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| #803417 (Received by flhurricane at: 4:46 AM 24.Sep.2015) 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 |
