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Tropical Storm Beta Forecast Discussion Number 11
2020-09-20 10:51:27| National Hurricane Center (Atlantic)
Issued at 400 AM CDT Sun Sep 20 2020 000 WTNT42 KNHC 200851 TCDAT2 Tropical Storm Beta Discussion Number 11 NWS National Hurricane Center Miami FL AL222020 400 AM CDT Sun Sep 20 2020 Beta has generally changed little overnight. The storm remains strongly sheared with deep convection confined to the north and northeast of the low-level center. A combination of flight-level, SFMR, and dropsonde data from the Air Force Hurricane Hunters a few hours ago supports keeping the 50-kt initial intensity. 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 was caught in weak steering currents for nearly a day, but it now seems to be moving. The current initial motion estimate is west-northwestward at 3 kt. A mid-level ridge currently centered over the southwestern U.S. is forecast to slide eastward, which should cause Beta to move a little faster to the west-northwestward later today. This motion should continue for a couple of days, taking the storm to the Texas coast on Monday or Monday night. After Beta moves inland, the ridge is forecast to move away as a shortwave trough approaches from the west. This change in the pattern is expected to cause Beta to slow down, or even stall, on Tuesday and then turn northeastward after that. The NHC track forecast lies near the middle of the guidance envelope, and it is fairly similar to the previous prediction. The tropical storm is currently in an environment of about 20 kt of southwesterly wind shear and a fair amount of dry air, especially on the west side of the circulation. Since the shear is not expected to decrease much while Beta moves into an even drier and more stable air mass, little change in intensity is expected through landfall in 36 to 48 hours. After Beta moves inland, steady weakening is forecast due to a combination of land interaction, dry air, and an increase in southwesterly shear. The models are in fair agreement, and this forecast is largely an update of the previous one. 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 20/0900Z 27.0N 92.7W 50 KT 60 MPH 12H 20/1800Z 27.2N 93.4W 50 KT 60 MPH 24H 21/0600Z 27.6N 94.6W 50 KT 60 MPH 36H 21/1800Z 28.2N 95.6W 50 KT 60 MPH 48H 22/0600Z 28.8N 96.2W 45 KT 50 MPH...INLAND 60H 22/1800Z 29.2N 96.2W 35 KT 40 MPH...INLAND 72H 23/0600Z 29.7N 95.5W 30 KT 35 MPH...INLAND 96H 24/0600Z 31.4N 92.9W 25 KT 30 MPH...INLAND 120H 25/0600Z 33.8N 90.2W 20 KT 25 MPH...POST-TROP/INLAND $$ Forecaster Cangialosi
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