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Tropical Storm Beta Forecast Discussion Number 8
2020-09-19 17:38:30| National Hurricane Center (Atlantic)
Issued at 1000 AM CDT Sat Sep 19 2020 000 WTNT42 KNHC 191538 TCDAT2 Tropical Storm Beta Discussion Number 8...Corrected NWS National Hurricane Center Miami FL AL222020 1000 AM CDT Sat Sep 19 2020 Corrected 96 and 120 h intensities Morning visible imagery indicates that Beta has become significantly sheared, with the low-level center mostly exposed well to the southwest of the main convective area. A new convective band is currently forming near the center over the northwestern quadrant. An Air Force Reserve Hurricane Hunter aircraft reported pressures near 994-995 mb, along with flight-level and SFMR winds that justify keeping the initial intensity at 50 kt. The storm appears to be starting its westward turn, and the initial motion is now 315/3. The mid- to upper-level trough over the northwestern Gulf of Mexico is lifting out, with a mid-level ridge building to the north of Beta. This should cause the storm to turn more westward during the next 6-12 h. After that, a slow west- northwestward to northwestward motion should develop, with the center now forecast to be near or over the middle Texas coast in about 72 h. Beyond 72 h, another mid-latitude trough moving through the central United States should cause Beta to recurve slowly to the northeast. The new NHC forecast is shifted a little to the west and now explicitly shows landfall on the Texas coast in about 72 h. This new forecast is a little to the north of the consensus models through 72 h and close to them after that time. There are lots of uncertainties in the intensity forecast. First, the global models suggest that the ongoing southwesterly shear may not subside that much even though the first trough is lifting out. Second, GOES-16 air mass imagery shows abundant upper-level dry air over the western Gulf of Mexico, including near the storm center. Third, surface observations show a drier low-level airmass in place over southeastern Texas, and some of this may get entrained into the storm. The intensity guidance has trended downward to the point where none of the models are currently forecasting Beta to become a hurricane. Based on this and the current storm structure, the intensity forecast has been adjusted to show little change in strength today, followed by slow strengthening to a hurricane by 48-60 h. However, this forecast is well above the guidance, and additional downward adjustments may be needed later today. A Tropical Storm Warning is being issued for parts of the Texas and Louisiana coasts at this time. A Hurricane Warning could still be issued later today or tonight depending on later intensity forecasts. Key Messages: 1. The expected slow motion of Beta has the potential to produce a long duration rainfall event along the western Gulf Coast. The potentially prolonged period of rainfall could cause flash, urban, and river flooding, especially where tide levels are above normal. 2. Life-threatening storm surge and hurricane-force winds are possible along portions of the northwestern Gulf Coast, with tropical storm conditions expected by late this weekend. Storm Surge and Hurricane watches and Tropical Storm warnings are in effect, and residents in these areas should ensure they have their hurricane plan in place and follow advice given by local officials. FORECAST POSITIONS AND MAX WINDS INIT 19/1500Z 26.6N 92.6W 50 KT 60 MPH 12H 20/0000Z 26.9N 93.0W 50 KT 60 MPH 24H 20/1200Z 27.1N 93.8W 55 KT 65 MPH 36H 21/0000Z 27.5N 94.8W 60 KT 70 MPH 48H 21/1200Z 27.8N 95.5W 65 KT 75 MPH 60H 22/0000Z 28.2N 96.2W 65 KT 75 MPH 72H 22/1200Z 28.6N 96.2W 55 KT 65 MPH...INLAND 96H 23/1200Z 29.5N 95.0W 45 KT 50 MPH...INLAND 120H 24/1200Z 30.5N 93.0W 30 KT 35 MPH...INLAND $$ Forecaster Beven
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