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Tropical Storm IDA Forecast Discussion Number 3
2015-09-19 04:54:02| National Hurricane Center (Atlantic)
Issued at 1100 PM AST FRI SEP 18 2015 000 WTNT45 KNHC 190253 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 3 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 PM AST FRI SEP 18 2015 The depression's cloud pattern has been increasing in organization. A primarily convective band over the eastern half of the circulation has been increasing in curvature and the cloud tops are cooling significantly. The initial wind speed is increased to 35 kt in agreement with the latest CI numbers from TAFB and SAB and UW-CIMSS ADT value. Westerly shear associated with an upper-level trough, extending from the eastern subtropical Atlantic into the eastern Caribbean, could increase over Ida during the next day or so and slow the cyclone's rate of intensification. Global models agree that a relative reduction in the shear should occur in 2 to 3 days when a piece of the upper-level trough cuts off near the Greater Antilles, which could allow Ida to strengthen, perhaps even more than forecast. Late in the forecast period, the uncertainty in the intensity forecast increases considerably, with the ECMWF calling for more northwesterly shear and less favorable thermodynamic variables in the storm's environment than in the GFS. The new intensity forecast is only slightly higher than the previous one through 72 hours. The forecast after that time is of low confidence and essentially levels Ida's intensity off in agreement with the SHIPS model output. Recent fixes suggest that the cyclone's motion has been more northwesterly (or 305/06) since earlier today. Ida is being steered by east-southeasterly to southeasterly flow underneath a subtropical ridge migrating westward with the cyclone, and some increase in forward speed is predicted by the models during the next day or so. In about 48 hours, a deep-layer trough amplifying southwestward across the eastern Atlantic will cause steering currents surrounding Ida to collapse, which should result in the cyclone's forward motion coming to a halt. With increasing northerly or northwesterly flow aloft, Ida should begin to meander in about 4 days and even drift southward or southeastward by day 5. The new track forecast is adjusted to the right some throughout the period, primarily due to the more northwesterly initial motion but also in the direction of the multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 19/0300Z 13.7N 37.5W 35 KT 40 MPH 12H 19/1200Z 14.7N 38.9W 40 KT 45 MPH 24H 20/0000Z 15.8N 40.9W 45 KT 50 MPH 36H 20/1200Z 16.9N 43.2W 50 KT 60 MPH 48H 21/0000Z 18.1N 45.0W 50 KT 60 MPH 72H 22/0000Z 20.1N 47.4W 60 KT 70 MPH 96H 23/0000Z 20.7N 48.0W 55 KT 65 MPH 120H 24/0000Z 20.3N 47.8W 55 KT 65 MPH $$ Forecaster Kimberlain
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