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Hurricane Harvey Forecast Discussion Number 21
2017-08-25 16:53:45| National Hurricane Center (Atlantic)
Issued at 1000 AM CDT Fri Aug 25 2017 000 WTNT44 KNHC 251453 TCDAT4 Hurricane Harvey Discussion Number 21 NWS National Hurricane Center Miami FL AL092017 1000 AM CDT Fri Aug 25 2017 Multiple observing platforms indicate that Harvey's structure is evolving this morning. The hurricane has developed concentric eyewalls, as observed in data from the WSR-88D Doppler radars in Brownsville and Corpus Christi, and an Air Force Reserve Hurricane Hunter plane reported double wind maxima with diameters of 12 and 28 n mi. The aircraft data indicate that the central pressure has continued to fall--now down to 947 mb--but the maximum winds have not increased further. This discrepancy is not surprising given hurricane's current structure. Intensity forecasts become complicated once a hurricane develops concentric eyewalls, and fluctuations in intensity become more likely. If an eyewall replacement occurs, then Harvey's intensity could decrease a bit while at the same time the overall wind field increases in size. However, the hurricane remains in an environment for intensification, and strengthening beyond the current intensity is still possible before the center reaches land. But in the end, the hurricane's exact intensity at landfall does not change the fact that catastrophic flooding due to a prolonged period of heavy rainfall and/or storm surge is expected at the coast and well inland across much of southern and southeastern Texas. Slow weakening is expected after Harvey crosses the coast since the center is not expected to move very far inland, and the cyclone is likely to maintain tropical storm status through Wednesday. Harvey has not quite begun to slow down, and the initial motion estimate is 315/9 kt. Strong mid-level ridging building over the western United States is still expected to impede Harvey's forward motion in the coming days, and the track guidance continues to show the hurricane meandering or stalling near or just inland of the Texas coast in 36-48 hours. Harvey could begin moving slowly eastward on days 4 and 5 due to the influence of a mid-level shortwave trough digging southward over the upper Midwest, but at this time it is too early to say whether the center will definitely re-emerge over the northwestern Gulf of Mexico. Key Messages: 1. Harvey is expected to be a major hurricane when it makes landfall tonight, bringing life-threatening storm surge, rainfall, and wind hazards to portions of the Texas coast. Preparations should be rushed to completion in the warning areas as tropical-storm-force winds are arriving on the coast, and conditions will continue to deteriorate through the rest of today and tonight. 2. A Storm Surge Warning is in effect for much of the Texas coast. Life-threatening storm surge flooding could reach heights of 6 to 12 feet above ground level at the coast between the north entrance of the Padre Island National Seashore and Sargent. For a depiction of areas at risk, see the Storm Surge Watch/Warning Graphic at hurricanes.gov. Due to the slow motion of Harvey and a prolonged period of onshore flow, water levels will remain elevated for several days. 3. Catastrophic and life-threatening flooding is expected across the middle and upper Texas coast from heavy rainfall of 15 to 25 inches, with isolated amounts as high as 35 inches, through Wednesday. Please refer to products from your local National Weather Service office and the NOAA Weather Prediction Center for more information on the flooding hazard. 4. The Potential Storm Surge Flooding Map is available on the NHC website. This product depicts a reasonable worst-case scenario - the amount of inundation that has a 10 percent chance of being exceeded at each individual location. This map best represents the flooding potential in those locations within the watch and warning areas. FORECAST POSITIONS AND MAX WINDS INIT 25/1500Z 26.7N 96.0W 95 KT 110 MPH 12H 26/0000Z 27.6N 96.8W 105 KT 120 MPH 24H 26/1200Z 28.4N 97.3W 90 KT 105 MPH...INLAND 36H 27/0000Z 28.8N 97.5W 70 KT 80 MPH...INLAND 48H 27/1200Z 28.9N 97.6W 55 KT 65 MPH...INLAND 72H 28/1200Z 28.3N 96.8W 35 KT 40 MPH...INLAND 96H 29/1200Z 28.5N 95.5W 40 KT 45 MPH...OVER WATER 120H 30/1200Z 30.0N 94.5W 40 KT 45 MPH...INLAND $$ Forecaster Berg
