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| #803331 (Received by flhurricane at: 10:49 PM 20.Sep.2015) TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 11 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 PM AST SUN SEP 20 2015 Ida is showing new signs of life. A new burst of intense convection has developed over and east of the center, suggesting that westerly shear over the cyclone could be decreasing. Although Ida's cloud pattern has recently become better organized, an ASCAT pass from 2358 UTC indicates that there has been no increase in winds, with peak uncontaminated winds of 37 kt. Thus the initial wind speed is held at 40 kt. A large upper-level low northeast of the Lesser Antilles is moving away from Ida, resulting in a gradual relaxation of the westerly shear over the cyclone while the surrounding flow gradually becomes more diffluent. All other factors being equal, this change in the upper-air pattern should favor some intensification during the next 24 hours or so, perhaps more than forecast. After 36 hours, a mid- to upper-level trough amplifying southwestward from the eastern Atlantic should induce an inhospitable environment of strong northwesterly shear, drier mid-tropospheric air and confluence over Ida. The storm's slow movement by that time could also cause enhanced oceanic upwelling and locally cooler waters underneath the cyclone. It is hard to imagine that these conditions would not induce weakening, yet global models show Ida re-strengthening from days 3 to 5 likely, probably due to a baroclinic forcing arising from the storm's interaction with the trough. The new intensity forecast shows more intensification in the short term, and is above all of the intensity guidance. Given the larger than normal uncertainty in the extended range, the intensity is essentially flatlined not far from the multi-model consensus. Recent fixes suggest that Ida's forward speed is decreasing, and the initial motion estimate is 310/12. Ida is quickly approaching a weakness in the low- to mid-level subtropical ridge along 50W, which should result in the cyclone's forward speed decelerating during the next 12 to 24 hours. Steering currents should collapse in about 36 hours in response to the eastern Atlantic trough deepening southwestward, and Ida should suddenly come to a halt and then meander with some eastward component of motion for a few days. As the effects of the trough diminish by day 4, a majority of the track guidance shows Ida moving faster toward the north or north- northwest between two mid-level ridges. The GFS is left of nearly all the guidance, however, showing Ida encountering a blocking ridge that would cause a more westerly track. The details of the track forecast between days 3 to 5 are still in greater than normal doubt, and thus the track is hardly adjusted through 72 hours, but is shifted toward the north and east after that time toward the multi-model consensus without the GFS. FORECAST POSITIONS AND MAX WINDS INIT 21/0300Z 18.9N 47.0W 40 KT 45 MPH 12H 21/1200Z 20.2N 48.0W 45 KT 50 MPH 24H 22/0000Z 21.0N 48.4W 50 KT 60 MPH 36H 22/1200Z 21.4N 48.3W 50 KT 60 MPH 48H 23/0000Z 21.2N 48.0W 50 KT 60 MPH 72H 24/0000Z 20.6N 47.5W 45 KT 50 MPH 96H 25/0000Z 21.3N 47.4W 45 KT 50 MPH 120H 26/0000Z 23.2N 48.6W 50 KT 60 MPH $$ Forecaster Kimberlain |
