6 Sep Thu 2018

FLORENCE WEAKENING FOR NOW BUT STILL A STRONG HURRICANE

Hurricane Florence Discussion Number 28
NWS National Hurricane Center Miami FL AL062018
500 AM AST Thu Sep 06 2018

Southwesterly shear continues to affect Florence. Since the last
advisory, the cloud pattern has become more asymmetric, and cloud
tops surrounding the ragged eye have warmed. AMSR imagery around
0430 UTC indicated that the hurricane’s eye is tilted slightly
southwest to northeast with height, but the low- to mid-level
inner-core was mostly intact. Based on an average of Final-T and
CI numbers from TAFB and SAB, the initial intensity has been lowered
to 100 kt, and objective estimates are even lower.

Given the wind shear and current appearance of Florence, additional
weakening in the short-term seems likely, and all of the intensity
guidance agrees. From 24-72 h, the spread increases with the
statistical models generally showing continued gradual weakening for
another day or two, while the dynamical models show little change or
slight restrengthening. By day 5, all of the guidance calls for
Florence to restrengthen. The guidance envelope and consensus is
lower with this model cycle, but the NHC intensity forecast has only
been adjusted slightly lower, mainly in the first 72 h of the
forecast. The official forecast is now a little above the IVCN
intensity consensus and HCCA at most forecast hours, generally
favoring the stronger solution of the dynamical models.

Florence has continued to track northwestward, and the initial
motion is 315/10 kt. The cyclone is still forecast to gradually
turn westward over the next 48 h, in response to a building
mid-level ridge to its north. Beyond that time, a mid-latitude
trough over the northwestern Atlantic could create enough of a
weakness in the ridge to steer the hurricane farther north, closer
to Bermuda, as shown by the latest GFS. However, the ECMWF and
UKMET suggest that the ridge will not be significantly affected, and
Florence will move more westward. The ensembles from the GFS and
ECMWF do not clearly favor one solution over another, and in fact
many members track Florence somewhere in-between. While little
change was made to the previous forecast and the new official track
forecast remains close to the various consensus aids, the model
spread has increased and confidence in the forecast is low.

There is still considerable model ensemble spread for Florence’s
track beyond day 5. Given the large uncertainty at these time
ranges, it is far too soon to speculate what, if any, impacts
Florence may have on the U.S. East Coast next week. Regardless of
Florence’s eventual track, large swells emanating from the hurricane
will reach Bermuda beginning on Friday and portions of the U.S. East
Coast this weekend, resulting in life-threatening surf and rip
currents.

FORECAST POSITIONS AND MAX WINDS

INIT 06/0900Z 24.1N 47.9W 100 KT 115 MPH
12H 06/1800Z 24.8N 49.1W 90 KT 105 MPH
24H 07/0600Z 25.4N 50.8W 90 KT 105 MPH
36H 07/1800Z 25.6N 52.2W 90 KT 105 MPH
48H 08/0600Z 25.6N 53.6W 90 KT 105 MPH
72H 09/0600Z 26.1N 56.1W 95 KT 110 MPH
96H 10/0600Z 27.2N 59.0W 105 KT 120 MPH
120H 11/0600Z 29.0N 63.5W 110 KT 125 MPH

$$
Forecaster Zelinsky