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Tropical Storm Hanna Forecast Discussion Number 7
2020-07-24 16:59:42| National Hurricane Center (Atlantic)
Issued at 1000 AM CDT Fri Jul 24 2020 000 WTNT43 KNHC 241459 TCDAT3 Tropical Storm Hanna Discussion Number 7 NWS National Hurricane Center Miami FL AL082020 1000 AM CDT Fri Jul 24 2020 Reports from a NOAA reconnaissance aircraft this morning and nearby ship D5DY4 indicate that Hanna has strengthened a little. However, the aircraft data also showed that Hanna's center had moved or reformed a little farther north near the northern edge of the convective cloud mass. A partial SSMI/S pass around 1231Z suggested that a mid-level eye feature could be forming, but it also possible that a dry slot may be intruding into the cloud shield from the northwest and west. The initial intensity of 40 kt is based on aircraft SFMR surface winds of 38-40 kt, and the 1200Z D5DY5 ship report of 48 kt at 89 meters elevation, which reduces to a 38-kt 10-meter wind speed. Even with the earlier northwestward jump in the center position, reconnaissance and microwave satellite data indicate that Hanna's motion is still west-northwestward or 285/08 kt. There remains no significant change to the previous official track forecast or reasonings over the past couple of days. The latest NHC model guidance remains in good agreement that mid-level ridge will build to the north and northwest of Hanna over the next couple of days, resulting in the cyclone turning westward by tonight and on Saturday. It should then turn west-southwestward Saturday night and Sunday. The new NHC forecast track continues to show the center making landfall along the south-central coast of Texas within the tropical storm warning area Saturday afternoon or evening, which is in good agreement with the various consensus models. Hanna's convective cloud shield remains very asymmetrical with the bulk of the convection confined to the southern semicircle despite the otherwise symmetrical and expanding upper-level outflow pattern. More recently, some deep convection has developed near the center and the aforementioned possible mid-level eye feature. Hanna is forecast to remain in relatively low vertical wind shear regime and over SSTs of 30C or more, a combination that typically favors significant intensification. However, nearby dry mid-level air noted in 1200Z soundings from Corpus Christi and Brownsville has been eroding and preventing convection from developing in the northwest quadrant and near the center, which has inhibited strengthening over the past couple of days despite the low shear conditions. The latest global model guidance shows the dry air mixing out in about 24 h just prior to landfall, which should allow for at least gradual strengthening until landfall occurs in about 30 h or so. However, if an eyewall forms during the next 12 h, then it is possible that Hanna could be near 60 kt when it makes landfall. The new NHC intensity forecast is similar to the previous advisory, and is a blend of the IVCN and HCCA consensus intensity forecasts. Key Messages 1. Hanna is forecast to strengthen and it is expected to bring tropical-storm-force winds to portions of the Texas coast, where a tropical storm warning is in effect. 2. Hanna is expected to produce heavy rains across portions of southern Texas. These rains could result in flash flooding and isolated minor to moderate river flooding. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 27.2N 93.2W 40 KT 45 MPH 12H 25/0000Z 27.5N 94.6W 45 KT 50 MPH 24H 25/1200Z 27.4N 96.2W 55 KT 65 MPH 36H 26/0000Z 27.3N 97.6W 50 KT 60 MPH...INLAND 48H 26/1200Z 27.0N 99.0W 35 KT 40 MPH...INLAND 60H 27/0000Z 26.3N 100.4W 25 KT 30 MPH...INLAND 72H 27/1200Z 25.7N 101.6W 15 KT 15 MPH...POST-TROP/REMNT LOW 96H 28/1200Z...DISSIPATED $$ Forecaster Stewart
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