDS INIT 15/0900Z 17.6N 29.3W 30 KT 35 MPH 12H 15/1800Z 17.8N 31.2W 30 KT 35 MPH 24H 16/0600Z 18.0N 33.7W 30 KT 35 MPH 36H 16/1800Z 17.9N 36.4W 30 KT 35 MPH 48H 17/0600Z 17.6N 39.0W 30 KT 35 MPH 72H 18/0600Z 16.7N 43.8W 30 KT 35 MPH 96H 19/0600Z 16.5N 49.0W 35 KT 40 MPH 120H 20/0600Z 17.0N 53.5W 40 KT 45 MPH $$ Forecaster Berg
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