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Tropical Storm Beta Forecast Discussion Number 12
2020-09-20 16:59:30| National Hurricane Center (Atlantic)
Issued at 1000 AM CDT Sun Sep 20 2020 000 WTNT42 KNHC 201459 TCDAT2 Tropical Storm Beta Discussion Number 12 NWS National Hurricane Center Miami FL AL222020 1000 AM CDT Sun Sep 20 2020 Beta remains a sheared tropical cyclone whose internal structure and convective pattern remained unchanged from the previous advisory, and essentially unchanged over the past 24 hours. The cyclone is going through another bursting phase with the strongest convection displaced into the northeastern quadrant. An Air Force Reserve reconnaissance aircraft investigating Beta this morning has found maximum 850-mb flight-level winds of 61 kt in some rather vigorous thunderstorms in the north of the center, along with believable SFMR surface winds of 45-47 kt in the northeastern quadrant where 45-kt winds were reported by ship KGSG at 0800 UTC. The aircraft also found that the central pressure was down a little bit to 996 mb. Based on these data, the initial intensity is being maintained at 50 kt. It should be noted that the wind field is quite asymmetric with the strongest winds located in the deep convection and farther to the northeast behind an old frontal boundary. Beta remains trapped in weak steering currents and the initial motion is still quite slow at 300/03 kt. Beta is forecast to remain embedded in weak steering currents for the next 48 h or so, caught between a mid-level ridge located over Florida and another ridge situated over the U.S. Southern Plains. Thereafter, the ridge over the Southern Plains if expected to break down while the ridge over Florida amplifies northward and westward across the southern U.S., resulting in a very gradual increase in forward speed toward the north by late Tuesday and then toward the northeast on Wednesday. Beta is forecast to degenerate into a remnant low pressure system by early Wednesday and dissipate inland over the lower Mississippi by late Friday or Saturday. Due to the continued southwest to westerly shear expected to affect Beta, which will keep the convection confined to the northeastern and eastern quadrants, the official forecast track is located along the eastern or right side of the track guidance envelope, and is the right of all of the consensus aids, toward the middle-to-upper Texas coast. Beta is expected to remain under the influence of 15-20 kt of deep layer vertical wind shear, which is enough to keep the cyclone from strengthening much, if any, but not enough to weaken or dissipate the cyclone before it makes landfall. As a result, the intensity is forecast to remain steady at 50 kt until landfall, followed by slower-than-normal weakening for an inland tropical cyclone due to its expected proximity to the Gulf where onshore rainbands could brings higher squalls along the coast. By 72 h, Beta is forecast to weaken fairly quickly into a remnant low since the system will be much farther inland by that time. The intensity model guidance remain in decent agreement, so the new NHC intensity forecast is identical to the previous advisory and is similar to the HCCA consensus model. Key Messages: 1. The expected slow motion of Beta will produce a long duration rainfall event from the middle Texas coast to southern Louisiana. Flash, urban, and river flooding is likely. Rainfall will then spread northward into the lower Mississippi River Valley by mid-week where flash, urban, and river flooding is possible. 2. There is the danger of life-threatening storm surge near times of high tide through Tuesday along portions of the Texas and Louisiana coasts within the storm surge warning areas. Residents in these areas should follow advice given by local officials. 3. Tropical-storm-force winds are occurring along portions of the northwestern Gulf Coast within the tropical storm warning area in Louisiana. These winds will spread westward to the Texas coast later today and Monday. FORECAST POSITIONS AND MAX WINDS INIT 20/1500Z 27.2N 93.0W 50 KT 60 MPH 12H 21/0000Z 27.6N 93.9W 50 KT 60 MPH 24H 21/1200Z 28.1N 95.0W 50 KT 60 MPH 36H 22/0000Z 28.6N 95.8W 50 KT 60 MPH 48H 22/1200Z 29.0N 96.0W 45 KT 50 MPH...INLAND 60H 23/0000Z 29.6N 95.7W 40 KT 45 MPH...INLAND 72H 23/1200Z 30.2N 94.7W 30 KT 35 MPH...INLAND 96H 24/1200Z 32.1N 92.1W 20 KT 25 MPH...POST-TROP/INLAND 120H 25/1200Z 34.4N 89.4W 20 KT 25 MPH...POST-TROP/INLAND $$ Forecaster Stewart
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