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Hurricane Hanna Forecast Discussion Number 11
2020-07-25 16:58:29| National Hurricane Center (Atlantic)
Issued at 1000 AM CDT Sat Jul 25 2020 000 WTNT43 KNHC 251458 TCDAT3 Hurricane Hanna Discussion Number 11 NWS National Hurricane Center Miami FL AL082020 1000 AM CDT Sat Jul 25 2020 NOAA Doppler radar data from Corpus Christi and Brownsville, Texas, along with reconnaissance data from the NOAA Hurricane Hunters indicate that Hanna has continued to strengthen this morning. A well-defined but fairly large 30-35-nmi-wide eye has formed and recent dropsonde data from the eye indicate that the central pressure has dropped to 978 mb. The aircraft measured a peak 700-mb flight-level wind speed of 77 kt, which equates to about 69 kt at the surface. During the past hour, Doppler velocities have sharply increased with numerous patches of 85-90 kt between 7000-8000 ft common in the northern eyewall. Although these values would typically correspond to surface winds of about 75 kt, the Doppler velocity are occurring in low reflectivity regions of 25-30 dBZ, so the full effect of those winds are likely not reaching the surface. This could be why the highest SFMR surface wind observed by the aircraft was only 63 kt. The initial intensity has been increased to 70 kt, which is a blend of the aircraft flight-level, radar, and SFMR surface wind speed estimates. The center position is a tad north of the recon fixes due to some southward tilt of the eye caused by northerly shear. This has resulted in a slower westward motion of 270/06 kt. The ridge to the north of the hurricane has been slowly building westward and southwestward based on upper-air data over the past 24 hours. This slow ridging pattern is expected to continue for the next 48 hours, resulting in Hanna gradually turning toward the west-southwest by late this afternoon or evening, with landfall occurring in about 12 h along south Texas coast. After landfall, Hanna should continue its west-southwestward motion until dissipation over the mountainous terrain of northeastern Mexico in 48-60 h. The new NHC track forecast is similar to the previous advisory track, and lies close to the tightly packed consensus model guidance. Although the deep-layer vertical wind shear is expected to remain out of the north-northwest to north at about 20 kt, the relatively large and stable eye, along with the expected convective vigor of the hurricane, could result in a little more strengthening just before landfall occurs. After landfall, rapid weakening of the wind field is expected. However, the weakening peak winds will have no negative effect on the likelihood for heavy rainfall or the possibility of isolated tornadoes developing. Key Messages 1. Life-threatening storm surge is occurring along portions of the Texas coast from Port Mansfield to Sargent, where a Storm Surge Warning is in effect. Residents in these locations should follow advice given by local emergency officials. 2. Hurricane conditions are expected along the Texas coast from Port Mansfield to Mesquite Bay, where a Hurricane Warning is in effect. Tropical storm conditions are expected to first reach the coast within the warning area this morning. 3. Hanna is expected to produce heavy rains across portions of southern Texas and northeastern Mexico. These rains could result in life-threatening flash flooding and isolated minor to moderate river flooding. FORECAST POSITIONS AND MAX WINDS INIT 25/1500Z 27.1N 96.3W 70 KT 80 MPH 12H 26/0000Z 26.9N 97.6W 70 KT 80 MPH...INLAND 24H 26/1200Z 26.4N 99.1W 45 KT 50 MPH...INLAND 36H 27/0000Z 25.9N 100.6W 25 KT 30 MPH...INLAND 48H 27/1200Z 25.3N 102.1W 15 KT 15 MPH...POST-TROP/REMNT LOW 60H 28/0000Z...DISSIPATED $$ Forecaster Stewart
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