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Tropical Storm Isaac Discussion Number  17
NWS National Hurricane Center Miami FL       AL092018
500 PM AST Tue Sep 11 2018

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

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