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Issued at 1000 AM CDT Fri Aug 25 2017 000 WTNT44 KNHC 251453 TCDAT4 Hurricane Harvey Discussion Number 21 NWS National Hurricane Center Miami FL AL092017 1000 AM CDT Fri Aug 25 2017 Multiple observing platforms indicate that Harvey's structure is evolving this morning. The hurricane has developed concentric eyewalls, as observed in data from the WSR-88D Doppler radars in Brownsville and Corpus Christi, and an Air Force Reserve Hurricane Hunter plane reported double wind maxima with diameters of 12 and 28 n mi. The aircraft data indicate that the central pressure has continued to fall--now down to 947 mb--but the maximum winds have not increased further. This discrepancy is not surprising given hurricane's current structure. Intensity forecasts become complicated once a hurricane develops concentric eyewalls, and fluctuations in intensity become more likely. If an eyewall replacement occurs, then Harvey's intensity could decrease a bit while at the same time the overall wind field increases in size. However, the hurricane remains in an environment for intensification, and strengthening beyond the current intensity is still possible before the center reaches land. But in the end, the hurricane's exact intensity at landfall does not change the fact that catastrophic flooding due to a prolonged period of heavy rainfall and/or storm surge is expected at the coast and well inland across much of southern and southeastern Texas. Slow weakening is expected after Harvey crosses the coast since the center is not expected to move very far inland, and the cyclone is likely to maintain tropical storm status through Wednesday. Harvey has not quite begun to slow down, and the initial motion estimate is 315/9 kt. Strong mid-level ridging building over the western United States is still expected to impede Harvey's forward motion in the coming days, and the track guidance continues to show the hurricane meandering or stalling near or just inland of the Texas coast in 36-48 hours. Harvey could begin moving slowly eastward on days 4 and 5 due to the influence of a mid-level shortwave trough digging southward over the upper Midwest, but at this time it is too early to say whether the center will definitely re-emerge over the northwestern Gulf of Mexico. Key Messages: 1. Harvey is expected to be a major hurricane when it makes landfall tonight, bringing life-threatening storm surge, rainfall, and wind hazards to portions of the Texas coast. Preparations should be rushed to completion in the warning areas as tropical-storm-force winds are arriving on the coast, and conditions will continue to deteriorate through the rest of today and tonight. 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. Due to the slow motion of Harvey and a prolonged period of onshore flow, water levels will remain elevated for several days. 3. Catastrophic 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, through 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 25/1500Z 26.7N 96.0W 95 KT 110 MPH 12H 26/0000Z 27.6N 96.8W 105 KT 120 MPH 24H 26/1200Z 28.4N 97.3W 90 KT 105 MPH...INLAND 36H 27/0000Z 28.8N 97.5W 70 KT 80 MPH...INLAND 48H 27/1200Z 28.9N 97.6W 55 KT 65 MPH...INLAND 72H 28/1200Z 28.3N 96.8W 35 KT 40 MPH...INLAND 96H 29/1200Z 28.5N 95.5W 40 KT 45 MPH...OVER WATER 120H 30/1200Z 30.0N 94.5W 40 KT 45 MPH...INLAND $$ Forecaster Berg

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