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Tropical Storm IDA Forecast Discussion Number 17
2015-09-22 16:33:26| National Hurricane Center (Atlantic)
Issued at 1100 AM AST TUE SEP 22 2015 000 WTNT45 KNHC 221433 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 17 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 AM AST TUE SEP 22 2015 Ida continues to be a sheared cyclone with the low-level center located to the northwest of an area of very deep convection. The upper-level outflow in the southern half of the cyclone has become better defined since yesterday, but it is not existent elsewhere. Dvorak numbers are decreasing and do not support 40 kt at this time. However, I would wait for scatterometer or any other data, if available, to reduce the winds, if necessary. The cyclone is embedded within an uncommon flow pattern. It is located at the southern end of a mid-level trough, which is forcing the cyclone to move east-southeastward at about 7 kt, and also causing shear. The evolution of this trough will be crucial for the future of Ida. Unanimously, global models keep the cyclone drifting generally eastward for the next 2 days or so, while embedded within the trough. After that time, all models forecast the trough to lift out, leaving Ida south of an amplifying ridge. This forecast pattern should result in a decrease of the shear with favorable conditions for strengthening, and a slow motion of the cyclone toward the north-northwest. It is interesting to note the current agreement of all global models with this scenario, and at long range, the GFS and ECMWF primarily forecast a significant strengthening of Ida. Given the uncommon pattern and the currently hostile environment, the NHC forecast keeps the cyclone with the same intensity for the next day or two, and allows for some intensification once Ida becomes detached from the trough. The NHC track forecast is in the middle of the guidance envelope, and follows the trend of the ECMWF and GFS consensus. FORECAST POSITIONS AND MAX WINDS INIT 22/1500Z 20.5N 47.5W 40 KT 45 MPH 12H 23/0000Z 20.5N 47.0W 40 KT 45 MPH 24H 23/1200Z 20.5N 46.3W 40 KT 45 MPH 36H 24/0000Z 20.0N 45.5W 40 KT 45 MPH 48H 24/1200Z 20.5N 45.0W 40 KT 45 MPH 72H 25/1200Z 22.0N 45.5W 45 KT 50 MPH 96H 26/1200Z 23.5N 46.5W 50 KT 60 MPH 120H 27/1200Z 25.5N 47.0W 55 KT 65 MPH $$ Forecaster Avila
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