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Tropical Storm IDA Forecast Discussion Number 24
2015-09-24 10:43:36| National Hurricane Center (Atlantic)
Issued at 500 AM AST THU SEP 24 2015 000 WTNT45 KNHC 240843 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 24 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 AM AST THU SEP 24 2015 Ida's low-level center is exposed to the west of a relatively small cluster of deep convection due to 25 kt of west-northwesterly shear. By themselves, satellite intensity estimates would no longer support Ida's status as a tropical storm, but the partial ASCAT passes from last evening had just enough wind that I'm hesitant to downgrade the system to a tropical depression just yet. Vertical shear is expected to decrease only slightly during the next couple of days, and along with abundant mid-level dry air, the environment will not be particularly favorable for strengthening. The GFDL, which brings Ida to hurricane strength in 4 days, remains an outlier and is discounted as a reasonable solution. Conversely, the ECMWF and GFS global models show Ida weakening as a result of the unfavorable environment, with the GFS even showing the cyclone degenerating to a trough by day 4 or 5. Because of these more believable scenarios, the NHC intensity forecast is a little lower than the previous one at the end of the forecast period, and overall is very close to the Florida State Superensemble. Ida had been moving east-southeastward during the past 12 hours, but the current motion estimate is 090/5 kt. Ida remains located within the base of a mid-tropospheric trough, but it should become detached from the trough during the next 24 hours and turn northward as low- to mid-level ridging develops over the eastern Atlantic. Now that the global models have been trending toward a weaker system by days 3 through 5, they show Ida becoming blocked by a surface high over the north Atlantic and moving westward at a faster forward speed by the end of the forecast period. The updated NHC track forecast is a little faster than and southwest of the previous forecast at days 4 and 5 to account for the model trends, but it is otherwise very similar before those times. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 19.6N 45.5W 35 KT 40 MPH 12H 24/1800Z 20.2N 45.4W 35 KT 40 MPH 24H 25/0600Z 20.8N 45.4W 35 KT 40 MPH 36H 25/1800Z 21.6N 45.8W 35 KT 40 MPH 48H 26/0600Z 22.5N 46.5W 35 KT 40 MPH 72H 27/0600Z 23.8N 48.2W 40 KT 45 MPH 96H 28/0600Z 23.5N 50.0W 40 KT 45 MPH 120H 29/0600Z 23.0N 52.5W 40 KT 45 MPH $$ Forecaster Berg
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