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Tropical Storm IDA Forecast Discussion Number 13
2015-09-21 16:39:20| National Hurricane Center (Atlantic)
Issued at 1100 AM AST MON SEP 21 2015 000 WTNT45 KNHC 211439 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 13 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 AM AST MON SEP 21 2015 Ida has been producing a large cluster of very deep convection overnight and this morning. The tropical cyclone, however, is still being affected by shear as the low-level center can be seen in visible satellite imagery just northwest of the convection, and this was confirmed by a recent scatterometer pass. The initial wind speed is maintained at 45 kt, which is based on the latest TAFB Dvorak T-number and the recent ASCAT data that revealed winds of around 40 kt. The upper-level wind pattern is forecast to become a little more conducive for strengthening during the next 12 to 24 hours and the NHC forecast calls for some slight intensification. After that time, northwesterly shear is expected to increase as an upper-level trough to the northeast of Ida retrogrades westward. This could cause some weakening, but most of the intensity guidance shows little change in strength during this time, and the NHC forecast maintains an intensity of 55 kt from 24 to 72 hours. Later in the forecast period, the global models take the upper-level trough northeastward and develop a more favorable upper-air pattern over the tropical cyclone, which should allow for strengthening by days 4 and 5. The new NHC intensity forecast is similar to the previous advisory, and is close to the SHIPS and LGEM models. The forward motion of Ida continues to decrease and is now 330/6 kt. The steering currents around the tropical cyclone are forecast to further weaken during the next day or so as the mid- to upper-level trough retrogrades westward. Ida should become nearly stationary tonight, then meander eastward or east- southeastward on Tuesday and Wednesday as the mid- to upper-level flow becomes northwestward. The dynamical models have come into better agreement on this scenario, including the latest GFS run that shifted eastward and is now close to the ECMWF. After 72 hours, when the trough moves northeastward, Ida should turn northwestward at a faster forward speed. The updated NHC track has been adjusted eastward and is close to a blend of the ECMWF, GFS, and GFS ensemble mean. FORECAST POSITIONS AND MAX WINDS INIT 21/1500Z 20.6N 48.0W 45 KT 50 MPH 12H 22/0000Z 21.1N 48.1W 50 KT 60 MPH 24H 22/1200Z 21.3N 47.7W 55 KT 65 MPH 36H 23/0000Z 21.2N 47.3W 55 KT 65 MPH 48H 23/1200Z 20.7N 46.7W 55 KT 65 MPH 72H 24/1200Z 20.8N 46.1W 55 KT 65 MPH 96H 25/1200Z 22.0N 46.8W 60 KT 70 MPH 120H 26/1200Z 24.0N 48.0W 65 KT 75 MPH $$ Forecaster Brown
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