Irma Becomes A Category 3 Hurricane Over The Eastern Atlantic

000 WTNT41 KNHC 020850 TCDAT1 Hurricane Irma Discussion Number 12 NWS National Hurricane Center Miami FL AL112017 500 AM AST Sat Sep 02 2017 Irma appears to have weakened a little during the last several hours. The eye has become cloud filled once again, and the convective pattern is not as impressive as it was yesterday. A blend of the latest Dvorak classifications from TAFB/SAB and ADT values from CIMSS at the University of Wisconsin support lowering the initial wind speed a little to 95 kt. It is interesting to note that a ship (BATFR17) passed within 50 n mi to the west of the center of Irma and has only reported winds of about 40 kt, indicating that the core of Irma is compact. The observed fluctuations in strength during the past day or so are likely to continue for about another day while Irma remains over marginally warm waters and in fairly close proximity to dry air. Eyewall replacement cycles, like the one observed yesterday, could occur, but forecasting the timing and duration of these are not possible. After 24 hours, Irma is expected to move over progressively warmer waters and into a more moist environment. These more favorable conditions combined with low to moderate wind shear should allow the hurricane to strengthen. The NHC intensity forecast follows the consensus aids HCCA and IVCN, and it is fairly similar to the previous advisory. Irma is now moving due west at 12 kt. A subtropical high pressure system to the north of the hurricane is expected to strengthen and build westward during the next couple of days. This pattern should cause Irma to move west-southwestward during that time. Thereafter, a turn back to the west and then west-northwest is predicted in the 3-5 day time period when Irma moves on the south and southwest sides of the high. Although the models agree on the overall scenario, there remains about 200 n mi north-south spread among the best-performing models on day 5. The NHC track forecast has been adjusted to the south at the longer-range points, and it is about halfway between the latest runs of the GFS and ECMWF models. KEY MESSAGES: 1. Irma is expected to be a major hurricane when it moves closer to 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 02/0900Z 19.0N 41.8W 95 KT 110 MPH 12H 02/1800Z 18.8N 43.7W 100 KT 115 MPH 24H 03/0600Z 18.3N 46.1W 100 KT 115 MPH 36H 03/1800Z 17.7N 48.3W 105 KT 120 MPH 48H 04/0600Z 17.1N 50.4W 110 KT 125 MPH 72H 05/0600Z 17.0N 54.7W 110 KT 125 MPH 96H 06/0600Z 18.4N 59.3W 115 KT 130 MPH 120H 07/0600Z 21.0N 64.5W 115 KT 130 MPH

$$ Forecaster Cangialosi

000 ZCZC MIATCDAT1 ALL TTAA00 KNHC DDHHMM Hurricane Irma Discussion Number 13 NWS National Hurricane Center Miami FL AL112017 1100 AM AST Sat Sep 02 2017 Hurricane Irma continues to display an eye within a small central dense overcast in the visible and infrared imagery from the GOES and METEOSAT satellites. Dvorak current intensity numbers from TAFB, SAB, and the CIMSS ADT have not changed, so the intensity remains 95 kt. The environment in which Irma is embedded shows mixed signals for the next few days. While the vertical shear remains low through Monday, the SSTs are lukewarm and mid-level humidities are dry. (Indeed an overnight SSMIS microwave pass showed a distinct finger of dry air wrapping around the south side of Irma not far from its inner core.) However, after the waters warm and the atmosphere moistens at days 3-5, the shear is forecast to go up. The guidance has substantial spread between Category 2 and 4 by the end of the forecast period. The official intensity forecast is based upon a blend of the LGEM/DSHP statistical techniques and the HWRF dynamical model and is unchanged from the previous advisory. Irma is finally moving slightly south of due west at 13 kt. A large, building Azores-Bermuda high should turn Irma toward the west-southwest at a slower rate of speed during the next two days. By days 3 to 5, Irma should be rounding the southwestern periphery of the high and start turning back toward the west-northwest. The track guidance is in tight agreement with this scenario and the official forecast is a simply an update from the previous advisory. Irma is a small hurricane, as observed by ship BATFR17 and the overnight ASCAT scatterometer pass with tropical-storm-force winds extending out at most about 60 nm. The official size forecast is based upon the RVCN multi-model consensus technique. KEY MESSAGES: 1. Irma is expected to be a major hurricane when it moves closer to 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 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 02/1500Z 18.8N 43.3W 95 KT 110 MPH 12H 03/0000Z 18.5N 45.0W 95 KT 110 MPH 24H 03/1200Z 18.0N 47.4W 100 KT 115 MPH 36H 04/0000Z 17.4N 49.5W 105 KT 120 MPH 48H 04/1200Z 17.1N 51.6W 110 KT 125 MPH 72H 05/1200Z 17.4N 56.0W 110 KT 125 MPH 96H 06/1200Z 19.2N 60.6W 115 KT 130 MPH 120H 07/1200Z 22.0N 66.0W 115 KT 130 MPH $$ Forecaster Landsea NNNN

