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Tropical Storm IDA Forecast Discussion Number 20
2015-09-23 10:45:59| National Hurricane Center (Atlantic)
Issued at 500 AM AST WED SEP 23 2015 000 WTNT45 KNHC 230845 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 20 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 AM AST WED SEP 23 2015 Shortwave infrared satellite imagery and a 0413 UTC GCOM microwave pass suggest that multiple low-level swirls are revolving around Ida's center, which has been fixed to the west-northwest of an ongoing burst of deep convection. This pattern is the consequence of 30 kt of west-northwesterly shear, as analyzed by UW-CIMSS. Maximum winds remain 35 kt based on earlier ASCAT data, and it appears that Ida's intensity probably won't change much during the next couple of days. Vertical shear is forecast to gradually weaken, but it likely won't fall to less than 15 kt for at least another 48 hours. Some strengthening is possible beginning on day 3 once the shear has decreased. With the exception of the GFDL, the intensity models are in close agreement through day 4, with some more spread by day 5. Based on this latest guidance, no changes were made to the previous NHC intensity forecast for this advisory. Although Ida's exact center is difficult to locate, the entire circulation has seemingly been drifting southward since the previous advisory. This is probably for good reason, as Ida is embedded within northerly flow near the base of a mid-tropospheric trough which stretches across the eastern Atlantic. The estimated initial motion is 180/2 kt, but Ida will likely gain an eastward component soon while being tugged by the eastern Atlantic trough. The subtropical ridge is expected build westward over the eastern Atlantic by 48 hours, which should impart a slightly faster northward motion through the end of the forecast period. The spread in the 00 UTC track guidance decreased on this forecast cycle compared to earlier runs, with the UKMET model being the only obvious outlier. Given the tight clustering, the NHC official track forecast was again adjusted westward, mainly on days 3-5, and lies very close to the various multi-model consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 23/0900Z 20.4N 47.2W 35 KT 40 MPH 12H 23/1800Z 20.4N 46.3W 35 KT 40 MPH 24H 24/0600Z 20.5N 45.8W 35 KT 40 MPH 36H 24/1800Z 21.0N 45.7W 35 KT 40 MPH 48H 25/0600Z 21.7N 45.9W 35 KT 40 MPH 72H 26/0600Z 23.0N 47.0W 40 KT 45 MPH 96H 27/0600Z 25.0N 47.5W 45 KT 50 MPH 120H 28/0600Z 26.0N 47.0W 50 KT 60 MPH $$ Forecaster Berg
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