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Hurricane Irma Forecast Discussion Number 31
2017-09-06 22:57:15| National Hurricane Center (Atlantic)
Issued at 500 PM AST Wed Sep 06 2017 000 WTNT41 KNHC 062057 TCDAT1 Hurricane Irma Discussion Number 31 NWS National Hurricane Center Miami FL AL112017 500 PM AST Wed Sep 06 2017 The eye of Irma passed over the northernmost Virgin Islands earlier this afternoon and it is now located just to their northwest. The satellite and radar presentation of the hurricane remains extremely impressive. An Air Force Reserve reconnaissance aircraft reported peak SFMR winds of 156 kt and flight-level winds of 164 kt during its mission this afternoon. Although there have been no SFMR or flight-level winds as high as what was observed yesterday, the initial intensity remains 160 kt, due to the potential of undersampling. Dropsonde observations in the eye indicated that the pressure rose a few millibars this morning, but the most recent aircraft report shows that the pressure has fallen to 914 mb. The hurricane remains on a west-northwestward motion at about 14 kt. A high pressure ridge over the western Atlantic is forecast to keep Irma on a west-northwestward course over the next 48 to 72 hours. The track guidance is still in good agreement during that period, and little change to the NHC forecast was required. By the weekend, a shortwave trough diving southward over the east-central United States is expected to cause Irma to turn northwestward and northward. The 12Z guidance has generally shifted slightly westward, closer to the previous NHC forecast. As a result, little overall change was made to the 4-5 day track forecast. The NHC track is once again close to the HFIP corrected consensus model. This is also near the 12Z GEFS ensemble mean, but a little east of the latest ECMWF ensemble mean. Users are reminded that the average NHC track errors at days 4 and 5 are about 175 and 225 miles, respectively. Low vertical wind shear and warm waters along the forecast track of Irma should allow it to remain a very powerful hurricane during the next several days, and the intensity forecast is again near the upper-end of the guidance and is the same as the previous advisory through 96 hours. Increasingly southwesterly shear and potential land interaction late in the period is expected to cause some decrease in Irma's strength by day 5. Efforts to provide the forecast models with as much data as possible continue, with 6-hourly NWS balloon launches across much of the continental United States, and the NOAA G-IV aircraft currently sampling the environment around the storm. KEY MESSAGES: 1. Irma is a potentially catastrophic category 5 hurricane and will bring life-threatening wind, storm surge, and rainfall hazards to Puerto Rico tonight, the northern coast of Hispaniola Thursday, and the Turks and Caicos and southeastern and central Bahamas Thursday and Friday. 2. Hurricane watches are in effect for the northwestern Bahamas and much of Cuba. Irma is likely to bring dangerous wind, storm surge, and rainfall to portions of these areas on Friday and Saturday. 3. The threat of direct hurricane impacts in Florida over the weekend and early next week has increased. Hurricane watches could be issued for portions of the Florida Keys and the Florida peninsula on Thursday. FORECAST POSITIONS AND MAX WINDS INIT 06/2100Z 18.8N 65.4W 160 KT 185 MPH 12H 07/0600Z 19.6N 67.6W 155 KT 180 MPH 24H 07/1800Z 20.7N 70.4W 150 KT 175 MPH 36H 08/0600Z 21.5N 73.1W 145 KT 165 MPH 48H 08/1800Z 22.2N 75.6W 140 KT 160 MPH 72H 09/1800Z 23.2N 79.0W 135 KT 155 MPH 96H 10/1800Z 26.0N 80.2W 125 KT 145 MPH 120H 11/1800Z 31.0N 81.0W 105 KT 120 MPH $$ Forecaster Brown
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