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Tropical Storm IDA Forecast Discussion Number 7
2015-09-20 04:54:42| National Hurricane Center (Atlantic)
Issued at 1100 PM AST SAT SEP 19 2015 000 WTNT45 KNHC 200254 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 7 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 PM AST SAT SEP 19 2015 Ida is badly sheared tropical cyclone. The center, a well-defined swirl of low clouds, remains well removed from the nearest deep convection to east. This cloud pattern is consistent with UW-CIMSS shear analyses indicating about 20 kt of westerly vertical shear over the cyclone. A partial ASCAT overpass at 0016 UTC showed believable 37-kt wind well east of the center, and the initial intensity estimate is kept at 35 kt. Global models show strong westerly shear persisting over Ida for the next day or so. With so much shear likely, little to no intensification is expected to occur. A respite in the shear is forecast by 36 hours when a piece of an upper-level trough pinches off near the Greater Antilles and drifts westward, which may allow Ida an opportunity to intensify some. The lower-shear environment should not last long, however, since a large mid- to upper-level trough is forecast to dig southwestward into the central Atlantic on top of Ida in 2 to 3 days, with the ECMWF model showing the potential for a complex trough interaction to take place. Assuming that Ida survives, the environment looks less than ideal, with confluent and strong northwesterly winds aloft and the possibility of enhanced oceanic upwelling underneath the cyclone. Weakening is therefore considered the most likely scenario, and it would not be surprising to see Ida become a remnant low during this time frame. The new intensity forecast is lower than the previous one, closest to the HWRF model. An alternate but less likely possibility is that Ida could restrengthen due to baroclinic forcing as shown in the ECMWF solution. Ida has been moving faster and more westerly because of its more shallow nature, and the initial motion estimate is 290/13. A subtropical ridge migrating westward with the cyclone should keep it on a general west-northwestward track for another day or so. Steering currents are expected to collapse in a couple of days in response to the large mid- to upper-level trough amplifying southwestward in the vicinity of Ida. The lack of steering should cause Ida to come to a halt in about 3 days, with the track forecast after that dependent on the depth of the storm. A deeper cyclone would move faster toward the north or northeast as shown in the ECMWF, while a weaker and shallower cyclone would move westward ad depicted in the GFS. The official forecast is between these two extremes but leans more toward the GFS, given the current intensity forecast, and shows a slow northerly motion consistent with the previous forecast. FORECAST POSITIONS AND MAX WINDS INIT 20/0300Z 15.8N 42.4W 35 KT 40 MPH 12H 20/1200Z 16.7N 44.1W 35 KT 40 MPH 24H 21/0000Z 17.8N 46.1W 40 KT 45 MPH 36H 21/1200Z 18.9N 47.6W 45 KT 50 MPH 48H 22/0000Z 19.8N 48.4W 50 KT 60 MPH 72H 23/0000Z 19.9N 48.5W 45 KT 50 MPH 96H 24/0000Z 19.9N 48.5W 40 KT 45 MPH 120H 25/0000Z 20.3N 48.4W 40 KT 45 MPH $$ Forecaster Kimberlain
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