| 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 Tropical Storm Beta Advisory Number 13 NWS National Hurricane Center Miami FL AL222020 400 PM CDT Sun Sep 20 2020 ...BETA MOVING A LITTLE FASTER TOWARD THE CENTRAL TEXAS COAST... ...OUTER RAINBANDS SPREADING FARTHER INLAND OVER THE TEXAS COASTAL PLAIN... SUMMARY OF 400 PM CDT...2100 UTC...INFORMATION ---------------------------------------------- LOCATION...27.7N 94.0W ABOUT 120 MI...195 KM SSE OF GALVESTON TEXAS ABOUT 155 MI...250 KM ESE OF PORT OCONNOR TEXAS MAXIMUM SUSTAINED WINDS...60 MPH...95 KM/H PRESENT MOVEMENT...WNW OR 295 DEGREES AT 6 MPH...9 KM/H MINIMUM CENTRAL PRESSURE...996 MB...29.42 INCHES WATCHES AND WARNINGS -------------------- CHANGES WITH THIS ADVISORY: None. SUMMARY OF WATCHES AND WARNINGS IN EFFECT: A Storm Surge Warning is in effect for... * Port Aransas, Texas to Rockefeller Wildlife Refuge, Louisiana, including Copano Bay, Aransas Bay, San Antonio Bay, Matagorda Bay, Galveston Bay, Sabine Lake, and Lake Calcasieu A Tropical Storm Warning is in effect for... * Port Aransas Texas to Morgan City Louisiana A Tropical Storm Watch is in effect for... * Baffin Bay to Port Aransas Texas A Storm Surge Warning means there is a danger of life-threatening inundation, from rising water moving inland from the coastline, during the next 36 hours in the indicated locations. For a depiction of areas at risk, please see the National Weather Service Storm Surge Watch/Warning Graphic, available at hurricanes.gov. This is a life-threatening situation. Persons located within these areas should take all necessary actions to protect life and property from rising water and the potential for other dangerous conditions. Promptly follow evacuation and other instructions from local officials. A Tropical Storm Warning means that tropical storm conditions are expected somewhere within the warning area within 36 hours. A Tropical Storm Watch means that tropical storm conditions are possible within the watch area, in this case within the next 36 hours. For storm information specific to your area, including possible inland watches and warnings, please monitor products issued by your local National Weather Service forecast office. DISCUSSION AND OUTLOOK ---------------------- At 400 PM CDT (2100 UTC), the center of Tropical Storm Beta was located by an Air Force Reserve reconnaissance aircraft and NOAA Doppler weather radars near latitude 27.7 North, longitude 94.0 West. Beta is moving toward the west-northwest near 6 mph (9 km/h), and this general motion is forecast to continue for the next day or so. A decrease in forward speed and a sharp turn to the north and northeast is expected Monday night and Tuesday. On the forecast track, the center of Beta will continue to move toward the central coast of Texas and will likely move inland by Monday night, and remain close to the coast of southeastern Texas on Tuesday. Data from the reconnaissance aircraft indicate that maximum sustained winds are near 60 mph (95 km/h) with higher gusts. Little change in strength is forecast during the next couple of days before Beta reaches the Texas coast. Weakening is anticipated once Beta moves inland. Tropical-storm-force winds extend outward up to 195 miles (315 km) from the center. The estimated minimum central pressure based on data from the Hurricane Hunter aircraft is 996 mb (29.42 inches). HAZARDS AFFECTING LAND ---------------------- Key messages for Beta can be found in the Tropical Cyclone Discussion under AWIPS header MIATCDAT2 and WMO header WTNT42 KNHC. STORM SURGE: The combination of a dangerous storm surge and the tide will cause normally dry areas near the coast to be flooded by rising waters moving inland from the shoreline. The water could reach the following heights above ground somewhere in the indicated areas if the peak surge occurs at the time of high tide... San Luis Pass, TX to Sabine Pass, TX including Galveston Bay...3-5 ft Port Aransas, TX to San Luis Pass, TX including Copano Bay, Aransas Bay, San Antonio Bay, and Matagorda Bay2-4 ft Sabine Pass, TX to Rockefeller Wildlife Refuge, LA including Sabine Lake and Calcasieu Lake2-4 ft Rockefeller Wildlife Refuge, LA to Ocean Springs, MS including Vermilion Bay, Lake Borgne, Lake Pontchartrain, and Lake Maurepas...1-3 ft Baffin Bay, TX to Port Aransas, TX including Corpus Christi Bay and Baffin Bay... 1-3 ft Mouth of the Rio Grande to Baffin Bay, TX...1-2 ft The deepest water will occur along the immediate coast in areas of onshore winds, where the surge will be accompanied by large and dangerous waves. Surge-related flooding depends on the relative timing of the surge and the tidal cycle, and can vary greatly over short distances. For information specific to your area, please see products issued by your local National Weather Service forecast office. WIND: Tropical storm conditions are occurring in the tropical storm warning area along the southwestern Louisiana coast and will spread westward to the warning areas in Texas tonight through early Monday. Tropical storm conditions are possible within the tropical storm watch area along the south Texas coast on Monday. RAINFALL: Through Thursday, Beta is expected to produce rainfall accumulations of 6 to 12 inches with isolated totals of 15 inches from the middle Texas coast to southern Louisiana. Rainfall totals of 3 to 5 inches expected northward into the ArkLaTex region and east into the Lower Mississippi Valley. Flash and urban flooding is likely, as well as minor to isolated moderate river flooding. TORNADOES: A tornado or two could occur Monday near the middle-to-upper Texas coast or the southwestern Louisiana coast. SURF: Swells are increasing and reaching the coast of Texas and the Gulf Coast of Mexico, generated by a combination of Beta and a cold front entering the northern Gulf of Mexico. These swells are likely to cause life-threatening surf and rip current conditions. Please consult products from your local weather office. NEXT ADVISORY ------------- Next intermediate advisory at 700 PM CDT. Next complete advisory at 1000 PM CDT. $$ Forecaster Stewart |