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Hurricane FLORENCE


Hurricane Florence Discussion Number  29
NWS National Hurricane Center Miami FL       AL062018
1100 AM AST Thu Sep 06 2018

Vertical shear has increased since yesterday, which has caused a
degradation of Florence's structure and a decrease in its maximum
winds.  The cloud-filled eye has been eroded over the past hour or
so, and the deep convection is no longer symmetric, with the
low-level circulation peeking out from under the higher clouds.
Dvorak CI numbers from TAFB and SAB have fallen to T5.0-5.5 (90-100
kt), while the objective numbers from UW-CIMSS support 70-90 kt.
The initial intensity is set at 90 kt, near the middle of this wide
range of estimates.

The intensity forecast has been somewhat of a self-defeating
prophecy due to the nuances of the environmental shear.  Even though
Florence was able to rapidly intensify yesterday in an area just
south of a zone of strong shear, the hurricane's stronger-than-
expected intensity caused it to move more poleward, into that
stronger shear.  Right now, shear analyses range anywhere from
25-30 kt, and the latest available guidance suggests that this
level of shear should continue for another 12-24 hours.  As a
result, continued weakening is forecast over the next day or so.
After 36 hours, Florence is likely to encounter an upper-level
environment that is more conducive for reintensification.  The NHC
forecast is adjusted downward toward the newest consensus aids,
especially during the first 48 hours, but it still shows Florence
reaching major hurricane strength again by days 4 and 5.

Florence's forward motion has slowed just a little to 9 kt toward
the northwest (315 degrees).  A mid-level ridge is building to the
north, which is likely to cause the hurricane to turn westward by
36 hours, with that motion continuing through about day 3.  After
that time, there is still considerable uncertainty in the evolution
of the steering pattern over the western Atlantic, especially on day
4.  On one hand, the GFS and HWRF dig a strong shortwave trough over
Atlantic Canada by Monday, creating a break in the ridge which would
allow Florence to turn northwestward.  On the other hand, the ECMWF
and UKMET both have weaker troughs and maintain stronger ridging
over the northwestern Atlantic, allowing Florence to maintain a
westward or west-northwestward course.  All the models show a
mid-level high over the western Atlantic by Tuesday.  Due to typical
biases among these models in the part of the Atlantic, we prefer to
be between the GFS and ECMWF solutions at this time, which places
the official NHC track forecast close to the TVCN multi-model
consensus and just north of HCCA.

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


INIT  06/1500Z 24.6N  48.6W   90 KT 105 MPH
 12H  07/0000Z 25.2N  49.8W   80 KT  90 MPH
 24H  07/1200Z 25.6N  51.3W   75 KT  85 MPH
 36H  08/0000Z 25.6N  52.6W   75 KT  85 MPH
 48H  08/1200Z 25.7N  54.0W   80 KT  90 MPH
 72H  09/1200Z 26.3N  56.4W   90 KT 105 MPH
 96H  10/1200Z 28.0N  59.5W  100 KT 115 MPH
120H  11/1200Z 29.5N  65.0W  105 KT 120 MPH

Forecaster Berg