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Hurricane Irma Forecast Discussion Number 20
2017-09-04 10:54:56| National Hurricane Center (Atlantic)
Issued at 500 AM AST Mon Sep 04 2017 000 WTNT41 KNHC 040854 TCDAT1 Hurricane Irma Discussion Number 20 NWS National Hurricane Center Miami FL AL112017 500 AM AST Mon Sep 04 2017 Irma's signature in infrared satellite images has been improving over the past few hours. An eye has appeared and is warming, and the central dense overcast has become more symmetric. Although the satellite presentation is better than when a NOAA plane investigated Irma last evening, the initial intensity will be held at 100 kt for now. Another NOAA P3 aircraft is just now beginning to sample the hurricane and should provide an updated intensity estimate during the next few hours. A strong, stationary mid-tropospheric high centered over the central Atlantic is forcing Irma on a west-southwestward course, and the initial motion estimate remains 255/12 kt. Irma will begin rounding the southwestern edge of the high soon, which will allow the hurricane to turn westward later today and then west- northwestward in 36-48 hours. Down the road, a large mid-latitude trough is expected to dig southward over the eastern United States during the next 72 hours, but the global models have been trending toward quickly lifting the trough out over New England and eastern Canada on days 4 and 5, with the subtropical ridge building westward toward Florida. As a result, it's becoming increasingly likely that Irma would maintain a west-northwestward heading on days 3 through 5, and the track guidance shifted significantly westward on this cycle during that period. Remarkably, the track models are very tightly clustered through day 5, which increases the confidence in the westward shift of the latest NHC forecast. All environmental factors suggest that Irma will at least maintain its intensity for the entire 5-day forecast period, if not strengthen gradually. Oceanic heat content values increase along Irma's forecast path, mid-level moisture will be more abundant, and vertical shear appears to be generally low. As a result, the NHC intensity forecast continues to call for gradual intensification through at least 72 hours, with a possibility for some slight weakening by days 4 and 5 (but still as a major hurricane). As with any major hurricane, Irma's intensity may fluctuate around these forecast intensities, but the overall trend is for a gradual increase of the maximum winds, assuming Irma's core does not move over any of the islands in the Greater Antilles. Users are reminded to not focus on the exact forecast track since strong winds and heavy rainfall extend well away from the center. In addition, average NHC track errors are about 175 and 225 statute miles at days 4 and 5, respectively. KEY MESSAGES: 1. Irma is expected to affect the northeastern Leeward Islands within a couple of days as a major hurricane, accompanied by dangerous wind, storm surge, and rainfall impacts, along with rough surf and rip currents. Hurricane watches are in effect for portions of the Leeward Islands, and additional hurricane or tropical storm watches or warnings will likely be issued later today. Residents in these areas should monitor the progress of Irma and listen to advice given by officials. 2. Irma is expected to remain a dangerous major hurricane through the upcoming week and could directly affect the British and U.S. Virgin Islands, Puerto Rico, Hispaniola, the Turks and Caicos, the Bahamas, and Cuba. Residents in all of these areas should monitor the progress of Irma and listen to advice given by officials. Tropical storm or hurricane watches will likely be issued for the British and U.S. Virgin Islands and Puerto Rico later today. 3. It is too early to determine what direct impacts Irma might have on the continental United States. Regardless, everyone in hurricane-prone areas should ensure that they have their hurricane plan in place, as we are now near the peak of the season. FORECAST POSITIONS AND MAX WINDS INIT 04/0900Z 16.9N 52.3W 100 KT 115 MPH 12H 04/1800Z 16.7N 53.9W 105 KT 120 MPH 24H 05/0600Z 16.6N 56.2W 110 KT 125 MPH 36H 05/1800Z 17.0N 58.6W 115 KT 130 MPH 48H 06/0600Z 17.8N 61.2W 120 KT 140 MPH 72H 07/0600Z 19.6N 66.8W 120 KT 140 MPH 96H 08/0600Z 21.0N 72.0W 115 KT 130 MPH 120H 09/0600Z 22.0N 76.5W 110 KT 125 MPH $$ Forecaster Berg
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