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Tropical Storm IDA Forecast Discussion Number 19
2015-09-23 04:52:27| National Hurricane Center (Atlantic)
Issued at 1100 PM AST TUE SEP 22 2015 000 WTNT45 KNHC 230252 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 19 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 PM AST TUE SEP 22 2015 Last-light visible imagery, microwave imagery, and a timely ASCAT pass around 00Z showed that the center of Ida is located on the northeastern side of the larger gyre seen earlier today, just west of a new burst of deep convection. This structure is consistent with the 30 kt of westerly shear seen in UW-CIMSS satellite analyses. The initial intensity is set to 35 kt based on data from the ASCAT pass and in agreement with the latest Dvorak classification from TAFB. Little change in intensity is expected during the next 48 hours, with moderate to strong shear forecast to continue. By 72 hours the shear decreases a bit, which should allow for a little strengthening. The new NHC intensity forecast is close to a blend of the latest SHIPS, LGEM, and HWRF models. The initial position results in a relocation of the center to the northeast of the previous advisory with a slow east-southeastward drift as the initial motion. Ida is currently embedded in the base of a mid-latitude trough which should result in a continued slow east-southeastward motion during the next day or so followed by a gradual turn toward the northeast and then the north by 48 hours as the trough moves away. A slightly faster north-northwestward motion is forecast at days 3 through 5 as a mid-level ridge builds to the north of the tropical cyclone. During the latter portion of the forecast period much of the guidance has shifted westward, now showing a weaker Ida remaining south of the mid-latitude westerlies through day 5. The NHC track during this time has been nudged to the left of the previous one adjusted for the initial position. However, out of respect for continuity the NHC forecast remains close to the ECMWF on the eastern side of the guidance envelope and is well right of the consensus aids. Given the large shift in the guidance this cycle, confidence is quite low in the track forecast late in the period. FORECAST POSITIONS AND MAX WINDS INIT 23/0300Z 20.9N 46.8W 35 KT 40 MPH 12H 23/1200Z 20.7N 46.4W 35 KT 40 MPH 24H 24/0000Z 20.5N 45.8W 35 KT 40 MPH 36H 24/1200Z 21.0N 45.3W 35 KT 40 MPH 48H 25/0000Z 21.6N 45.1W 35 KT 40 MPH 72H 26/0000Z 23.0N 45.5W 40 KT 45 MPH 96H 27/0000Z 25.0N 46.5W 45 KT 50 MPH 120H 28/0000Z 26.0N 47.0W 50 KT 60 MPH $$ Forecaster Brennan
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