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Hurricane Harvey Forecast Discussion Number 20
2017-08-25 10:47:15| National Hurricane Center (Atlantic)
Issued at 400 AM CDT Fri Aug 25 2017 000 WTNT44 KNHC 250847 TCDAT4 Hurricane Harvey Discussion Number 20 NWS National Hurricane Center Miami FL AL092017 400 AM CDT Fri Aug 25 2017 The satellite presentation has improved during the past several hours with an intermittent eye feature surrounded by a ring of very deep convection. There are various cyclonically curved convective bands primarily to the north of the eye and the outflow is fair. NOAA and Air Force Hurricane Hunter planes penetrated the eye various times during the past several hours, and the most significant data were a flight-level peak wind of 103 knots, and a peak SFMR surface wind of 88 kt. The central pressure dropped to 967 mb. Based on these data, the initial intensity was adjusted upward to 90 kt. Another reconnaissance plane will be in the eye of Harvey shortly. Since Harvey is embedded within light shear and moving over warm waters, additional strengthening is anticipated before landfall in about 24 hours. Thereafter, gradual weakening is forecast but since a good portion of the circulation will remain over water, the weakening process could be slower than normal. Radar and reconnaissance fixes indicate that Harvey is moving toward the northwest or 320 degrees at 8 kt. The hurricane is on the western edge of a persistent area of high pressure over the eastern Gulf of Mexico, and this pattern will maintain the current hurricane motion until landfall. Once Harvey is inland over Texas, the steering currents are forecast to collapse and the cyclone should begin to meander, prolonging the flooding conditions for several days. The track guidance between now and landfall is very consistent and there is high confidence in the track forecast. After landfall, the track models show large variability and the confidence is low. In any case, NHC forecast depicts a slow moving tropical cyclone near or over Texas for the next five days. Once again, it is very critical that users not focus on the exact forecast track of Harvey, since cycle-to-cycle adjustments are likely. All locations within the hurricane and storm surge warning areas should be preparing for the possibility of major hurricane-force winds and life-threatening storm surge. Key Messages: 1. Harvey is expected to be a major hurricane at landfall, bringing life-threatening storm surge, rainfall, and wind hazards to portions of the Texas coast. Preparations to protect life and property should be completed this morning, as tropical-storm-force winds will first arrive in the hurricane and storm surge warning areas later today. 2. A Storm Surge Warning is in effect for much of the Texas coast. Life-threatening storm surge flooding could reach heights of 6 to 12 feet above ground level at the coast between the north entrance of the Padre Island National Seashore and Sargent. For a depiction of areas at risk, see the Storm Surge Watch/Warning Graphic at hurricanes.gov. 3. Devastating and life-threatening flooding is expected across the middle and upper Texas coast from heavy rainfall of 15 to 25 inches, with isolated amounts as high as 35 inches, from today through next Wednesday. Please refer to products from your local National Weather Service office and the NOAA Weather Prediction Center for more information on the flooding hazard. 4. The Potential Storm Surge Flooding Map is available on the NHC website. This product depicts a reasonable worst-case scenario - the amount of inundation that has a 10 percent chance of being exceeded at each individual location. This map best represents the flooding potential in those locations within the watch and warning areas. FORECAST POSITIONS AND MAX WINDS INIT 25/0900Z 25.9N 95.4W 90 KT 105 MPH 12H 25/1800Z 26.9N 96.3W 105 KT 120 MPH 24H 26/0600Z 28.0N 97.1W 105 KT 120 MPH 36H 26/1800Z 28.5N 97.4W 65 KT 75 MPH...INLAND 48H 27/0600Z 28.5N 97.5W 60 KT 70 MPH...INLAND 72H 28/0600Z 28.3N 97.0W 35 KT 40 MPH...INLAND 96H 29/0600Z 28.5N 96.0W 35 KT 40 MPH...OVER WATER 120H 30/0600Z 29.5N 95.0W 35 KT 40 MPH...INLAND $$ Forecaster Avila
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Hurricane Harvey Forecast Discussion Number 19
