re: Hurricane Florence - Catastrophic Flooding Potential

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NWS National Hurricane Center Miami FL
AL062018
1100 PM AST
Fri Sep 07 2018

Florence remains a sheared tropical cyclone. Satellite images indicate that the low-level center is partially exposed on the southwesterly edge of a large convective mass, with the overall circulation somewhat elongated from southwest to northeast. A blend of the latest Dvorak wind speed estimates from TAFB/SAB and the CIMSS SATCON gives a value of 50 kt for this advisory. While the winds at 200 mb are already from an easterly direction near the center of Florence, there is significant shear from northwesterly winds from 300-500 mb, undercutting the outflow layer. This shear is forecast to relax by the global models over the next 36 hours as an anticyclone builds to the north of the storm, which should promote some strengthening by Sunday. After 48 hours, the deep-layer flow becomes easterly near the cyclone, with very little shear while the system is over very warm waters. This pattern favors significant intensification, and most of the guidance brings Florence back to a category 4 hurricane in 4 or 5 days. The intensity forecast is very similar to the last one, and is raised slightly at days 3 and 4 to come into better agreement with the guidance. It is interesting to note that even with a lower initial intensity, the guidance is higher than the last cycle, which speaks to the strength of the signal for intensification in the long range. The initial motion estimate is 265 degrees at 6 kt. Florence is expected to continue moving slowly westward for the next 48 hours under the influence of a weak mid-level ridge over the western Atlantic. By days 4 and 5, an exceptionally strong blocking ridge is forecast to develop between Bermuda and the Northeast U.S. and build westward, keeping Florence on a west-northwestward trajectory with a notable increase in forward speed by the end of the forecast period. It feels like a broken record to mention that the overall guidance envelope keeps shifting southwestward, and the official forecast is moved in that direction. Unfortunately with such a large well-defined steering current from the ridge becoming likely, the extended-range risk to the United States keeps rising, which is confirmed by the majority of the latest ensemble guidance.

Key Messages:

1. Regardless of Florence's eventual track, large swells are affecting Bermuda and will begin to affect portions of the U.S. East Coast this weekend, resulting in life-threatening surf and rip currents.

2. The risk of other direct impacts associated with Florence along the U.S. East Coast next week has increased. However, there is still very large uncertainty in model forecasts of Florence's track beyond day 5, making it too soon to determine the exact location, magnitude, and timing of these impacts. Interests near and along the U.S. East Coast should monitor the progress of Florence through the weekend and ensure they have their hurricane plans in place.

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Tropical Depression Nine Discussion Number 2
NWS National Hurricane Center Miami FL
AL092018
1100 PM AST
Fri Sep 07 2018

The depression continues to be impeded by moderate easterly shear with the center of circulation decoupled to the east of a deep convective banding feature. The initial intensity is held at 30 kt based on subjective intensity estimates from TAFB and SAB. The dynamical-statistical intensity guidance, as well as the large-scale models, indicate that the shear should diminish to less than 10 kt within the next 24 hours. A more favorable upper-wind pattern along with warm oceanic temperatures is expected to promote steady strengthening commencing after that time. The intensity forecast is based on a blend of the IVCN and the HCCA consensus models and remains below the stronger HWRF at days 4 and 5.

The depression has been meandering during the past several hours, as a result of weak low- to mid-level steering currents created by a deep-layer trough digging southward over the eastern Atlantic. In 12 to 24 hours, a westward motion with a gradual increase in forward speed is forecast thereafter as the trough lifts northeast, and the subtropical ridge re-establishes to the north of the cyclone. The NHC track forecast lies closest to the HCCA and TVCN guidance and takes the system toward the Lesser Antilles by the end of the forecast period.

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