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Tropical Storm Laura Forecast Discussion Number 18
2020-08-24 10:57:45| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Mon Aug 24 2020 000 WTNT43 KNHC 240857 TCDAT3 Tropical Storm Laura Discussion Number 18 NWS National Hurricane Center Miami FL AL132020 500 AM EDT Mon Aug 24 2020 Satellite data indicate that Laura has moved back over the warm waters of the Caribbean Sea south of Cuba. Modest northerly vertical wind shear has kept the bulk of the deep convection displaced into the southern semicircle of Laura's circulation. Earlier ASCAT-B/-C scatterometer passes around 0210Z-0250Z revealed a small circulation just offshore southeastern Cuba. However, this feature is considered to be a leeside vortex, possibly having developed in response to the long-fetch southerly low-level flow passing over Jamaica, and not the primary low-level center associated with Laura. The initial intensity is being maintained at 55 kt based on 43-46 kt wind vectors located just offshore southeastern Cuba that were present in the aforementioned ASCAT data, and allowing for some undersampling by the scatterometer instrument. The initial motion estimate is 290/18 kt. The deep-layer Bermuda-Azores ridge to the north of the cyclone is forecast by all of the models to build and expand westward across the Bahamas, Florida, and into the eastern and central Gulf of Mexico over the new few days. This dominant steering flow pattern is expected to keep Laura moving west-northwestward just offshore the southern coast of Cuba today, followed by the cyclone emerging over the southeastern Gulf of Mexico on Tuesday. By late Tuesday and early Wednesday, a mid- to upper-level trough located over the south-central U.S. is forecast to pinch off and dig southwestward and merge with the remnants of Tropical Storm Marco over the western Gulf of Mexico, eroding the western extent of the ridge in the process. This is expected to cause Laura to slow down and gradually turn northwestward and then northward toward the northwestern Gulf coast by late Wednesday and early Thursday. The latest NHC model guidance is in excellent agreement on this developing synoptic pattern, and the official forecast is similar to but slightly left or south of the previous advisory track to account for the expected northerly to northeasterly wind shear keeping the convection and low-level center displaced to the south for the next 24 hours or so. On the forecast track, Laura is expected to remain just offshore the southern Cuba today, move into the Gulf of Mexico by early Tuesday morning, and approach the U..S northwest Gulf coast area Wednesday night and early Thursday. Since Laura's center is expected to remain over water just south of Cuba, there is the possibility that some slight strengthening could occur today or tonight before the cyclone reaches the Gulf of Mexico in 24 hours. After that, Laura will be moving over the very warm and deep waters of Gulf Stream and Loop Current located over the southeastern Gulf, which could trigger a brief period of rapid intensification. The GFS and ECMWF models, along with the statistical and corrected-consensus models, only strengthen the cyclone to a peak intensity around 75 kt. In contrast, the HRWF and HMON models bring Laura to major hurricane strength by 60 hours. Given the very favorable environmental conditions of high SSTs near 31 deg C and low vertical shear values less than 10 kt after 24-36 hours, subsequent intensity forecasts might have to trend more toward the regional models. But for now, the official intensity forecast will continue to follow a blend of the regional and global model intensity forecasts, and lies at the high-end of the intensity guidance at 60 and 72 hours. Users are again reminded to not to focus on the exact details of the track or intensity forecasts as the average NHC track error at 72 h is around 100 miles and the average intensity error is around 15 mph (13 kt). In addition, winds, storm surge, and rainfall hazards will extend far from the center. Key Messages: 1. Tropical storm conditions are expected across much of Cuba today. Heavy rainfall is likely across Cuba and Jamaica today, and these rains could cause mudslides and life-threatening flash and urban flooding. Tropical storm conditions are expected in the Dry Tortugas, and the Middle and Lower Florida Keys later today. 2. While the details of the long-range track and intensity forecasts remain uncertain, Laura is forecast to strengthen over the Gulf of Mexico and there is an increasing risk of dangerous storm surge, wind, and rainfall impacts along portions of the U.S. Gulf Coast by the middle of the week. This could result in a prolonged period of hazardous weather for areas that are likely to be affected by Marco. Interests along the Gulf Coast should monitor the progress of Laura and Marco and updates to the forecast during the next couple of days. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 20.8N 78.9W 55 KT 65 MPH 12H 24/1800Z 21.7N 81.5W 55 KT 65 MPH 24H 25/0600Z 22.9N 84.6W 60 KT 70 MPH 36H 25/1800Z 24.5N 87.6W 65 KT 75 MPH 48H 26/0600Z 26.1N 90.2W 75 KT 85 MPH 60H 26/1800Z 28.0N 92.3W 85 KT 100 MPH 72H 27/0600Z 29.8N 93.3W 90 KT 105 MPH 96H 28/0600Z 35.4N 91.5W 35 KT 40 MPH...INLAND 120H 29/0600Z 37.6N 82.1W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Stewart
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