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Tropical Storm Beta Forecast Discussion Number 13

2020-09-20 22:57:35| National Hurricane Center (Atlantic)

Issued at 400 PM CDT Sun Sep 20 2020 000 WTNT42 KNHC 202057 TCDAT2 Tropical Storm Beta Discussion Number 13 NWS National Hurricane Center Miami FL AL222020 400 PM CDT Sun Sep 20 2020 After a short-lived intense burst of deep convection a couple of hours ago, which helped to spin up a mid-level eye feature in radar imagery, Beta's convection has waned somewhat and the eye feature has become less distinct. Doppler velocity values of 60-65 kt between 15,000-20,000 ft were noted when the vortex column looked its best, but that spin up of the circulation also generated a significant amount of dry air entrainment that is now evident by a pronounced slot wrapping into the center from the north and northeast, which has likely caused the recent decrease in the inner-core convection. An Air Force Reserve reconnaissance aircraft was investigating Beta during the time of the aforementioned convective burst, and the low-level center was located about 18-20 nmi east of the radar eye feature, and the surface dropsonde measured west winds of 39 kt beneath the calm 850-mb center. These data indicate that vortex column possesses a significant amount of vertical tilt, which is not suggestive of an intensifying tropical cyclone. The aircraft found that the central pressure has remained at around 996 mb and also measured an 850-mb flight-level maximum wind of 60 kt, thus the initial intensity is being held at 50 kt. After accounting for the westward jump in the low-level center due to its recent reformation, the initial motion estimate is 295/05 kt. There is no significant change to the previous forecast track or reasoning. Beta is expected to remain within weak steering currents for the next couple of days, caught between a mid-level ridge over Florida and a weaker ridge located over the U.S. Southern Plains. Thereafter, the ridge over Florida become the dominant steering feature by amplifying northward and northwestward across the southeastern U.S. by early Tuesday, nudging Beta northward by late Tuesday, then followed by a faster northeastward motion on Wednesday through Friday. Due to the continued southwest to westerly shear expected to affect the cyclone, which will keep the convection and associated best pressure falls confined to the northeastern and eastern quadrants or near the Texas coast. As the result, the new NHC forecast track lies near the previous advisory track, and it located along the right side of the track guidance envelope, but not as far right as the new GFS-ECMWF (GFEX) consensus model. which keeps Beta over water for the next several days. Excluding the recent weakening of the vertical wind shear, which allowed Beta to undergo that bursting phase, the cyclone is forecast to remain under the influence of 15-20 kt or greater deep-layer shear throughout the forecast period, which is strong enough to keep the cyclone from strengthening much, but not enough to weaken or dissipate the cyclone before landfall occurs in 24 hours or so. Therefore, the intensity is expected to remain steady at 50 kt until landfall, although 5-kt fluctuations could occur which are in the forecast statistical noise. Slower-than-normal weakening for an inland tropical cyclone is expected due to Beta's proximity to the Gulf where brisk onshore flow could bring strong squalls over the Gulf into the coast. By day 3, Beta should weaken fairly quickly into a remnant low since the system will be located much farther and away from the influence of the Gulf of Mexico. The cyclone is expected to degenerate into a remnant low pressure system by early Wednesday and dissipate inland over the lower Mississippi Valley area by late Friday or Saturday. The intensity model guidance remains in good agreement on this developing forecast scenario, so no significant changes were made to the previous intensity forecast. 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 also spread northward into the ArkLaTex region and east into the Lower Mississippi Valley 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 20/2100Z 27.7N 94.0W 50 KT 60 MPH 12H 21/0600Z 28.1N 95.0W 50 KT 60 MPH 24H 21/1800Z 28.6N 96.1W 50 KT 60 MPH 36H 22/0600Z 29.0N 96.7W 45 KT 50 MPH...INLAND 48H 22/1800Z 29.4N 96.4W 40 KT 45 MPH...INLAND 60H 23/0600Z 29.7N 95.7W 35 KT 40 MPH...INLAND 72H 23/1800Z 30.0N 94.7W 30 KT 35 MPH...INLAND 96H 24/1800Z 32.0N 91.8W 20 KT 25 MPH...POST-TROP/INLAND 120H 25/1800Z 34.4N 89.5W 20 KT 25 MPH...POST-TROP/INLAND $$ Forecaster Stewart

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