Hurricane Florence Discussion Number 55

Hurricane Florence Discussion Number  55
NWS National Hurricane Center Miami FL       AL062018
1100 PM EDT Wed Sep 12 2018

Satellite data and reports from Hurricane Hunter aircraft indicate
significant changes in the structure of Florence and the
environment near the storm since the last advisory.  Microwave
satellite imagery shows that the convection on the southern side of
the storm has been disrupted, and reports from an Air Force Reserve
Hurricane Hunter aircraft indicate the eyewall now wraps less than
50 percent of the way around the center.  The aircraft also reports
that the hurricane again has concentric wind maxima, the inner at a
radius 20-30 n mi and the outer at 50-60 n mi.  The convection seems
to have been affected by 20-25 kt of southerly vertical wind shear,
most of which appears to be due to strong winds between 200-250 mb
seen in dropsonde data from the NOAA G-IV jet.  The central
pressure has risen to 957 mb, and the maximum 700-mb flight-level
winds reported so far are 103 kt.  Based on the latter data, the
initial intensity reduced to a probably generous 95 kt.

The initial motion is 315/15.  During the next 12-36 hours, the
hurricane is expected to turn toward the west-northwest and west
with a decrease in forward speed as it moves into an area of
weakening steering currents near and over the southeastern United
States.  The new forecast track now brings the center onshore in
southern North Carolina near the 36 h point.  After landfall, the
cyclone should move slowly westward to west-southwestward through
the 72 h point, then it should turn northwestward to northward by
the end of the forecast period as it moves through the Appalachian
Mountains.  The new forecast track lies between the HCCA corrected
consensus model and the other consensus aids, and it is nudged just
a little to the north of the previous track.

The dynamical models forecast the current shear to subside after
6-12 h as Florence moves farther from an upper-level low currently
near northeastern Florida.  This, combined with sea surface
temperatures near 29C, would allow a last chance for strengthening
before landfall.  However, the storm structure, particularly the
large outer wind maxima, would likely be slow to respond to the
more favorable environment.  The pre-landfall part of the intensity
forecast thus calls for little change in strength, but given the
uncertainties the confidence in this is low.  After landfall,
Florence should gradually weaken during the 36-48 h period while
the center is near the coast, then weaken more quickly when the
center moves farther inland.

While Florence has weakened below major hurricane intensity, the
wind field of the hurricane continues to grow in size.  This
evolution will produce storm surges similar to that of a more
intense, but smaller, hurricane, and thus the storm surge values
seen in the previous advisory are still valid.  The threat of
rainfall has also not diminished, and these impacts will cover a
large area regardless of exactly where the center of Florence moves.


Key Messages:

1. A life-threatening storm surge is now highly likely along
portions of the coastlines of South Carolina and North Carolina, and
a Storm Surge Warning is in effect for a portion of this area.  All
interests in these areas should complete preparations and follow any
advice given by local officials.

2. Life-threatening, catastrophic flash flooding and prolonged
significant river flooding are likely over portions of the Carolinas
and the southern and central Appalachians late this week into early
next week, as Florence is expected to slow down as it approaches the
coast and moves inland.

3. Damaging hurricane-force winds are likely along portions of the
coasts of South Carolina and North Carolina, and a Hurricane Warning
is in effect. Strong winds could also spread inland into portions
of the Carolinas.

4. Large swells affecting Bermuda, portions of the U.S. East
Coast, and the northwestern and central Bahamas will continue this
week, resulting in life-threatening surf and rip currents.


FORECAST POSITIONS AND MAX WINDS

INIT  13/0300Z 32.0N  73.7W   95 KT 110 MPH
 12H  13/1200Z 33.1N  75.3W   95 KT 110 MPH
 24H  14/0000Z 33.9N  76.7W   95 KT 110 MPH
 36H  14/1200Z 34.2N  78.0W   80 KT  90 MPH...INLAND
 48H  15/0000Z 34.1N  78.7W   60 KT  70 MPH...INLAND
 72H  16/0000Z 33.5N  80.5W   40 KT  45 MPH...INLAND
 96H  17/0000Z 34.0N  83.0W   25 KT  30 MPH...INLAND
120H  18/0000Z 37.5N  83.5W   20 KT  25 MPH...POST-TROP/REMNT LOW

$$
Forecaster Beven

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