2017-08-25 04:35:42| National Hurricane Center (Atlantic)
Issued at 1000 PM CDT Thu Aug 24 2017 000 WTNT44 KNHC 250235 TCDAT4 Hurricane Harvey Discussion Number 19 NWS National Hurricane Center Miami FL AL092017 1000 PM CDT Thu Aug 24 2017 Harvey's rapid intensification seems to have slowed for the moment, as an eye seen earlier in satellite imagery has lost definition during the past few hours. In addition, reports from an Air Force Reserve Hurricane Hunter aircraft show that 700-mb flight level winds are in the 75-80 kt range, with reliable surface wind estimates from the Stepped Frequency Microwave Radiometer remaining near 75 kt. The aircraft also reported that the central pressure has slowly fallen to 973 mb inside the 16 n mi wide eye. Based on these data, the initial intensity remains 75 kt, and this could be a little conservative. Harvey has turned a little to the left since the last advisory with the initial motion now 315/9. A mid-level anticyclone over the eastern Gulf of Mexico is expected to steer Harvey generally northwestward with a decreasing forward speed for the next 36-48 h, with the center now forecast to make landfall on the middle Texas coast in about 36 h. This part of the new forecast track has been nudged a little to the left based mainly on the initial position and motion. After landfall, the cyclone is likely to get stuck between the Gulf anticyclone and a stronger anticyclone over the western United States, with little motion likely from 48-96 h. A slow eastward motion appears likely by 120 h due to the influence of a trough in the westerlies digging into the eastern United States. There is some spread in the guidance at that time, with the GFS showing Harvey drifting slowly eastward and the ECMWF showing a faster motion. The new forecast track splits this difference of 5-day forecasts and lies near the consensus models. It is unclear why the intensification has slowed, although it is possibly due to some dry air seen earlier wrapping around the north side of the core convection. Otherwise, Harvey remains in an favorable environment for further strengthening with low vertical shear and high oceanic heat content. The intensity forecast will use the scenario that rapid intensification will resume tonight, with Harvey becoming a major hurricane before landfall in Texas. After landfall, the intensity forecast is based on the scenario that Harvey will weaken over land, but it will remain close enough to the coast so that the weakening will be slower than normal. Thus, the forecast keeps Harvey as a tropical storm from 72-120 h. It is critical that users not focus on the exact forecast track of Harvey, since cycle-to-cycle adjustments are likely. All locations within the hurricane and storm surge warning areas should be preparing for the possibility of major hurricane-force winds and life-threatening storm surge. Key Messages: 1. Harvey is expected to be a major hurricane at landfall, bringing life-threatening storm surge, rainfall, and wind hazards to portions of the Texas coast. Preparations to protect life and property should be completed by tonight, as tropical-storm-force winds will first arrive in the hurricane and storm surge warning areas on Friday. 2. A Storm Surge Warning is in effect for much of the Texas coast. Life-threatening storm surge flooding could reach heights of 6 to 12 feet above ground level at the coast between the north entrance of the Padre Island National Seashore and Sargent. For a depiction of areas at risk, see the Storm Surge Watch/Warning Graphic at hurricanes.gov. 3. Devastating and life-threatening flooding is expected across the middle and upper Texas coast from heavy rainfall of 15 to 25 inches, with isolated amounts as high as 35 inches, from Friday through next Wednesday. Please refer to products from your local National Weather Service office and the NOAA Weather Prediction Center for more information on the flooding hazard. 4. The Potential Storm Surge Flooding Map is available on the NHC website. This product depicts a reasonable worst-case scenario - the amount of inundation that has a 10 percent chance of being exceeded at each individual location. This map best represents the flooding potential in those locations within the watch and warning areas. FORECAST POSITIONS AND MAX WINDS INIT 25/0300Z 25.2N 94.6W 75 KT 85 MPH 12H 25/1200Z 26.1N 95.6W 95 KT 110 MPH 24H 26/0000Z 27.2N 96.5W 110 KT 125 MPH 36H 26/1200Z 28.1N 97.1W 100 KT 115 MPH...INLAND 48H 27/0000Z 28.6N 97.3W 70 KT 80 MPH...INLAND 72H 28/0000Z 28.5N 97.5W 35 KT 40 MPH...INLAND 96H 29/0000Z 28.5N 97.0W 35 KT 40 MPH...INLAND 120H 30/0000Z 29.5N 95.0W 35 KT 40 MPH...INLAND $$ Forecaster Beven
