1 Sep Sat 2018

Tropical Storm Florence Discussion Number 8
NWS National Hurricane Center Miami FL AL062018
500 AM AST Sat Sep 01 2018

A recent SSMIS microwave pass has revealed that the cyclone has
become much better organized with more pronounced convective
banding. Satellite intensity estimates are wide ranging, spanning
from T1.5/25 kt from SAB to T3.4/53 kt from the UW-CIMSS ADT. Since
the numbers overall have increased, and the structure has improved,
the initial intensity is raised to 35 kt, in closest agreement to
the TAFB estimate, but this could be conservative. This makes the
depression Tropical Storm Florence, the sixth named storm of the
season.

Florence is moving west-northwestward, or 285/12 kt, and should
maintain this motion for the next 48 hours while traveling along the
southern periphery of the subtropical ridge. After 48 hours, the
storm is expected to reach a break in the ridge, causing it to slow
down and turn northwestward by day 5. The biggest change noted
among the track models on this cycle was a westward shift in the
overall guidance envelope. The European model, in particular, swung
significantly to the left, showing a weaker Florence not feeling the
break in the ridge quite as much. I’d like to see this trend
continue before making a significant change to the forecast, so for
now the updated NHC track prediction is only nudged westward. That
said, the models appear to be trending away from a definitive
recurvature scenario.

The environment is mixed with positives and negatives for Florence’s
strengthening. For the first 3 days, vertical shear over Florence
should be quite low, but the cyclone will also be moving through a
less-than-ideal thermodynamic environment with marginal sea surface
temperatures and decreasing mid-level moisture. After 3 days, the
thermodynamic environment should begin to improve, but then the
shear is forecast to strengthen. These conflicting signals point to
only gradual strengthening, and the suite of intensity models aren’t
too far off from one another, nor from the previous NHC forecast.
For that reason, no significant changes were made in this forecast
package.

FORECAST POSITIONS AND MAX WINDS

INIT 01/0900Z 14.5N 26.7W 35 KT 40 MPH
12H 01/1800Z 15.0N 28.7W 40 KT 45 MPH
24H 02/0600Z 15.7N 31.2W 45 KT 50 MPH
36H 02/1800Z 16.4N 33.7W 50 KT 60 MPH
48H 03/0600Z 17.1N 36.3W 55 KT 65 MPH
72H 04/0600Z 18.7N 41.4W 60 KT 70 MPH
96H 05/0600Z 21.0N 45.5W 60 KT 70 MPH
120H 06/0600Z 23.5N 48.5W 55 KT 65 MPH

$$
Forecaster Berg