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Hurricane Irma Forecast Discussion Number 22
2017-09-04 22:38:02| National Hurricane Center (Atlantic)
Issued at 500 PM AST Mon Sep 04 2017 000 WTNT41 KNHC 042037 TCDAT1 Hurricane Irma Discussion Number 22 NWS National Hurricane Center Miami FL AL112017 500 PM AST Mon Sep 04 2017 Irma remains an impressive hurricane in satellite imagery. The eye has become a little smaller and cloud filled this afternoon, perhaps the result of an ongoing eyewall replacement. An Air Force reserve reconnaissance aircraft reported a double-eyewall structure and double wind maximums during the first pass through Irma but noted that the eyewalls had consolidated somewhat during their second pass through the center. The aircraft measured flight-level wind of 121 kt in the northeast eyewall and SFMR winds of 113 kt. Based on these reports, the peak intensity has been increased to 115 kt, making Irma a category four hurricane. The hurricane will be moving through an environment of low vertical wind shear, a moist mid-level atmosphere, and increasing upper-ocean heat content. These conditions favor intensification and the intensity guidance continues to call for some additional strengthening during the next couple of days. However, there are likely to be eyewall cycles that are difficult to predict, which could result in some fluctuations in intensity. Barring land interaction with the islands of the Greater Antilles, Irma is forecast to remain a powerful hurricane throughout the 5-day forecast period. Irma has been moving a little south of due west today, and the longer-term motion estimate is 265/11 kt. The hurricane will reach the southwestern portion of a strong mid-level ridge that is centered over the central Atlantic later today or tonight. This should result in a westward, then west-northwestward turn over the next 24 to 36 hours. This motion is expected to bring the hurricane near or over the northern Leeward Islands on Tuesday night or early Wednesday. A large mid-latitude trough that is predicted to deepen over the eastern U.S. during the next few days is forecast to lift northeastward late in the week, which is expected to cause the subtropical ridge over the western Atlantic to build westward. As a result, Irma is predicted to remain on a general west-northwestward heading on days 3 through 5. The dynamical model guidance is in excellent agreement through 72 hours, with some increase in spread late in the period, however the typically more reliable ECMWF and GFS are in very good agreement through day 5, and the new NHC track forecast lies very close to those models. Six hourly upper-air soundings began at 1800 UTC today over the central United States to better sample the upstream mid-latitude trough. In addition, the NOAA G-IV aircraft is currently sampling the environment around Irma, and these data will be included in tonight's 0000 UTC model runs. 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 a dangerous major hurricane, accompanied by life-threatening wind, storm surge, and rainfall impacts. Hurricane warnings are in effect for portions of the Leeward Islands. Preparations should be rushed to completion, as tropical-storm force winds are expected to first arrive in the hurricane warning area by late Tuesday. 2. Irma could directly affect the British and U.S. Virgin Islands and Puerto Rico as a dangerous major hurricane later this week. Hurricane watches have been issued for these areas, and tropical- storm-force winds could arrive in these areas by early Wednesday. 3. Irma could directly affect Hispaniola, the Turks and Caicos, the Bahamas, and Cuba as a dangerous major hurricane later this week. Residents in these areas should monitor the progress of Irma and listen to advice given by officials. 4. There is an increasing chance of seeing some impacts from Irma in the Florida Peninsula and the Florida Keys later this week and this weekend. Otherwise, it is still too early to determine what direct impacts Irma might have on the continental United States. However, everyone in hurricane-prone areas should ensure that they have their hurricane plan in place. FORECAST POSITIONS AND MAX WINDS INIT 04/2100Z 16.7N 54.4W 115 KT 130 MPH 12H 05/0600Z 16.6N 56.2W 125 KT 145 MPH 24H 05/1800Z 17.0N 58.7W 130 KT 150 MPH 36H 06/0600Z 17.8N 61.3W 130 KT 150 MPH 48H 06/1800Z 18.7N 64.1W 125 KT 145 MPH 72H 07/1800Z 20.4N 69.7W 120 KT 140 MPH 96H 08/1800Z 21.6N 74.8W 115 KT 130 MPH 120H 09/1800Z 23.0N 79.0W 115 KT 130 MPH $$ Forecaster Brown
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