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Hurricane Isaac Discussion Number  10
NWS National Hurricane Center Miami FL       AL092018
1100 PM AST Sun Sep 09 2018

Isaac's IR satellite presentation has generally improved since the
last advisory. While there is no evidence of an eye in conventional
satellite imagery, the most recent microwave imagery around 2100 UTC
indicated the presence of a pinhole eye. Dvorak intensity estimates
are T4.0 from both TAFB and SAB, and on this basis Isaac has been
upgraded to a hurricane, the 5th of the 2018 Atlantic season and 3rd
in the past few days.

I must stress that all aspects of the forecast are highly uncertain
at this time. The dynamical track model spread is nearly 1000 miles
at day 5, while the intensity guidance spread is nearly 100 kt.
Although the NHC track and intensity forecasts have not been changed
very much, this should not be interpreted as increasing confidence
in the forecast.

With regard to track, there is a clear split in the model guidance
regarding the speed of Isaac. The GFS and ECMWF models forecast that
a mid-level ridge to the north will accelerate the hurricane
westward for the next 36 h, with a westward motion continuing
through day 5 as the ridge is reinforced in the wake of Florence. On
the other hand, the UKMET and HWRF models show a weakness developing
in the ridge which causes Isaac to move much slower, and eventually
turn northward. The NHC forecast has been favoring the former
solution, and continues to do so with this advisory since I see no
reason to make a drastic change to the forecast at this point.

Isaac is a very small hurricane, which makes the intensity forecast
particularly tricky since small storms are susceptible to rapid
changes of intensity, up and down. Since the wind shear is expected
to remain low, and Isaac is forecast to remain over fairly warm SSTs
for the next 48 h, the current forecast of further intensification
seems reasonable, and the NHC forecast is still at the top end of
the guidance envelope. From 72 h onward, there could be a sudden,
and substantial increase in wind shear associated first with an
upper-level trough to the north, and later with the outflow of
Hurricane Florence.  Assuming this shear affects Isaac as expected,
the tiny hurricane would likely weaken quickly, perhaps even faster
than currently shown in the forecast.  By the end of the forecast
period, the NHC forecast remains a fair amount below the intensity
consensus, closer to the weakening solutions of the GFS and ECMWF.

Key Messages:
1. Isaac is a small hurricane and uncertainty in the forecast is
higher than normal.  Although Isaac is forecast to begin weakening
by Tuesday 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  10/0300Z 14.5N  41.6W   65 KT  75 MPH
 12H  10/1200Z 14.7N  43.5W   75 KT  85 MPH
 24H  11/0000Z 14.9N  46.3W   80 KT  90 MPH
 36H  11/1200Z 15.0N  49.3W   85 KT 100 MPH
 48H  12/0000Z 15.0N  52.0W   85 KT 100 MPH
 72H  13/0000Z 15.2N  57.3W   75 KT  85 MPH
 96H  14/0000Z 15.5N  63.5W   60 KT  70 MPH
120H  15/0000Z 15.5N  70.0W   50 KT  60 MPH

$$
Forecaster Zelinsky

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