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Tropical Depression Harvey Forecast Discussion Number 14
2017-08-24 04:38:36| National Hurricane Center (Atlantic)
Issued at 1000 PM CDT Wed Aug 23 2017 000 WTNT44 KNHC 240238 TCDAT4 Tropical Depression Harvey Discussion Number 14 NWS National Hurricane Center Miami FL AL092017 1000 PM CDT Wed Aug 23 2017 Reports from an Air Force Reserve Hurricane Hunter aircraft suggest that Harvey's structure has consolidated a little bit, with the pressure falling to 1002 mb and the radius of maximum winds shrinking to 60 n mi. However, there have been no reliable reports of winds indicating that the cyclone has reached tropical-storm strength, and the convective cloud pattern remains ragged. Based on these data, the initial intensity is held at 30 kt. Perhaps the most significant finding of the aircraft is that the center was to the east of the previous advisory position. The initial motion is a rather uncertain 325/2. A weak mid-level ridge to the northeast of Harvey should cause the cyclone to move on a northwestward or north-northwestward track through 48-72 h, and this motion should bring the center inland over southern Texas. 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. There is a major spread amongst the large-scale models from 72-120 h, with the UKMET forecasting a slow northward motion into eastern Texas, the GFS a southwestward motion over Texas, and the ECMWF a cyclonic loop that moves the system back over the Gulf of Mexico. Given the uncertainty, the new forecast track shows a slow motion over southern Texas during that time, with a change from the small right turn of the previous forecast to a small left turn. Overall, the track lies a little to the right of the consensus models through 72 h. An upper-level low seen in water vapor imagery over the northwestern Gulf of Mexico appears to be weakening, and as this happens Harvey should end up in a low shear environment. This should allow strengthening over deep warm water in the western Gulf of Mexico. The new intensity forecast is adjusted slightly to show Harvey explicitly becoming a hurricane in 48 h, and some additional strengthening could occur between the 48 h point and landfall. After landfall, Harvey is expected to weaken. However, the forecast track keeps the system close enough to the Texas coast that this weakening is likely to be slower than normal. Since Harvey has moved slower than anticipated during the past 6-12 h, warnings are not yet required for portions of the Texas coast. Warnings are likely to be issued sometime on Thursday. 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 24/0300Z 21.9N 92.6W 30 KT 35 MPH 12H 24/1200Z 22.8N 93.1W 35 KT 40 MPH 24H 25/0000Z 24.1N 94.1W 40 KT 45 MPH 36H 25/1200Z 25.4N 95.4W 50 KT 60 MPH 48H 26/0000Z 26.6N 96.3W 65 KT 75 MPH 72H 27/0000Z 28.5N 97.5W 50 KT 60 MPH...INLAND 96H 28/0000Z 28.5N 98.0W 35 KT 40 MPH...INLAND 120H 29/0000Z 28.0N 98.0W 30 KT 35 MPH...INLAND $$ Forecaster Beven
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