Hurricane Isaac Discussion Number  11
NWS National Hurricane Center Miami FL       AL092018
500 AM AST Mon Sep 10 2018

Isaac has generally changed little during the past several hours.
The small hurricane continues to have a central dense overcast
feature with some fragmented bands to the south of the center.  The
Dvorak classifications remain 4.0/65 kt from both TAFB and SAB, and
therefore, the initial intensity is held at 65 kt.

Isaac is moving westward at 11 kt.  The models remain in poor
agreement this cycle as they are split into two camps.  The UKMET,
HWRF, and a few other models show Isaac recurving near or to the
east of the Lesser Antilles toward a developing trough over the
central Atlantic.  Conversely, the GFS and ECMWF have been very
consistent in showing a due westward motion across the Lesser
Antilles and into the Caribbean as the cyclone moves on the south
side of a strengthening mid-level ridge.  The NHC track forecast
continues to favor the usually more reliable GFS and ECMWF models,
and this forecast is near a blend of those aids.  It should be
noted, however, that confidence in this track forecast is low given
the model spread.

The intensity forecast is also quite uncertain.  One of the complex
factors is Isaac's compact size as small hurricanes like this one
are more prone to quick intensity changes, both up and down.  In
addition, the intensity guidance is in poor agreement as solutions
range from steady weakening from this point onward to the HWRF that
shows Isaac becoming a major hurricane.  Given that the wind shear
is expected to remain less than 10 kt for another day or two,
strengthening is forecast during that time period.  After that,
however, an increase in shear first from a trough over the central
Atlantic and later from the outflow from Florence should result in
weakening when the system moves across the Lesser Antilles and into
the Caribbean Sea.  The NHC intensity forecast is not too different
from the previous one and lies above the model consensus in the
short term, but below it at the later forecast times.  This
forecast is in best agreement with the latest LGEM guidance.

Key Messages:

1. Isaac is a small hurricane and uncertainty in the forecast is
higher than normal.  Although Isaac is forecast to begin weakening
when it approaches the Lesser Antilles, it is still expected to be
at or near hurricane intensity when it reaches the islands.

2. Interests in the Lesser Antilles should continue to monitor Isaac
during the next few days.

FORECAST POSITIONS AND MAX WINDS

INIT  10/0900Z 14.7N  42.7W   65 KT  75 MPH
12H  10/1800Z 14.8N  44.6W   75 KT  85 MPH
24H  11/0600Z 15.0N  47.4W   75 KT  85 MPH
36H  11/1800Z 15.0N  50.3W   80 KT  90 MPH
48H  12/0600Z 15.0N  53.0W   75 KT  85 MPH
72H  13/0600Z 15.3N  58.0W   65 KT  75 MPH
96H  14/0600Z 15.5N  64.0W   50 KT  60 MPH
120H  15/0600Z 15.6N  69.2W   40 KT  45 MPH

$$
Forecaster Cangialosi