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Tropical Storm IDA Forecast Discussion Number 14
2015-09-21 22:34:48| National Hurricane Center (Atlantic)
Issued at 500 PM AST MON SEP 21 2015 000 WTNT45 KNHC 212034 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 14 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 PM AST MON SEP 21 2015 Satellite data indicate that the low- and mid-level centers of Ida have become separated, with the low-level center exposed well to the northwest of the deep convection. This appears to be the result of some unanticipated mid-level shear that is occurring below the typical 200 mb outflow layer. The latest TAFB Dvorak classification and objective ADT estimates from UW/CIMSS support an initial intensity of 40-45 kt, and based on the earlier ASCAT data the advisory wind speed is held at 45 kt, although this could be generous. The forecast upper-level wind pattern over the next couple of days, which features continued mid-level shear along with an increase in upper-level northwesterly winds, should prevent strengthening. In fact, Ida could weaken during the next 2-3 days, and perhaps not survive as a tropical cyclone. The NHC forecast assumes that Ida will maintain tropical cyclone status, and that upper-level winds will become a little more conducive for strengthening by late in the period. The updated NHC intensity forecast is lower than the previous one, but still shows some increase in strength at 96 and 120 h. Ida continues to move north-northwestward, but recent visible satellite imagery suggest that the forward speed is decreasing. The track forecast reasoning remains unchanged from the previous advisory. The steering currents around the tropical storm are expected to weaken very soon, and Ida is expected to meander eastward or east-southeastward over the next couple of days. After that time, Ida should begin a northwestward or north-northwestward motion as a mid- to upper-level trough lifts out to the northeast. The track guidance has become quite divergent this cycle with very large spread between a faster and more eastward ECMWF solution, and a slower and more westward GFS track. For now, the official forecast is near the middle of the guidance envelope, closest to the GFS ensemble mean and FSU Superensemble. FORECAST POSITIONS AND MAX WINDS INIT 21/2100Z 21.2N 48.8W 45 KT 50 MPH 12H 22/0600Z 21.6N 48.7W 45 KT 50 MPH 24H 22/1800Z 21.6N 48.1W 45 KT 50 MPH 36H 23/0600Z 21.1N 47.3W 45 KT 50 MPH 48H 23/1800Z 20.8N 46.7W 45 KT 50 MPH 72H 24/1800Z 21.5N 46.8W 45 KT 50 MPH 96H 25/1800Z 23.0N 47.8W 50 KT 60 MPH 120H 26/1800Z 25.0N 48.5W 55 KT 65 MPH $$ Forecaster Brown
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