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Tropical Storm IDA Forecast Discussion Number 9
2015-09-20 16:52:41| National Hurricane Center (Atlantic)
Issued at 1100 AM AST SUN SEP 20 2015 000 WTNT45 KNHC 201452 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 9 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 AM AST SUN SEP 20 2015 The low-level center of Ida remains exposed to the west of the deep convection, due to moderate westerly shear that has been affecting the tropical cyclone during the past couple of days. A recent ASCAT overpass revealed winds of 35 to 40 kt over the eastern portion of the circulation so the initial intensity has been increased to 40 kt. The ASCAT data also indicate that the area of tropical-storm-force winds is larger than previous estimated, and the wind radii have been adjusted accordingly. Ida continues to move west-northwestward or 300 degrees at 13 kt. The forward motion of the tropical cyclone is forecast to slow down during the next day or two, as a blocking ridge strengthens to the north of the system over the central Atlantic. All of the dynamical models show Ida becoming stationary or meandering over the central Atlantic between 48 and 96 hours. The NHC forecast follows this scenario and shows the tropical cyclone stationary for a couple of days. Late in the forecast period, the ridge is expected to weaken, which should allow a northwestward or northward motion to begin, however, the track guidance is quite divergent at that time. Given the large spread in the guidance late in the period, the NHC forecast remains near the multi-model consensus at days 4 and 5. The westerly shear that has been affecting Ida is expected to decrease during the next day or so. This should allow for some strengthening, and the NHC forecast is near the IVCN consensus model and the previous advisory. After 48 hours, the global models are suggesting that an upper-level trough to the north of Ida may cause an increase in westerly shear. As a result, the updated official forecast shows no change in intensity late in the period. FORECAST POSITIONS AND MAX WINDS INIT 20/1500Z 17.3N 44.8W 40 KT 45 MPH 12H 21/0000Z 18.2N 46.3W 40 KT 45 MPH 24H 21/1200Z 19.5N 47.7W 45 KT 50 MPH 36H 22/0000Z 20.4N 48.5W 50 KT 60 MPH 48H 22/1200Z 20.8N 48.7W 50 KT 60 MPH 72H 23/1200Z 20.8N 48.7W 50 KT 60 MPH 96H 24/1200Z 20.8N 48.7W 50 KT 60 MPH 120H 25/1200Z 21.2N 49.0W 50 KT 60 MPH $$ Forecaster Brown
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