ISAAC EXPECTED TO BRING TROPICAL STORM CONDITIONS TO PORTIONS OF THE LESSER ANTILLES

Tropical Storm Isaac Discussion Number 21
NWS National Hurricane Center Miami FL AL092018
500 PM AST Wed Sep 12 2018

The satellite presentation of Isaac hasn’t changed much in the past
several hours, with the center exposed on the west side of a
re-developing band of convection. On the last pass of the NOAA
Hurricane Hunter aircraft, peak flight-level winds were 61 kt and
SFMR values were about 50 kt. These data support an initial wind
speed estimate of 50 kt. It is interesting to note that the
surface center is estimated to be a tenth of a degree or two south
of the well-defined swirl on satellite–a function of the fast
forward speed and high shear.

The shear is forecast to continue for the next day or two, and
gradual weakening is anticipated, in line with the model
guidance. Given the strong shear, it is possible that Isaac could
degenerate into a wave near the Lesser Antilles. After that time,
Isaac could escape the upper-level trough over the central Atlantic
that has been shearing the cyclone, and find a more conducive
environment. More of the model guidance is suggesting some
reintensification of the cyclone in the western Caribbean Sea, with
the biggest hold-out being the ECMWF. Continuity dictates that the
forecast not change as much as the guidance, most of which is
showing a hurricane at long range, so the compromise solution is no
longer dissipating the cyclone and flat-lining the intensity at
long range. It almost goes without saying that this is a
low-confidence intensity forecast.

Isaac has accelerated to near 17 kt over the past 12 hours, and the
shorter-term motion is closer to 20 kt with the convection-less
center. A mid-tropospheric high is forecast to persist for the next
several days, which should keep the cyclone moving westward, albeit
at a slightly slower pace. The model guidance has shifted a bit to
the north on this cycle, but is mostly in response to the initial
latitude. The NHC track forecast is moved northward as well but
still lies on the south side of the consensus. At long range, it
makes sense when forecasting a weaker system to stay on the left
and faster side of the consensus, so that’s where the new NHC
prediction lies.

Key Messages:

1. Isaac is expected to remain at tropical storm intensity when it
moves across the Lesser Antilles on Thursday, and tropical storm
warnings are in effect for Martinique, Dominica, and Guadeloupe.
Tropical storm watches are in effect for Montserrat, St. Kitts and
Nevis, Antigua, and Saba and St. Eustatius. Interests on those
islands should follow any advice given by their local officials.

2. Life-threatening flash flooding is possible with Isaac. The
storm is expected to produce total rainfall accumulations of 2 to 4
inches with isolated amounts up 8 inches across Martinique,
Dominica and Guadeloupe. Rainfall amounts of 1 to 2 inches with
isolated amounts to 4 inches are expected across Puerto Rico and the
southern United States Virgin Islands.

FORECAST POSITIONS AND MAX WINDS

INIT 12/2100Z 15.4N 56.6W 50 KT 60 MPH
12H 13/0600Z 15.6N 59.0W 50 KT 60 MPH
24H 13/1800Z 15.8N 61.8W 45 KT 50 MPH
36H 14/0600Z 16.0N 64.8W 45 KT 50 MPH
48H 14/1800Z 16.0N 67.6W 40 KT 45 MPH
72H 15/1800Z 16.0N 72.5W 40 KT 45 MPH
96H 16/1800Z 16.5N 76.5W 40 KT 45 MPH
120H 17/1800Z 17.5N 80.5W 40 KT 45 MPH

$$
Forecaster Blake

FLORENCE MOVING STEADILY TOWARD THE CAROLINA COASTAL AREAS… …LIFE-THREATENING STORM SURGE AND RAINFALL EXPECTED.

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.

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 12/2100Z 30.9N 72.5W 105 KT 120 MPH
12H 13/0600Z 32.1N 74.1W 110 KT 125 MPH
24H 13/1800Z 33.4N 75.9W 110 KT 125 MPH
36H 14/0600Z 33.9N 77.1W 105 KT 120 MPH
48H 14/1800Z 34.0N 77.9W 100 KT 115 MPH…NEAR THE COAST
72H 15/1800Z 33.6N 79.2W 70 KT 80 MPH…NEAR THE COAST
96H 16/1800Z 34.0N 81.7W 30 KT 35 MPH…INLAND
120H 17/1800Z 35.6N 83.4W 20 KT 25 MPH…POST-TROP/INLAND

$$
Forecaster Stewart

JOYCE FORMS IN THE NORTH ATLANTIC

Subtropical Storm Joyce Discussion Number 1
NWS National Hurricane Center Miami FL AL102018
500 PM AST Wed Sep 12 2018

The strong low pressure system which the NHC has been monitoring in
the north Atlantic for a couple of days has developed a cyclonically
curved band of deep convection near the center, and scatterometer
data indicate that the winds are near 40 kt. Since the system is
still vertically stacked with an upper-low, it is then classified
as a subtropical cyclone at this time. However, the cyclone is
gaining organization while it is acquiring tropical characteristics.
The NHC forecast calls for Joyce to transform into a tropical system
in about 12 to 24 hours. Since the ocean is warm, some slight
strengthening is possible during the next 3 days. After that time,
Joyce will be over cooler waters and should then be absorbed by a
larger extratropical low.

Joyce has been moving southwestward or 225 degrees at 5 kt, steered
by the northerly flow around the subtropical high which has been
steering Florence and to west of the mid-latitude trough which
is forcing Helene to recurve. In a couple of days, global models
indicate that the steering pattern will change as the trough
amplifies, and Joyce should then turn toward the northeast with an
increase in forward speed.

FORECAST POSITIONS AND MAX WINDS

INIT 12/2100Z 34.3N 41.9W 40 KT 45 MPH
12H 13/0600Z 33.7N 42.6W 40 KT 45 MPH…TROPICAL STORM
24H 13/1800Z 33.0N 43.5W 40 KT 45 MPH
36H 14/0600Z 32.5N 43.5W 45 KT 50 MPH
48H 14/1800Z 32.5N 43.5W 50 KT 60 MPH
72H 15/1800Z 34.0N 41.0W 50 KT 60 MPH
96H 16/1800Z 37.0N 35.0W 40 KT 45 MPH
120H 17/1800Z…ABSORBED

$$
Forecaster Avila

PAUL BECOMES A REMNANT LOW