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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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