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Tropical Storm Beta Forecast Discussion Number 17
2020-09-21 22:42:53| National Hurricane Center (Atlantic)
Issued at 400 PM CDT Mon Sep 21 2020 000 WTNT42 KNHC 212042 TCDAT2 Tropical Storm Beta Discussion Number 17 NWS National Hurricane Center Miami FL AL222020 400 PM CDT Mon Sep 21 2020 Beta's convective cloud structure has continued to erode since the previous advisory as cold-air stratocumulus clouds have wrapped around the entire and into the system center. Most of the cloud tops are barely reaching the freezing level, with the exception of a small convective burst that has recently developed near and to the northwest of the low-level center. The initial intensity has been lowered to 40 kt and is based on data from the last Air Force Reserve reconnaissance leg that indicated peak SFMR surface winds of 40-42 kt northwest of the center and a dropsonde-measured central pressure of 999-1000 mb. The initial motion estimate 310/04 kt. Beta is expected to move onshore the central Texas coast later tonight, and then stalling along or just inland from the coast during the 12-24 hour period when the steering currents collapse due to a complete break down of a weak ridge over the Gulf of Mexico. A weak trough to the west of the cyclone is then forecast to nudge Beta east-northeastward toward the Gulf of Mexico in the 36-60 hour period, with the cyclone possibly reaching the warm Gulf waters by 48 hours. By 72 hours and beyond, the approaching mid-level trough Beta is expected to move Beta a little faster toward the northeast until the cyclone dissipates over the Lower Mississippi Valley area by day 5. The latest NHC track guidance has shifted a little farther to right or east of the previous forecast track, with most of the models now taking Beta back out over the western Gulf of Mexico by 24 hours. As a result, the new NHC official track forecast has been nudged a little farther to the right of the previous, but remains to the left or west of the NOAA-HCCA consensus model and the UKMET model. West-southwesterly wind shear of 15-20 kt is expected to affect Beta for the next 36 hours or so, followed by a gradual increase in the shear thereafter. That unfavorable flow regime, along with land interaction, should induce a slow weakening trend throughout the forecast period. The NHC intensity forecast remains a little above the available model guidance through 48 hours since Beta is forecast to remain very close to or over the Gulf of Mexico where convective rain bands containing tropical-storm-force winds could possibly move onshore the central and upper Texas coastal areas. Key Messages: 1. The expected slow motion of Beta will produce a long duration rainfall event from the middle Texas coast to southeast Louisiana. Flash, urban, and minor river flooding is likely. Rainfall will also spread northward into the ArkLaTex region and east into the Lower Mississippi Valley and portions of the Southeast through the end of the week. Flash, urban, and isolated minor 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 will spread westward across the Texas coast later this evening and continue into Tuesday. FORECAST POSITIONS AND MAX WINDS INIT 21/2100Z 28.2N 96.1W 40 KT 45 MPH 12H 22/0600Z 28.5N 96.6W 35 KT 40 MPH...INLAND 24H 22/1800Z 28.7N 96.5W 35 KT 40 MPH...INLAND 36H 23/0600Z 28.9N 95.8W 35 KT 40 MPH...NEAR TEXAS COAST 48H 23/1800Z 29.2N 94.7W 30 KT 35 MPH...OVER WATER 60H 24/0600Z 29.7N 93.3W 25 KT 30 MPH...OVER WATER 72H 24/1800Z 30.8N 91.6W 20 KT 25 MPH...INLAND 96H 25/1800Z 34.1N 88.7W 20 KT 25 MPH...POST-TROP/INLAND 120H 26/1800Z...DISSIPATED INLAND $$ Forecaster Stewart
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