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Hurricane Harvey Forecast Discussion Number 18
2017-08-24 22:46:10| National Hurricane Center (Atlantic)
Issued at 400 PM CDT Thu Aug 24 2017 000 WTNT44 KNHC 242046 TCDAT4 Hurricane Harvey Discussion Number 18 NWS National Hurricane Center Miami FL AL092017 400 PM CDT Thu Aug 24 2017 Since the issuance of the special advisory at 1800 UTC, data from a dropsonde released by a NOAA Hurricane Hunter aircraft indicated that Harvey's central pressure has dropped to 976 mb. However, the plane has not measured winds stronger than what was observed by the previous Air Force flight (although the NOAA P3 is flying a research mission at a nonstandard level). For now, the initial intensity will remain 75 kt pending data from another Air Force flight this evening. Harvey's initial motion estimate is 330/9 kt. A strengthening mid-level high over the eastern Gulf of Mexico is expected to push Harvey northwestward by tonight, and a northwest or north-northwest motion should continue up until landfall. However, a stronger mid-level high centered over the western United States is forecast to cause Harvey to slow down considerably and stall near or just inland of the Texas coast by days 3 and 4. After that time, the track models have trended toward showing Harvey moving slowly eastward by day 5, but it's too early to know if the center will remain over land or re-emerge over the northwestern Gulf of Mexico. The track guidance is very tightly clustered for the first 36-48 hours, which covers the period of landfall, but then there is much greater spread in the models after 48 hours when Harvey is forecast to slow down. Such guidance is usually an indication that the cyclone will move very little, which is reflected in the official forecast. Harvey remains in an environment for further strengthening. Vertical shear is low, and the hurricane will be moving over a warm eddy of relatively higher oceanic heat content within the next 12-24 hours. As a result, the NHC intensity forecast calls for additional intensification at least for the next 24 hours, with Harvey expected to attain major hurricane strength on Friday. There may be some fluctuations in intensity up until landfall due to the possibility of an eyewall replacement or cooling of the shallow shelf waters of Texas, but regardless Harvey is forecast to reach the coast as a major hurricane. The intensity forecast after 48 hours is tricky because it all depends on how far inland Harvey moves into Texas. If it stays stalls closer to the coast, as recent model trends have suggested, then the cyclone may be able to maintain a higher intensity with a greater proportion of its circulation remaining over water. For that reason, the updated NHC intensity forecast keeps Harvey as a tropical storm on days 3 through 5, but there is greater-than-normal uncertainty in this part of the forecast. It is critical that users not focus on the exact forecast track of Harvey, since cycle-to-cycle adjustment are likely. All locations within the hurricane and storm surge warning areas should be preparing for the possibility of major hurricane-force winds and life-threatening storm surge. Key Messages: 1. Harvey is expected to be a major hurricane at landfall, bringing life-threatening storm surge, rainfall, and wind hazards to portions of the Texas coast. Preparations to protect life and property should be completed by tonight, as tropical-storm-force winds will first arrive in the hurricane and storm surge warning areas on Friday. 2. A Storm Surge Warning is in effect for much of the Texas coast. Life-threatening storm surge flooding could reach heights of 6 to 12 feet above ground level at the coast between the north entrance of the Padre Island National Seashore and Sargent. For a depiction of areas at risk, see the Storm Surge Watch/Warning Graphic at hurricanes.gov. 3. Devastating and life-threatening flooding is expected across the middle and upper Texas coast from heavy rainfall of 15 to 25 inches, with isolated amounts as high as 35 inches, from Friday through next Wednesday. Please refer to products from your local National Weather Service office and the NOAA Weather Prediction Center for more information on the flooding hazard. 