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Hurricane Florence Discussion Number 54
NWS National Hurricane Center Miami FL AL062018
500 PM EDT Wed Sep 12 2018

Data from an Air Force Reserve hurricane hunter aircraft along with
satellite imagery and various intensity estimates indicate that
Florence has weakened instead of strengthening. However, while the
hurricane hasn’t strengthened in terms of peak winds, the inner-core
and outer wind fields have continued to expand, resulting in an
increase the cyclone’s total energy, which will create a significant
storm surge event. The upper-level outflow remains impressive and is
still expanding except toward the south.

Florence is moving toward the northwest or 315/14 kt. The new 12Z
global and regional model runs have come into much better agreement
on Florence moving steadily northwestward around a strong ridge
located between Bermuda and the U.S. mid-Atlantic region for the
next 48 hours or so. By late on day 2, Florence is forecast to
approach the southern portion of the North Carolina coast, then slow
down considerably and turn westward within collapsing steering flow,
with a very slow westward motion near the coasts of North and South
Carolina continuing into Friday and Saturday. Corrected-consensus
models HCCA and FSSE remain very close to each other and are quite
similar to the simple consensus model TVCA. Therefore, only a slight
eastward shift was needed to the previous forecast track through 36
hours or so, mainly due to the more eastward initial position based
on the reconnaissance fixes. At 48 hours and beyond, no significant
changes were required to the previous advisory track, which still
shows Florence moving slowly westward across South Carolina and
western North Carolina on day 4, followed by a slow northward motion
up the Appalachian mountain chain on day 5.

A narrow window of opportunity remains during the next 24 hours or
so for Florence to strengthen a little when the hurricane passes
over the warmer SSTs and deeper warm water/higher upper-ocean heat
content associated with the Gulf Stream, and low vertical shear
conditions of 5-10 kt will aid in any strengthening process.
However, significant strengthening is not anticipated due to
Florence’s large and expanding inner-core wind field. By 36 h
and beyond, decreasing ocean heat content along with the slowing
forward speed of Florence will likely produce cold upwelling beneath
the hurricane, inducing a gradual weakening trend. When Florence
moves over the shallow coastal shelf waters in 48-72 h, land
interaction and more significant upwelling are anticipated, which
should further enhance the weakening process. The NHC intensity
forecast remains near the higher statistical guidance through 48
hours, then follows the trend of the decay SHIPS model after that
time.

Although the maximum winds are expected to weaken a little more,
Florence is still expected to remain a dangerous major hurricane as
it approaches the coast. The threat to life from storm surge and
rainfall will not diminish, and these impacts will cover a large
area regardless of exactly where the center of Florence moves.