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Hurricane Florence Discussion Number  49
NWS National Hurricane Center Miami FL       AL062018
1100 AM AST Tue Sep 11 2018

Data from satellites and an Air Force Reserve reconnaissance
aircraft mission indicate that Florence has completed an overnight
eyewall replacement cycle (ERC). The recon data indicate that the
eye has now expanded to a diameter of 30-32 n mi, and this was
confirmed by an 1103Z SSMI/S microwave satellite image. The aircraft
provided various intensity estimates with a peak SFMR surface wind
of 113 kt noted in the northwest quadrant, a peak 700-mb
flight-level wind of 143 kt in the northeast quadrant, and a central
pressure of 950 mb. The 143-kt flight-level wind would normally
correlate to an equivalent surface wind of about 129 kt. However,
coincident SFMR surface winds were only 108 kt, indicating that the
weak convection that region of the hurricane was not vigorous enough
to bring down the strongest winds to the surface. The 950-mb central
pressures corespondents to about 113 kt. Based on a blend of all
these data, the initial intensity has been set to 115 kt.

The initial motion estimate is now 295/14 kt based on the recent
recon fix data. The broken record continues -- there is no
significant to the previous track forecast or reasoning. Although
the global and regional models continue to make minor shifts
northward and southward, the consensus models have changed little.
GOES-16 high-resolution water vapor imagery indicates that the
amplifying large-scale flow pattern across CONUS is inducing a
downstream ridge over the western Atlantic, with a high pressure
cell centered northwest of Bermuda. This blocking ridge pattern is
expected to keep Florence moving west-northwestward to northwest at
around 15 kt for the next 48 hours or so. However, embedded within
the large-scale flow is a weak shortwave trough over the central and
southern Plains that is expected to eject out northeastward and
weaken the ridging across the mid-Atlantic and southeastern U.S.,
causing Florence to slow down significantly in 72 hours as the
powerful hurricane approaches the Carolinas. On days 4 and 5, an
even slower motion or drift to the west and northwest is forecast,
which will exacerbate the heavy rainfall threat. The new NHC
forecast track is just an update of the previous one, and basically
lies the middle of the guidance envelope between the consensus
models TVCA to the north and HCCA and FSSE to the south.

Water vapor imagery indicates that Florence has finally developed
the much anticipated dual outflow pattern, with outflow jets noted
in the northwestern and eastern quadrants. The latter outflow jet is
flowing into an upper-level low, which is acting as an impressive
mass sink near 25N/49W. These two outflow channels are producing
significant deformation stretching across Florence's inner-core,
which should aid in the re-strengthening process. Now that the eye
has become stable with a diameter of about 30 n mi and since
Florence is expected to remain in a low-shear environment of around
5 kt and over above-average SSTs of 29.0-29.5 deg C, slow but steady
strengthening is expected for the next 24-36 hours. By 48 h and
beyond, Florence's slow forward speed, coupled with the large eye
and relatively shallow depth of the warm water should induce some
upwelling beneath the cyclone that will initiate a slow weakening
trend. By 72 hours the vertical wind shear is expected to increase
to near 20 kt from the southwest, which will cause more significant
weakening to occur. However, given the large overall wind field of
Florence along with the large eye, only gradual weakening is
expected. Once Florence moves inland, the slow forward speed of 3-5
kt will result in rapid spin down and weakening of the wind field.
The new official intensity forecast is above of all of the intensity
guidance based on the aforementioned very favorable synoptic outflow
pattern, and to maintain continuity with the previous forecast.

Key Messages:

1. A life-threatening storm surge is likely along portions of the
coastlines of South Carolina, North Carolina, and Virginia, and a
Storm Surge Watch is in effect for a portion of this area. All
interests from South Carolina into the mid-Atlantic region should
ensure they have their hurricane plan in place and follow any advice
given by local officials.

2. Life-threatening, catastrophic flash flooding and significant
river flooding is possible over portions of the Carolinas and
Mid-Atlantic states from 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 Watch
is in effect for a part of this area.  Damaging winds could also
spread well inland into portions of the Carolinas and Virginia.

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

FORECAST POSITIONS AND MAX WINDS

INIT  11/1500Z 26.7N  65.3W  115 KT 130 MPH
 12H  12/0000Z 27.7N  67.6W  120 KT 140 MPH
 24H  12/1200Z 29.4N  70.6W  130 KT 150 MPH
 36H  13/0000Z 31.1N  73.1W  130 KT 150 MPH
 48H  13/1200Z 32.6N  75.2W  120 KT 140 MPH
 72H  14/1200Z 34.2N  77.1W  105 KT 120 MPH
 96H  15/1200Z 35.0N  78.1W   40 KT  45 MPH...INLAND
120H  16/1200Z 35.5N  79.5W   25 KT  30 MPH...INLAND

$$
Forecaster Stewart

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