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Tropical Depression Harvey Forecast Discussion Number 13
2017-08-23 22:37:31| National Hurricane Center (Atlantic)
Issued at 400 PM CDT Wed Aug 23 2017 000 WTNT44 KNHC 232037 TCDAT4 Tropical Depression Harvey Discussion Number 13 NWS National Hurricane Center Miami FL AL092017 400 PM CDT Wed Aug 23 2017 High-resolution visible satellite images show that the cloud pattern of Harvey is a little better organized than it was this morning, but the system lacks distinct banding features. Surface synoptic observations, ASCAT data, and Dvorak classifications from TAFB and SAB indicated that the cyclone has not strengthened, so the current intensity is held at 30 kt. The global models predict that an upper-level low over the northwest Gulf of Mexico will essentially dissipate in a day or so. Therefore, Harvey is expected to remain in a relatively low-shear environment up to the Texas coast. Moreover, ocean analyses show that a warm eddy that broke off from the Loop Current has drifted westward across the Gulf to a location near the projected path of Harvey. This would also be conducive to strengthening, so it is likely that the system will become a hurricane prior to landfall, although this is not explicitly shown in the NHC forecast for which landfall is indicated between 48 and 72 hours. Based on the scatterometer data and geostationary satellite fixes the center hasn't moved much this afternoon, although recent imagery suggests a northwestward drift at about 320/2 kt. A weak mid-level ridge to the northeast of Harvey should cause the cyclone to move on a northwestward or north-northwestward track through 48 hours. Later, steering currents weaken as a ridge builds over the southwestern United States and a trough drops down from the Plains. As a result, Harvey should decelerate while making landfall and move very slowly just inland of the coast. Some of the track guidance models, such as the HWRF, have shifted southwestward in comparison to their previous run. The official track forecast is very close to the previous one through 48 hours and is a little slower and to the west after that time. This is very close to the latest dynamical model consensus, TVCN. It should be noted that synoptic surveillance data are currently being collected by the NOAA G-IV jet aircraft and these data will be assimilated into, and hopefully improve the forecasts by, the global models. Key Messages: 1. Harvey is likely to bring multiple hazards, including heavy rainfall, storm surge, and possible hurricane conditions to portions of the Texas coast beginning on Friday. 2. Heavy rainfall is likely to spread across portions of eastern Texas, Louisiana, and the lower Mississippi Valley from Friday through early next week and could cause life-threatening flooding. Please refer to products from your local National Weather Service office and the NOAA Weather Prediction Center for more information on the flooding hazard. 3. A Storm Surge Watch is in effect from Port Mansfield to High Island, Texas, indicating the possibility of life-threatening inundation from rising water moving inland from the coast during the next 48 hours. For a depiction of areas at risk, see the Storm Surge Watch/Warning Graphic at hurricanes.gov. 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. 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. FORECAST POSITIONS AND MAX WINDS INIT 23/2100Z 21.6N 92.6W 30 KT 35 MPH 12H 24/0600Z 22.4N 93.0W 35 KT 40 MPH 24H 24/1800Z 23.8N 93.9W 40 KT 45 MPH 36H 25/0600Z 25.1N 95.2W 50 KT 60 MPH 48H 25/1800Z 26.4N 96.3W 60 KT 70 MPH 72H 26/1800Z 28.6N 97.3W 50 KT 60 MPH...INLAND 96H 27/1800Z 29.0N 97.3W 35 KT 40 MPH...INLAND 120H 28/1800Z 29.0N 96.5W 35 KT 40 MPH...INLAND $$ Forecaster Pasch
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