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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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Hurricane Irma Forecast Discussion Number 21
2017-09-04 16:49:30| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Mon Sep 04 2017 000 WTNT41 KNHC 041449 TCDAT1 Hurricane Irma Discussion Number 21 NWS National Hurricane Center Miami FL AL112017 1100 AM AST Mon Sep 04 2017 The satellite presentation of Irma has improved markedly over the past 24 hours, with the eye becoming larger and much more distinct. A NOAA Hurricane Hunter aircraft has reported peak 700-mb flight level winds of 117 kt, SFMR winds of 107 kt, and dropsonde data that support a minimum pressure of around 944 mb. These data support an initial intensity of 105 kt. The aircraft also reported concentric eyewalls and a double wind maximum during the last couple of passes through the center, indicating that an eyewall replacement cycle has likely begun. Irma is expected to remain within a very favorable environment for strengthening during the next several days and additional intensification appears likely. However, eyewall replacement cycles are likely to cause some fluctuations in intensity during that time. The NHC forecast brings the hurricane to category 4 strength within the next 24 hours, and then maintains Irma as a powerful hurricane during the next 5 days, assuming that the core of the cyclone does not move over any of the Greater Antilles. Irma has been moving west-southwestward or 255/11 kt to the south of a strong mid-level ridge over the central Atlantic. The hurricane should turn westward later today or tonight, then west-northwestward Tuesday as it reaches the southwestern portion of the ridge. As mentioned in the previous NHC discussion, a large mid-latitude trough is expected to amplify over the eastern United States during the next few days. The global models are unanimous in lifting the trough out to the northeast by late in the week, allowing the Atlantic ridge to build westward on days 3 through 5. The track guidance has again shifted westward and southwestward at days 4 and 5, but the models remain in very good agreement through the forecast period. The updated NHC track forecast has been adjusted southwestward late in the period, and lies very near the consensus of the ECMWF, GFS, and HWRF models. Six hourly upper-air soundings will begin at 1800 UTC today over the central United States to better sample the upstream mid-latitude trough. In addition, the NOAA G-IV aircraft will begin sampling the environment around Irma this afternoon and evening, 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 as a dangerous major hurricane, accompanied by life-threatening wind, storm surge, and rainfall impacts, along with rough surf and rip currents. Hurricane warnings have been issued 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. In addition, rough surf and dangerous marine conditions will begin to affect the southeastern U.S. coast by later this week. 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, as we are now near the peak of the season. FORECAST POSITIONS AND MAX WINDS INIT 04/1500Z 16.8N 53.3W 105 KT 120 MPH 12H 05/0000Z 16.5N 54.8W 115 KT 130 MPH 24H 05/1200Z 16.7N 57.1W 120 KT 140 MPH 36H 06/0000Z 17.3N 59.6W 120 KT 140 MPH 48H 06/1200Z 18.2N 62.4W 125 KT 145 MPH 72H 07/1200Z 19.9N 68.0W 120 KT 140 MPH 96H 08/1200Z 21.2N 73.3W 115 KT 130 MPH 120H 09/1200Z 22.0N 77.8W 115 KT 130 MPH...NEAR THE COAST OF CUBA $$ Forecaster Brown
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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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Hurricane Irma Forecast Discussion Number 18
2017-09-03 22:52:50| National Hurricane Center (Atlantic)
Issued at 500 PM AST Sun Sep 03 2017 000 WTNT41 KNHC 032052 TCDAT1 Hurricane Irma Discussion Number 18 NWS National Hurricane Center Miami FL AL112017 500 PM AST Sun 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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Hurricane Irma Forecast Discussion Number 17
2017-09-03 16:52:19| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Sun Sep 03 2017 000 WTNT41 KNHC 031452 TCDAT1 Hurricane Irma Discussion Number 17 NWS National Hurricane Center Miami FL AL112017 1100 AM AST Sun Sep 03 2017 The eye of Irma is a little less distinct in geostationary satellite images this morning, which suggests that the intensity of the hurricane may be fluctuating yet again. However, the initial wind speed is maintained at 100 kt for this advisory, which is close to a consensus of the various objective and subjective satellite intensity estimates. The first reconnaissance mission, a NOAA P-3 Hurricane Hurricane aircraft, is scheduled to depart Barbados for tail Doppler radar mission into Irma late this afternoon and should provide additional information on Irma's intensity by this evening. A strong high pressure ridge over the central Atlantic is steering Irma west-southwestward or 255/12 kt. This general motion with some reduction in forward speed is expected during the next day or so. After that time, Irma is forecast to turn westward, then west-northwestward in about 72 hours as it approaches the western portion of the ridge. The various consensus aids are generally a little slower than the previous advisory, but there cross-track differences are small. As a result, the updated NHC track is very similar to the previous advisory, and is close to a consensus of the ECMWF, GFS, UKMET, and HWRF, but is not as far south as the latest runs of the UKMET or ECMWF. Irma is forecast to move over slightly warmer SSTs and into a moistening mid-level environment. These conditions, along with a favorable upper-level wind pattern, should allow for gradual strengthening during the next 2 to 3 days. However, eyewall replacement cycles could result in fluctuations in intensity during the next several days. While Irma is currently a small hurricane, the 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 near or over the northeastern Leeward Islands by the middle of this week, and could cause dangerous wind, storm surge, and rainfall impacts, along with rough surf and rip currents on some islands. Hurricane and tropical storm watches will likely be issued for some of these islands later today or tonight. Residents in these areas should monitor the progress of Irma and listen to advice given by officials. 2. Direct impacts from Irma are also possible in the Virgin Islands and Puerto Rico later this week, and tropical storm or hurricane watches could be issued for these islands by tomorrow. Residents in these areas should monitor the progress of Irma and listen to advice given by officials. 3. The possibility of direct impacts from Irma in Hispaniola, the Turks and Caicos, and the Bahamas later this week is increasing. Residents in these areas should monitor the progress of Irma and listen to advice given by officials. 4. 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 03/1500Z 17.7N 48.4W 100 KT 115 MPH 12H 04/0000Z 17.1N 50.1W 105 KT 120 MPH 24H 04/1200Z 16.6N 52.2W 110 KT 125 MPH 36H 05/0000Z 16.4N 54.3W 115 KT 130 MPH 48H 05/1200Z 16.7N 56.6W 120 KT 140 MPH 72H 06/1200Z 18.2N 61.8W 120 KT 140 MPH 96H 07/1200Z 20.4N 67.1W 115 KT 130 MPH 120H 08/1200Z 22.5N 72.0W 115 KT 130 MPH $$ Forecaster Brown
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