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Hurricane Irma Forecast Discussion Number 16
2017-09-03 10:49:19| National Hurricane Center (Atlantic)
Issued at 500 AM AST Sun Sep 03 2017 000 WTNT41 KNHC 030849 TCDAT1 Hurricane Irma Discussion Number 16 NWS National Hurricane Center Miami FL AL112017 500 AM AST Sun Sep 03 2017 Irma's convective pattern has improved slightly overnight. Very cold cloud tops completely encircle the eye, which has warmed and become a bit clearer in infrared satellite images. The initial intensity is raised to 100 kt based on a blend of the latest subjective and objective Dvorak estimates. Irma has been losing latitude since yesterday due to strong high pressure to its north, and the initial motion is now west- southwestward, or 255/13 kt. The hurricane is likely to continue moving on this trajectory for the next 36 hours, after which time it should gradually turn toward the west and then west-northwest on days 3-5 when it reaches the western extent of the ridge. The new NHC track forecast is very similar to the previous one during the first 48 hours, showing Irma bottoming out around 16.5N. However, the track guidance has shifted westward after 48 hours, delaying a turn toward the west-northwest, and this required a corresponding westward shift in the official forecast toward the multi-model consensus at the end of the forecast period. It should be noted that the official forecast still lies to the east of some of the better-performing models, such as the ECMWF, HWRF, and HCCA, so additional adjustment are possible in subsequent advisories. The environment ahead of Irma appears conducive for gradual strengthening for at least the next 2 to 3 days, with increasing sea surface temperatures and a moistening in the mid-levels of the atmosphere. We may still observe fluctuations in intensity, but overall, the model guidance seems to suggest a general upward trend with a peak in intensity possibly occurring around day 3. This type of intensification would coincide with the timing of Irma's west-southwest to westward motion, a pattern which we have observed in other west-southwestward-moving hurricanes in the past (i.e., Katrina, Joaquin, Fernanda, etc.). The NHC intensity forecast is bumped up slightly, showing a peak in intensity on day 3, and is largely a blend of the ICON intensity consensus and HCCA. While Irma is currently a small hurricane, the size guidance suggests it should grow in size during the next 72 h. This will affect how soon watches may be issued for portions of the Leeward and Virgin Islands, and interests on those islands should continue to monitor Irma's progress. KEY MESSAGES: 1. Irma is expected to be a major hurricane when it moves closer to the Lesser Antilles over the next few days, producing rough surf and rip currents. Irma could also cause dangerous wind, storm surge, and rainfall impacts on some islands, although it is too soon to specify where and when those hazards could occur. Residents in the Lesser Antilles should monitor the progress of Irma through the weekend and listen to any advice given by local officials. 2. It is much too early to determine what direct impacts Irma will have on the Bahamas and 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 03/0900Z 18.0N 47.5W 100 KT 115 MPH 12H 03/1800Z 17.5N 49.1W 100 KT 115 MPH 24H 04/0600Z 16.8N 51.3W 105 KT 120 MPH 36H 04/1800Z 16.5N 53.4W 110 KT 125 MPH 48H 05/0600Z 16.5N 55.7W 115 KT 130 MPH 72H 06/0600Z 17.8N 60.6W 120 KT 140 MPH 96H 07/0600Z 20.0N 66.0W 115 KT 130 MPH 120H 08/0600Z 22.5N 71.5W 115 KT 130 MPH $$ Forecaster Berg
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