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000 WTNT41 KNHC 012050 TCDAT1 Hurricane Irma Discussion Number 10 NWS National Hurricane Center Miami FL AL112017 500 PM AST Fri Sep 01 2017 Well, that eyewall replacement cycle didn't last long. Satellite images indicate that the cycle is now complete, with Irma displaying a larger warm eye. The initial wind speed has been set to 105 kt, in agreement with the CIMSS ADT. Since the eyewall cycle is finished, some strengthening is possible, although SSTs are currently pretty marginal. Although no increase in winds is shown in the short term, this could be conservative. During the next few days, water temperatures along the track will warm significantly, but there could be some higher shear, and of course more eyewall replacements. The forecast is a compromise between the statistical/dynamical models, which bring the intensity down then up again, versus the global models and hurricane models, which show a rather strong cyclone. The westward turn of Irma has begun, and the current motion is 275/12. A building mid-level high should cause the hurricane to turn west-southwestward tomorrow and continue through early next week. Later on, Irma should reach the southern periphery of the ridge, and begin to move west-northwestward. The biggest change since the last forecast is that the GFS and its ensemble has trended southwestward, toward the unwavering ECMWF model on the track forecast. Given the trend in guidance, little change is made to the official track prediction, although the corrected consensus models and the HWRF are now southwest of the latest forecast. KEY MESSAGES: 1. Irma is expected to be a major hurricane when it approaches the Lesser Antilles early next week, 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 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 01/2100Z 18.8N 39.1W 105 KT 120 MPH 12H 02/0600Z 18.9N 40.9W 105 KT 120 MPH 24H 02/1800Z 18.5N 43.4W 105 KT 120 MPH 36H 03/0600Z 17.9N 45.7W 105 KT 120 MPH 48H 03/1800Z 17.2N 47.9W 105 KT 120 MPH 72H 04/1800Z 16.4N 52.2W 110 KT 125 MPH 96H 05/1800Z 17.0N 56.5W 110 KT 125 MPH 120H 06/1800Z 19.0N 61.0W 115 KT 130 MPH $$ Forecaster Blake

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000 WTNT41 KNHC 020242 TCDAT1 Hurricane Irma Discussion Number 11 NWS National Hurricane Center Miami FL AL112017 1100 PM AST Fri Sep 01 2017 After developing a nearly clear eye during the afternoon hours, Irma appears to have once again temporarily peaked. A WindSat pass around 2100 UTC hinted at the beginning of another eyewall replacement cycle, which would be consistent with the observed cooling of the eye. However, the small size of the hurricane's inner core relative to the resolution of the microwave instrument makes it impossible for me to say for sure. The initial intensity has been lowered slightly to 100 kt, but it should be stressed that this is probably just another fluctuation, in what will likely be a long string of small changes in intensity over the next several days. Since we do not have the ability to predict such changes, the NHC forecast shows very gradual intensification throughout the forecast period, given the warm SSTs and increasing moisture content along the forecast track. The NHC forecast is near the intensity consensus, but slightly favors the higher dynamical models. The initial motion remains 275/12 kt. The hurricane has moved a little farther north than previously expected, and the track has been adjusted in that direction. Overall, the reasoning behind the track forecast has not changed, and Irma is still expected to turn west-southwestward on Saturday due to a building ridge over the central Atlantic. After about 72 h, there remains a large north-south spread in the guidance, with the GFS continuing to show a weaker ridge (and a northern track), while the ECMWF shows a stronger ridge/southern track. The ECMWF has performed better for Irma thus far, so my forecast continues to favor that solution. KEY MESSAGES: 1. Irma is expected to be a major hurricane when it approaches the Lesser Antilles early next week, 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 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 02/0300Z 19.1N 40.5W 100 KT 115 MPH 12H 02/1200Z 19.0N 42.3W 100 KT 115 MPH 24H 03/0000Z 18.5N 44.6W 105 KT 120 MPH 36H 03/1200Z 17.9N 46.9W 110 KT 125 MPH 48H 04/0000Z 17.3N 49.0W 110 KT 125 MPH 72H 05/0000Z 16.8N 53.3W 110 KT 125 MPH 96H 06/0000Z 18.0N 57.5W 115 KT 130 MPH 120H 07/0000Z 20.5N 62.5W 115 KT 130 MPH $$ Forecaster Zelinsky

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