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Tropical Storm Beta Forecast Discussion Number 10
2020-09-20 04:59:46| National Hurricane Center (Atlantic)
Issued at 1000 PM CDT Sat Sep 19 2020 000 WTNT42 KNHC 200259 TCDAT2 Tropical Storm Beta Discussion Number 10 NWS National Hurricane Center Miami FL AL222020 1000 PM CDT Sat Sep 19 2020 Beta's deep convection has been waning this evening. An Air Force Reserve Hurricane Hunter aircraft has found that the central pressure rose several millibars since this morning, but the plane also still measured surface winds around 50 kt from the SFMR instrument. The strongest winds appear to be located near on old frontal boundary which extends north and east of Beta's center, and in fact sustained tropical-storm-force winds are just grazing the coast of south-central Louisiana. The aircraft fixes indicated that Beta drifted north-northeastward during the day. Right now, the cyclone is embedded within weak steering currents between two areas of high pressure centered near the Bahamas and west Texas/southern New Mexico. The western high pressure area is expected to slide eastward across the Southern Plains into the southeastern U.S. through Sunday night, which should force Beta to move slowly toward the west-northwest toward the Texas coast. The center is now expected to reach the coast between 48-60 hours, at which point it is likely to recurve around the mid-level high and move slowly northeastward near or inland of the upper Texas coast on days 3 and 4. Because of Beta's drift today, the new guidance envelope has shifted a bit northward. For this forecast cycle, the NHC official forecast split the difference between the previous interpolated forecast and the HFIP Corrected Consensus guidance. This keeps the forecast to the east of the ECMWF, HCCA, and TVCN multi-model consensus during Beta's slow recurvature. The air mass behind the old front appears to have won out. Beta's circulation is embedded in a dry environment of mid-level relative humidities around 50 percent, and the air mass could actually become more stable as Beta moves closer to the Texas coast. On top of that, the moderate southwesterly shear is not expected to diminish much at all. As a result, the intensity guidance has continued to trend downward, and the new official forecast now flatlines Beta's intensity at 50 kt until landfall soon after 48 hours. This forecast still lies near the top end of the guidance envelope, closest to the SHIPS model. After 48 hours, weakening is expected assuming Beta's center remains over land, and it is now expected to become a remnant low over Louisiana by day 5. While the chances that Beta will become a hurricane continue to decrease, the Hurricane Watch for portions of the coast of Texas are being maintained out of an abundance of caution given the uncertainty in the forecast. Key Messages: 1. The expected slow motion of Beta has the potential to expand a long duration rainfall event from the Louisiana coast westward into southeast Texas on Sunday and northward into the Mid-South by mid next week. The potentially prolonged period of rainfall could cause flash, urban, and river flooding, especially where tide levels are above normal. 2. There is the danger of life-threatening storm surge near times of high tide from Sunday 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 expected to begin along portions of the northwestern Gulf Coast overnight within the tropical storm warning area in Louisiana and will spread westward to the Texas coast Sunday through Monday. Hurricane-force winds are possible along portions of the Texas coast late Monday and Monday night, where a hurricane watch is in effect. FORECAST POSITIONS AND MAX WINDS INIT 20/0300Z 26.8N 92.2W 50 KT 60 MPH 12H 20/1200Z 27.1N 93.0W 50 KT 60 MPH 24H 21/0000Z 27.4N 94.1W 50 KT 60 MPH 36H 21/1200Z 27.9N 95.2W 50 KT 60 MPH 48H 22/0000Z 28.4N 96.0W 50 KT 60 MPH 60H 22/1200Z 28.9N 96.3W 40 KT 45 MPH...INLAND 72H 23/0000Z 29.3N 96.0W 35 KT 40 MPH...INLAND 96H 24/0000Z 30.2N 94.1W 30 KT 35 MPH...INLAND 120H 25/0000Z 31.4N 92.1W 25 KT 30 MPH...POST-TROP/INLAND $$ Forecaster Berg
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