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000 WTNT41 KNHC 032052 TCDAT1 Hurricane Irma Discussion Number 18 NWS National Hurricane Center Miami FL AL112017 500 PM AST Sat Sep 03 2017 The satellite presentation of Irma has degraded slightly since this morning with the eye becoming less defined. There is evidence of some northerly flow beneath the cirrus outflow, which may be disrupting the inner core and preventing Irma from strengthening. The latest consensus of the Dvorak satellite estimates suggest that the 100 kt initial intensity could be a little generous, but with a NOAA aircraft headed into the hurricane it is best to wait until data from that mission is received before making any adjustment to the initial wind speed. Irma has been moving more westward since the previous advisory, but the longer-term motion estimate is still south of due west or 260/12 kt. A strong high pressure ridge over the central Atlantic should steer Irma westward to west-southwestward during the next couple of days. After that time, a turn toward the west-northwest should occur as Irma approaches the western portion of the Atlantic ridge. The cross-track spread of the guidance is still relatively small through day 5, but the ECMWF, UKMET, and HWRF are on the southern side of the guidance envelope, with the GFS near the middle of the envelope. The latest NHC track is once again near the consensus of these typically reliable models, which is between the southern edge of the guidance and the TCVN multi-model consensus. The updated track is not very different from the previous advisory, except at day 5 where it is slightly west of the previous forecast. Users are reminded to not focus on the exact forecast track since strong winds and heavy rainfall extend well away from the center. In fact, ASCAT data that arrived after the issuance of the previous advisory indicated that the size of the tropical-storm-force wind field has expanded, especially over the northern semicircle. As a result, the initial and forecast wind radii have been adjusted accordingly. KEY MESSAGES: 1. Irma is expected to impact the northeastern Leeward Islands by the middle of this week as a major hurricane, accompanied by dangerous wind, storm surge, and rainfall impacts, along with rough surf and rip currents. Hurricane watches have been issued for portions of the Leeward Islands and additional hurricane or tropical storm watches may be required on Monday. 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, and the Bahamas. Residents in all of these areas should monitor the progress of Irma and listen to advice given by officials. Tropical storm or hurricane watches could be issued for the British and U.S. Virgin Islands and Puerto Rico on Monday. 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 03/2100Z 17.6N 49.8W 100 KT 115 MPH 12H 04/0600Z 17.2N 51.4W 105 KT 120 MPH 24H 04/1800Z 16.8N 53.5W 110 KT 125 MPH 36H 05/0600Z 16.8N 55.8W 115 KT 130 MPH 48H 05/1800Z 17.3N 58.2W 120 KT 140 MPH 72H 06/1800Z 19.1N 63.5W 120 KT 140 MPH 96H 07/1800Z 21.2N 68.8W 115 KT 130 MPH 120H 08/1800Z 23.0N 73.5W 115 KT 130 MPH $$ Forecaster Brown

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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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