11 Sep Tue 2018 (UPDATE)

Microwave and visible satellite data show that the center of Isaac
is partially exposed on the western edge of the central dense
overcast. Perhaps Isaac is a little weaker than earlier, but a
blend of the latest subjective and objective estimates still gives
an uncertain initial wind speed of about 60 kt. We should have a
better idea of the maximum winds by tomorrow morning when a NOAA
Hurricane Hunter enters the storm, or if we get lucky with a
scatterometer pass this evening.

Isaac is barely holding on in the face of increasing shear, which
is much more evident in the 300-500 mb layer than a conventional
analysis at 200 mb. However, convection could redevelop near the
center while the storm moves over warming SSTs into a more moist and
unstable environment east of the Lesser Antilles. These
conflicting factors lead to a forecast of little change in the winds
of Isaac until the cyclone moves into the eastern Caribbean. I
should mention that the short-term trend on satellite makes me think
the intensity forecast is too high over the next 36 hours. Yet,
given our limited skill in forecasting intensity changes in marginal
environments, plus the 5 or 10 kt of initial wind speed uncertainty,
this is the course of least regret. We can see if this trend
becomes more consistent before making substantial changes on the
next advisory. Model guidance is very divergent in the long range
for Isaac, with the regional hurricane and statistical models
showing intensification, while the global models show continued
weakening. The global models have done fairly well with this
cyclone, so the forecast will lean more heavily on their weaker
solutions.

The latest initial motion estimate is 270/15, a little faster than
before. Isaac is forecast to move westward at roughly the same
speed for the next few days due to a mid-level ridge over the
central Atlantic Ocean. Model guidance is tightly clustered on this
cycle, with the UKMET now coming into good agreement with the rest
of the guidance. The only change to this forecast is to accelerate
Isaac roughly 1 kt over the course of the prediction, which over the
course of a 5-day forecast results in a sizable westward shift.
This forecast could still be too slow, since the GFS and ECMWF are
a bit ahead of the new NHC track prediction.

Key Messages:

1. Isaac is expected to be at or near hurricane intensity when it
approaches the Lesser Antilles on Thursday, and hurricane and
tropical storm watches have been issued for several islands.
Interests in those areas should follow any advice given by their
local officials, and warnings will likely be issued this evening.

2. Interests elsewhere in the Lesser Antilles should continue to
monitor Isaac during the next couple of days.

FORECAST POSITIONS AND MAX WINDS

INIT 11/2100Z 14.6N 51.3W 60 KT 70 MPH
12H 12/0600Z 14.6N 53.4W 60 KT 70 MPH
24H 12/1800Z 14.8N 56.2W 60 KT 70 MPH
36H 13/0600Z 15.0N 59.2W 60 KT 70 MPH
48H 13/1800Z 15.3N 62.3W 55 KT 65 MPH
72H 14/1800Z 15.5N 68.5W 50 KT 60 MPH
96H 15/1800Z 15.5N 73.5W 45 KT 50 MPH
120H 16/1800Z 15.5N 77.5W 45 KT 50 MPH

$$
Forecaster Blake

DANGEROUS FLORENCE EXPECTED TO BRING LIFE-THREATENING STORM SURGE AND RAINFALL TO PORTIONS OF THE CAROLINAS AND MID-ATLANTIC

Hurricane Florence Discussion Number 50
NWS National Hurricane Center Miami FL AL062018
500 PM AST Tue Sep 11 2018

Microwave satellite data indicate that Florence completed a full
eyewall replacement cycle (ERC) a few hours ago, and recent visible
and infrared imagery suggests that the eye has contracted slightly.
Outflow continues to expand in all quadrants, and the outflow jets
to the northwest and east have become better defined. Satellite
intensity estimates from TAFB, SAB, and UW-CIMSS ADT are all
T6.0/115 kt, and recent NHC objective intensity estimates are
T6.2/120 kt. Given that the eye has mostly cleared out and has also
warmed to near 19 deg C, the initial intensity has been bumped
upward to 120 kt, which could be conservative. All of the wind radii
had to be expanded/increased based on a blend of the earlier
reconnaissance data and a 1430 UTC ASCAT scatterometer pass.

The initial motion estimate is now 300/15 kt. There remains no
significant to the previous track forecast or reasoning. Overall,
the global and regional models have done a good job capturing the
evolving synoptic- scale flow pattern across CONUS, with an
amplifying trough moving onshore the the northwestern U.S. coast,
which is inducing downstream ridging across the northeastern U.S.
and the northwestern Atlantic Ocean. Water vapor imagery indicates
that the blocking high pressure northwest of Bermuda is continuing
to build and shift slowly eastward. The 12Z GFS model made a
significant shift to the west, the UKMET made a shift to the east,
and the ECMWF track has remained basically unchanged through 72
hours. As a result the consensus models have made only minor track
shifts to the west. What is noticeable is that all of the global and
regional models are indicating that the steering currents will
collapse by 72 h when Florence is approaching the southeast U.S.
coast. The weak steering currents are expected to continue through
the weekend, which makes the forecast track on days 3-5 quite
uncertain. The latest NHC forecast track is very similar to the
previous two advisory tracks, and lies the middle of the guidance
envelope between the consensus models TVCA to the north and the HCCA
and FSSE models to the south.

During the next 24 hours or so, Florence is expected to remain in a
very favorable environment consisting of low shear near 5 kt, an
expanding upper-level outflow pattern, and above-average SSTs of
29.0-29.5 deg C, which should result in additional strengthening.
By 48 h, the decreasing forward speed along with the large
inner-core wind field should induce some upwelling and gradual
weakening. Although the GFS- and ECMWF-based SHIPS intensity models
are indicating an increase in the southwesterly shear to near 20 kt,
this could be due to the SHIPS model capturing Florence’s own
strong outflow from the GFS and ECMWF model fields. Despite the
weakening shown at 72 hours, Florence is still expected to remain a
dangerous hurricane through landfall. After Florence moves inland,
the slow forward speed of less than 5 kt should cause a rapid spin
down and weakening of the hurricane’s circulation. The new NHC
intensity forecast is a little above the highest 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 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 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 likely 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 Warning
has been issued 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 11/2100Z 27.5N 67.1W 120 KT 140 MPH
12H 12/0600Z 28.7N 69.3W 130 KT 150 MPH
24H 12/1800Z 30.4N 72.1W 135 KT 155 MPH
36H 13/0600Z 32.1N 74.5W 130 KT 150 MPH
48H 13/1800Z 33.4N 76.2W 120 KT 140 MPH
72H 14/1800Z 34.5N 77.7W 100 KT 115 MPH…NEAR THE COAST
96H 15/1800Z 35.0N 78.8W 40 KT 45 MPH…INLAND
120H 16/1800Z 35.7N 81.7W 25 KT 30 MPH…INLAND

$$
Forecaster Stewart

POORLY ORGANIZED PAUL EXPECTED TO BECOME A REMNANT LOW LATER THIS WEEK