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Tropical Storm IDA Forecast Discussion Number 15
2015-09-22 04:50:21| National Hurricane Center (Atlantic)
Issued at 1100 PM AST MON SEP 21 2015 000 WTNT45 KNHC 220250 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 15 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 PM AST MON SEP 21 2015 Ida's cloud pattern has become increasingly disorganized. Satellite data indicate that the large separation between the cyclone's low- to mid-level centers persists as a result of a strong west- northwesterly shear. The nearest deep convection, coincident with the mid-level center, remains well removed to the southeast. The initial wind speed is conservatively lowered to 40 kt and is above the latest Dvorak intensity estimates. The tail of a large mid- to upper-level trough over the eastern Atlantic is forecast to sweep southward over Ida during the next 24 hours. Even stronger deep-layer northwesterly shear and confluence aloft associated with this feature should adversely affect the storm during the next few days, and cause weakening. One could envision Ida not surviving such a greatly inhospitable environment. However, global models depict Ida remaining a coherent feature and eventually re-strengthening, though Ida's forecast intensification occurs in different ways. The ECMWF has consistently shown Ida re- strengthening, seemingly as a consequence of baroclinic forcing from the trough interaction. Such an interaction could potentially result in Ida's taking on the characteristics of a hybrid cyclone. The GFS shows Ida becoming disentangled from the trough, and re-intensifying after the trough lifts out. Regardless of which scenario occurs, any re-strengthening of Ida as a tropical cyclone would likely be tempered by substantially drier air on the backside of the trough being entrained by the cyclone. The new intensity forecast is lower than the previous one in the short term due to the overall harsh environment affecting the storm. A modest increase in strength is shown at the end of the forecast, similar to the previous forecast. The definition of Ida's center has been deteriorating, and it has thus become harder to track. It appears though the the cyclone's forward speed has been decreasing, and the initial motion estimate is an uncertain 340/04. The deep layer west-northwesterly to northwesterly flow, associated with the trough overtaking Ida, could impart a slow eastward or east-southeastward motion for a couple of days. After that, the bulk of the track guidance shows Ida caught between two mid-level ridges, which should induce a north- northwestward and then northward motion at a gradually increasing forward speed. Only the GFS shows Ida encountering a blocking ridge with a track much farther to the west, a solution very different than its ensemble mean. The new track has been adjusted somewhat to the east overall through 72 hours and is a little faster than the previous one, in good agreement with a model consensus without the GFS. FORECAST POSITIONS AND MAX WINDS INIT 22/0300Z 21.7N 49.5W 40 KT 45 MPH 12H 22/1200Z 21.9N 49.3W 40 KT 45 MPH 24H 23/0000Z 21.5N 48.4W 35 KT 40 MPH 36H 23/1200Z 20.9N 47.5W 35 KT 40 MPH 48H 24/0000Z 20.9N 46.9W 35 KT 40 MPH 72H 25/0000Z 22.3N 47.1W 40 KT 45 MPH 96H 26/0000Z 24.4N 48.2W 45 KT 50 MPH 120H 27/0000Z 26.2N 48.9W 50 KT 60 MPH $$ Forecaster Kimberlain
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