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Tropical Depression Harvey Forecast Discussion Number 12
2017-08-23 16:50:32| National Hurricane Center (Atlantic)
Issued at 1000 AM CDT Wed Aug 23 2017 000 WTNT44 KNHC 231450 TCDAT4 Tropical Depression Harvey Discussion Number 12 NWS National Hurricane Center Miami FL AL092017 1000 AM CDT Wed Aug 23 2017 Air Force Reserve Hurricane Hunter data indicate that Harvey has regenerated in the Bay of Campeche, with a closed circulation noted and a central pressure of about 1006 mb. The initial wind speed is 30 kt based on SFMR data from the aircraft. Harvey is not well organized at the moment, with an asymmetric cloud pattern and a large radius of maximum wind. The environment, however, is conducive for intensification while Harvey moves over the very warm Gulf of Mexico waters in light-to-moderate shear. The biggest hindrance to intensification in the short term is the poor structure. Thus the NHC forecast will only show a gradual increase in wind speed during the first day, with a more significant intensification after that time. Although not explicitly forecast below, we are anticipating Harvey being a hurricane at landfall after the 48 hour forecast point. This forecast agrees well with the guidance, almost all of which shows a quickly intensifying cyclone approaching the Texas coast. Since the center has just recently formed, the initial motion is difficult to estimate, but the long-term motion is 310/8. Harvey is expected to move more slowly toward the northwest or north- northwest as it enters a weakness in the Atlantic subtropical ridge during the next day or so. The ridge slightly strengthens by late Thursday, which should cause a faster northwestward motion by then. Around the time of landfall, however, Harvey should enter an area of weaker steering currents near the upper Texas coast as high pressure rebuilds over the southwestern United States. The storm should slow down markedly over southeast Texas, and there is considerable uncertainty on exactly how fast Harvey moves out of that state ahead of the next mid-latitude trough. For now the NHC forecast will just drift Harvey generally toward the east at days 4 and 5, on the slow side of the model consensus. Hopefully later G-IV flights and special soundings over the southern United States will help clarify the long range forecast. Key Messages: 1. Harvey is likely to bring multiple hazards to portions of the Texas coast beginning on Friday. 2. Several days of heavy rainfall are likely across portions of eastern Texas, Louisiana, and the lower Mississippi Valley from Friday through early next week. This rainfall could cause life- threatening flooding. Please refer to products from your local National Weather Service office (www.weather.gov) and the NOAA Weather Prediction Center (www.wpc.ncep.noaa.gov) for more information on the flooding hazard. 3. The National Weather Service has issued a Storm Surge Watch from Port Mansfield to High Island, Texas. There is the possibility of life-threatening inundation from rising water moving inland from the coastline during the next 48 hours in these areas. For a depiction of areas at risk, please see the National Weather Service Storm Surge Watch/Warning Graphic, available at hurricanes.gov. 4. The Potential Storm Surge Flooding Map will also be available on the NHC website by 1200 PM CDT. Remember that the Potential Storm Surge Flooding Map does not represent a forecast of expected inundation, but rather depicts a reasonable worst-case scenario - the amount of inundation that has a 10 percent chance of being exceeded at each individual location. Because the Flooding Map is based on inputs that extend out only to about 72 hours, it best represents the flooding potential in those locations within the watch area. 5. Hurricane conditions are possible along the Texas coast from Port Mansfield to San Luis Pass. FORECAST POSITIONS AND MAX WINDS INIT 23/1500Z 21.5N 92.5W 30 KT 35 MPH 12H 24/0000Z 22.3N 93.2W 35 KT 40 MPH 24H 24/1200Z 23.8N 93.9W 40 KT 45 MPH 36H 25/0000Z 25.2N 94.9W 50 KT 60 MPH 48H 25/1200Z 26.7N 96.3W 60 KT 70 MPH 72H 26/1200Z 29.0N 97.3W 40 KT 45 MPH...INLAND 96H 27/1200Z 29.8N 96.9W 35 KT 40 MPH...INLAND 120H 28/1200Z 30.0N 95.0W 35 KT 40 MPH...INLAND $$ Forecaster Blake
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