Florence

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Hurricane Florence Discussion Number 46
NWS National Hurricane Center Miami FL AL062018
500 PM AST Mon Sep 10 2018

Unfortunately, the models were right. Florence has rapidly
intensified into an extremely dangerous hurricane, with 30-second
GOES-16 visible imagery showing well-defined eyewall mesovortices
rotating inside of the eye. A NOAA Hurricane Hunter aircraft found
peak SFMR winds of about 120 kt, with flight-level winds and
dropsonde measurements also supporting that value for the initial
wind speed estimate. Notably, the aircraft data also show the size
of the hurricane-force winds has doubled in the past 12 hours.

None of the guidance suggest that Florence has peaked in intensity,
and this is supported by a continuation of a low-shear environment,
and even warmer waters over the next 36 hours. Thus, the intensity
forecast is raised from the previous one, bringing Florence close
to category 5 strength tomorrow. Near landfall, the vertical wind
shear could increase, along with the increasing likelihood of
eyewall cycles. While the intensity forecast shows some weakening
of the maximum winds near landfall, the wind field is expected to
grow with time, which increases the storm surge and inland wind
threats. The bottom line is that there is high confidence that
Florence will be a large and extremely dangerous hurricane,
regardless of its exact intensity.

Florence has recently turned west-northwestward, still moving at 11
kt. The hurricane is expected to accelerate in that direction over
the next day or two due to building mid-level ridge over the
northwestern Atlantic Ocean. By late Wednesday, a turn toward the
northwest is forecast due to the orientation of the Atlantic ridge,
along with a decrease in forward speed due to a new ridge building
over the Great Lakes. There is a new player in the forecast as
well, with the disturbance over the northwestern Caribbean adding
some uncertainty in the ridge strength over the southeastern United
States. Perhaps it isn't surprising that the model spread has
increased on this cycle, with a small eastward shift overall. The
official forecast is nudged in the direction of the trend, but is
west of the model consensus. It is important not to focus on the
exact forecast track as average NHC errors at days 4 and 5 are about
140 and 180 n mi, respectively, and dangerous hazards will extend
well away from the center.

The NOAA G-IV jet will continue to conduct synoptic surveillance
missions every 12 h through at least Wednesday. In addition, special
0600 UTC and 1800 UTC radiosonde launches have been expanded to
additional upper-air stations across the U.S. are to collect extra
data for the numerical models.

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 will likely be issued for some of these areas by
Tuesday morning. 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 freshwater flooding is likely from a prolonged
and exceptionally heavy rainfall event, which may extend inland over
the Carolinas and Mid Atlantic for hundreds of miles 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
will likely be issued by Tuesday morning. 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 10/2100Z 25.4N 61.1W 120 KT 140 MPH
12H 11/0600Z 26.0N 63.2W 130 KT 150 MPH
24H 11/1800Z 27.0N 66.2W 135 KT 155 MPH
36H 12/0600Z 28.6N 69.3W 135 KT 155 MPH
48H 12/1800Z 30.4N 72.2W 130 KT 150 MPH
72H 13/1800Z 33.7N 77.0W 120 KT 140 MPH
96H 14/1800Z 35.6N 78.8W 50 KT 60 MPH...INLAND
120H 15/1800Z 36.5N 79.5W 25 KT 30 MPH...INLAND

$$
Forecaster Blake

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