4. The Potential Storm Surge Flooding Map is available on the NHC website. This product depicts a reasonable worst-case scenario - the amount of inundation that has a 10 percent chance of being exceeded at each individual location. This map best represents the flooding potential in those locations within the watch and warning areas. FORECAST POSITIONS AND MAX WINDS INIT 24/2100Z 24.7N 93.9W 75 KT 85 MPH 12H 25/0600Z 25.5N 94.8W 95 KT 110 MPH 24H 25/1800Z 26.7N 96.0W 110 KT 125 MPH 36H 26/0600Z 27.8N 96.7W 105 KT 120 MPH 48H 26/1800Z 28.5N 97.1W 85 KT 100 MPH...INLAND 72H 27/1800Z 28.7N 97.2W 45 KT 50 MPH...INLAND 96H 28/1800Z 28.5N 96.5W 35 KT 40 MPH...INLAND 120H 29/1800Z 29.0N 95.0W 35 KT 40 MPH...OVER WATER $$ Forecaster Berg
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Hurricane Harvey Forecast Discussion Number 17
2017-08-24 19:48:09| National Hurricane Center (Atlantic)
Issued at 100 PM CDT Thu Aug 24 2017 000 WTNT44 KNHC 241748 TCDAT4 Hurricane Harvey Special Discussion Number 17 NWS National Hurricane Center Miami FL AL092017 100 PM CDT Thu Aug 24 2017 This special advisory is being issued to update Harvey's initial and forecast intensities through the 48-hour period. The storm surge values in the public advisory have also been updated based on the new intensity forecast. Recent data from Air Force Reserve and NOAA Hurricane Hunter aircraft indicate that Harvey's maximum sustained winds have increased to 75 kt. The Air Force plane measured maximum 850-mb flight-level winds of 88 kt and reliable SFMR winds around 75 kt. Based on the hurricane's current rate of intensification, the forecast intensities at 12 and 24 h have been raised by 15 kt, with a 10-kt increase at 36 h. There are no changes to the track or wind radii forecast with this special advisory. It is critical that users not focus on the exact forecast track of Harvey, since cycle-to-cycle adjustment are likely. All locations within the hurricane and storm surge warning areas should be preparing for the possibility of major hurricane-force winds and life-threatening storm surge. Note that this special advisory takes the place of the scheduled 100 PM CDT (1800 UTC) intermediate advisory. Key Messages: 1. Harvey has intensified rapidly, and is forecast to be a major hurricane at landfall, bringing life-threatening storm surge, rainfall, and wind hazards to portions of the Texas coast. Preparations to protect life and property should be completed by tonight, as tropical-storm-force winds will first arrive in the hurricane and storm surge warning areas on Friday. 2. A Storm Surge Warning is in effect for much of the Texas coast. Life-threatening storm surge flooding could reach heights of 6 to 12 feet above ground level at the coast between the north entrance of the Padre Island National Seashore and Sargent. For a depiction of areas at risk, see the Storm Surge Watch/Warning Graphic at hurricanes.gov. 3. Life-threatening flooding is expected across much of the Texas coast from heavy rainfall of 12 to 20 inches, with isolated amounts as high as 30 inches, from Friday through early next week. Please refer to products from your local National Weather Service office and the NOAA Weather Prediction Center for more information on the flooding hazard. 4. The Potential Storm Surge Flooding Map is available on the NHC website. This product depicts a reasonable worst-case scenario - the amount of inundation that has a 10 percent chance of being exceeded at each individual location. This map best represents the flooding potential in those locations within the watch and warning areas. FORECAST POSITIONS AND MAX WINDS INIT 24/1800Z 24.4N 93.6W 75 KT 85 MPH 12H 25/0000Z 24.9N 94.2W 85 KT 100 MPH 24H 25/1200Z 26.0N 95.3W 105 KT 120 MPH 36H 26/0000Z 27.3N 96.3W 110 KT 125 MPH 48H 26/1200Z 28.3N 97.0W 100 KT 115 MPH...INLAND 72H 27/1200Z 29.0N 97.7W 40 KT 45 MPH...INLAND 96H 28/1200Z 28.5N 97.5W 35 KT 40 MPH...INLAND 120H 29/1200Z 28.5N 96.5W 30 KT 35 MPH...INLAND $$ Forecaster Berg
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