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Tropical Storm IDA Forecast Discussion Number 5
2015-09-19 16:34:34| National Hurricane Center (Atlantic)
Issued at 1100 AM AST SAT SEP 19 2015 000 WTNT45 KNHC 191434 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 AM AST SAT SEP 19 2015 As was noted yesterday, Ida will have an obstacle course to navigate during the next several days, and the storm already appears to have encountered its first hurdle. Westerly shear, on the order of 20 kt according to UW-CIMSS analyses, has begun to affect the cyclone, and the low-level center is now exposed to the northwest of the deep convection. Dvorak CI numbers remain 2.5, and the initial intensity therefore remains 35 kt. The SHIPS guidance indicates that the vertical shear should be steady or even increase slightly during the next 24 hours, so only slow strengthening is anticipated during that time. The best window for more significant intensification appears to be between 36 and 72 hours when the shear is forecast to decrease. Another round of increased shear on days 4 and 5 should limit strengthening or induce weakening. The intensity models are split on Ida's future intensity, with the SHIPS and LGEM models only modestly strengthening the cyclone through day 5 while the GFDL and HWRF models bring Ida to hurricane strength by day 3. Due to the uncertainty of how Ida will interact with the complex environment around it, the NHC intensity forecast is a bit below the intensity consensus and is very similar to the previous forecast. Low- to mid-level ridging to the north and northeast of Ida is inducing a slightly faster west-northwestward motion, or 295/10 kt. The ridge is expected to weaken within the next 48 hours, leaving Ida between two deep-layer lows, one located over the eastern Atlantic and the other northeast of the Leeward Islands. This should cause Ida to become nearly stationary or meander by days 3 through 5. With the exception of the UKMET model, the other track models seem to be trending toward a solution in which the eastern Atlantic trough has at least some influence on Ida's motion, and the updated NHC track forecast now shows a slow eastward drift on days 4 and 5. Overall though, the NHC track forecast is very similar to the previous one and is close to the model consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 19/1500Z 14.8N 39.7W 35 KT 40 MPH 12H 20/0000Z 15.7N 41.4W 40 KT 45 MPH 24H 20/1200Z 16.9N 43.4W 45 KT 50 MPH 36H 21/0000Z 18.2N 45.1W 50 KT 60 MPH 48H 21/1200Z 19.3N 46.5W 55 KT 65 MPH 72H 22/1200Z 20.5N 47.5W 60 KT 70 MPH 96H 23/1200Z 20.5N 47.0W 60 KT 70 MPH 120H 24/1200Z 20.5N 46.5W 55 KT 65 MPH $$ Forecaster Berg
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