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ZCZC MIATCDAT4 ALL TTAA00 KNHC DDHHMM 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 NNNN