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| #803281 (Received by flhurricane at: 4:40 AM 19.Sep.2015) TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 AM AST SAT SEP 19 2015 The overall organization of Ida's cloud pattern continues to increase. Banding features are becoming more prominent in the southern part of the circulation, with a new band forming on the west side. In addition, a burst of convection has been ongoing during the past few hours near the apparent center. Satellite classifications still support an intensity of 35 kt, and this value is used as the initial wind speed. Satellite fixes suggest that the northwestward motion of Ida has sped up to 9 kt this morning. A mid-level ridge is expected to remain the dominant steering mechanism for the next couple of days, causing the storm to move to the west-northwest or northwest during that time. Models are in good agreement and only a slight northward adjustment was made to the official forecast then. Beyond 2 days, there is considerable uncertainty on how an amplifying trough over the east-central Atlantic will affect Ida. The cyclone seems likely to slow down due to steering currents collapsing, but there is poor agreement on whether the trough will pick up Ida, sending the storm east-northeastward, or leave the storm behind. For example, the normally reliable ECMWF and GFS models are going in opposite directions at day 5, with the ECMWF favoring a stronger trough solution while the GFS has Ida missing the trough and drifting westward. With so much uncertainty, I have elected to show a stationary Ida after day 3, preferring not to commit to either solution at this time. This forecast blends the previous forecast with the model consensus, although leans more heavily on the former. While the shear near Ida is low for now, most of the global models show an increase during the next day or so. This is expected to somewhat temper future strengthening, although since most of the other environmental factors are still favorable, a gradual intensification is forecast. The long-range intensity prediction, however, is very complex due to the potential interaction of the trough, which yields very different shear profiles, along with dry air aloft and possible upwelling of cooler water as Ida moves slowly. With the track more uncertain than average and these challenging factors, it seems prudent to remain conservative with the wind speed forecast. The official intensity forecast is close to the previous one, which ends up on the low side of the guidance at days 4/5. It almost goes without saying that these conditions are leading me to have a rather low confidence in the intensity forecast at 72 hours and beyond. FORECAST POSITIONS AND MAX WINDS INIT 19/0900Z 14.4N 38.5W 35 KT 40 MPH 12H 19/1800Z 15.3N 39.9W 40 KT 45 MPH 24H 20/0600Z 16.5N 42.2W 45 KT 50 MPH 36H 20/1800Z 17.6N 44.0W 50 KT 60 MPH 48H 21/0600Z 18.9N 45.6W 55 KT 65 MPH 72H 22/0600Z 20.5N 47.5W 60 KT 70 MPH 96H 23/0600Z 20.5N 47.5W 55 KT 65 MPH 120H 24/0600Z 20.5N 47.5W 55 KT 65 MPH $$ Forecaster Blake |
