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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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