| Track: http://www.nhc.noaa.gov/refresh/graphics_at4+shtml/?cone#contents 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 Hurricane Harvey Advisory Number 21 NWS National Hurricane Center Miami FL AL092017 1000 AM CDT Fri Aug 25 2017 ...OUTER RAINBAND FROM HARVEY SWIPING THE LOWER AND MIDDLE TEXAS COASTS... ...CATASTROPHIC FLOODING EXPECTED ACROSS PORTIONS OF SOUTHERN AND SOUTHEASTERN TEXAS... SUMMARY OF 1000 AM CDT...1500 UTC...INFORMATION ----------------------------------------------- LOCATION...26.7N 96.0W ABOUT 115 MI...185 KM SE OF CORPUS CHRISTI TEXAS ABOUT 120 MI...195 KM SSE OF PORT OCONNOR TEXAS MAXIMUM SUSTAINED WINDS...110 MPH...175 KM/H PRESENT MOVEMENT...NW OR 315 DEGREES AT 10 MPH...17 KM/H MINIMUM CENTRAL PRESSURE...947 MB...27.97 INCHES WATCHES AND WARNINGS -------------------- CHANGES WITH THIS ADVISORY: The Hurricane Watch south of Port Mansfield Texas to the Mouth of the Rio Grande has been discontinued. SUMMARY OF WATCHES AND WARNINGS IN EFFECT: A Storm Surge Warning is in effect for... * Port Mansfield to High Island Texas A Storm Surge Watch is in effect for... * South of Port Mansfield Texas to the Mouth of the Rio Grande A Hurricane Warning is in effect for... * Port Mansfield to Sargent Texas A Tropical Storm Warning is in effect for... * North of Sargent to High Island Texas * South of Port Mansfield Texas to the Mouth of the Rio Grande A Tropical Storm Watch is in effect for... * South of the Mouth of the Rio Grande to Boca de Catan Mexico A Storm Surge Warning means there is a danger of life-threatening inundation from rising water moving inland from the coastline in the indicated locations. For a depiction of areas at risk, please see the National Weather Service Storm Surge Watch/Warning Graphic, available at hurricanes.gov. This is a life-threatening situation. Persons located within these areas should take all necessary actions to protect life and property from rising water and the potential for other dangerous conditions. Promptly follow evacuation and other instructions from local officials. A Hurricane Warning means that hurricane conditions are expected somewhere within the warning area, in this case within the next 12 to 24 hours. Preparations to protect life and property should be rushed to completion. A Storm Surge Watch means there is a possibility of life- threatening inundation from rising water moving inland from the coastline in the indicated locations. Interests in southwestern Louisiana should continue to monitor the progress of this system. For storm information specific to your area in the United States, including possible inland watches and warnings, please monitor products issued by your local National Weather Service forecast office. For storm information specific to your area outside the United States, please monitor products issued by your national meteorological service. DISCUSSION AND 48-HOUR OUTLOOK ------------------------------ At 1000 AM CDT (1500 UTC), the eye of Hurricane Harvey was located by an Air Force Reserve Hurricane Hunter aircraft and NOAA Doppler radar near latitude 26.7 North, longitude 96.0 West. Harvey is moving toward the northwest near 10 mph (17 km/h), but its forward speed is expected to decrease significantly during the next couple of days. On the forecast track, Harvey will make landfall on the middle Texas coast tonight or early Saturday. Harvey is then likely to meander near or just inland of the middle Texas coast through the weekend. Maximum sustained winds are near 110 mph (175 km/h) with higher gusts. Some strengthening is possible, and Harvey is forecast to become a major hurricane before it reaches the middle Texas coast. Hurricane-force winds extend outward up to 35 miles (55 km) from the center, and tropical-storm-force winds extend outward up to 140 miles (220 km). A buoy located about 40 miles east of South Padre Island recently reported sustained winds of 42 mph (68 km/h) and a gust to 54 mph (86 km/h). The minimum central pressure based on data from the Air Force plane is 947 mb (27.97 inches). HAZARDS AFFECTING LAND ---------------------- RAINFALL: Harvey is expected to produce total rain accumulations of 15 to 25 inches and isolated maximum amounts of 35 inches over the middle and upper Texas coast through next Wednesday. During the same time period Harvey is expected to produce total rain accumulations of 5 to 15 inches in far south Texas and the Texas Hill Country over through southwest and central Louisiana. Rainfall of this magnitude will cause catastrophic and life-threatening flooding. STORM SURGE: The combination of a dangerous storm surge and the tide will cause normally dry areas near the coast to be flooded by rising waters moving inland from the shoreline. The water is expected to reach the following heights above ground if the peak surge occurs at the time of high tide... N Entrance Padre Island Natl Seashore to Sargent...6 to 12 ft Sargent to Jamaica Beach...5 to 8 ft Port Mansfield to N Entrance Padre Island Natl Seashore...5 to 7 ft Jamaica Beach to High Island...2 to 4 ft Mouth of the Rio Grande to Port Mansfield...2 to 4 ft High Island to Morgan City...1 to 3 ft The deepest water will occur along the immediate coast near and to the northeast of the landfall location, where the surge will be accompanied by large and destructive waves. Surge-related flooding depends on the relative timing of the surge and the tidal cycle, and can vary greatly over short distances. For information specific to your area, please see products issued by your local National Weather Service forecast office. WIND: All last-minute preparations should be rushed to completion since tropical storm conditions are likely just beginning in portions of the hurricane and tropical storm warning areas. Hurricane conditions are likely to begin within the hurricane warning area later today or tonight. Tropical storm conditions are likely to persist along portions of the coast through at least Sunday. SURF: Swells generated by Harvey are affecting the Texas, Louisiana, and northeast Mexico coasts. These swells are likely to cause life-threatening surf and rip current conditions. Please consult products from your local weather office. TORNADOES: A few tornadoes are possible later today through Saturday near the middle and upper Texas coast. NEXT ADVISORY ------------- Next intermediate advisory at 100 PM CDT. Next complete advisory at 400 PM CDT. $$ Forecaster Berg |