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Tropical Storm IDA Forecast Discussion Number 10
2015-09-20 22:45:43| National Hurricane Center (Atlantic)
Issued at 500 PM AST SUN SEP 20 2015 000 WTNT45 KNHC 202045 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 10 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 PM AST SUN SEP 20 2015 Deep convection associated with Ida has decreased in coverage and become somewhat less organized today. The tropical cyclone remains sheared with a small band of convection located well east of the exposed center. Although Dvorak satellite T-numbers have decreased this afternoon, the initial intensity remains 40 kt, which is based on the earlier ASCAT data. Ida is moving northwestward or 305 degrees at 15 kt, a little faster than this morning. Despite the recent increase in forward speed, Ida is expected to slow down during the next 24 hours, and become nearly stationary by Tuesday when a mid-level ridge builds to the north of the system. Ida is then forecast to meander for a couple of days, before the ridge slides eastward and allows the tropical cyclone to resume a northwestward motion. The track guidance is in good agreement during the first few days of the forecast, but again becomes quite divergent by the end of the forecast period. The ECMWF, which moves Ida more eastward during the next few days, is along the eastern edge of the guidance envelope. Meanwhile the UKMET and GFS show less of an eastern motion while the steering currents are weak, and take Ida more westward by day 5. The updated NHC track is between these scenarios and remains close to the multi-model consensus at 96 and 120 h. The westerly shear is still expected to decrease during the next day or so, but given the current organization of the system, Ida may not be able to take full advantage of the potentially more favorable upper-air pattern. Therefore, only a slight increase in intensity is predicted. After 48 hours, upper-level westerly winds associated with a trough are forecast to cause an increase in shear over the cyclone, which would likely cause weakening. The global models, however, generally deepen the cyclone between 72 and 120 h when Ida interacts with an upper-level trough. The official forecast shows little change in strength late in the period and lies between these differing possibilities. FORECAST POSITIONS AND MAX WINDS INIT 20/2100Z 18.4N 46.3W 40 KT 45 MPH 12H 21/0600Z 19.6N 47.5W 40 KT 45 MPH 24H 21/1800Z 20.9N 48.5W 45 KT 50 MPH 36H 22/0600Z 21.5N 48.7W 45 KT 50 MPH 48H 22/1800Z 21.5N 48.5W 45 KT 50 MPH 72H 23/1800Z 21.0N 48.2W 45 KT 50 MPH 96H 24/1800Z 20.9N 48.5W 45 KT 50 MPH 120H 25/1800Z 22.5N 49.5W 45 KT 50 MPH $$ Forecaster Brown
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