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Tropical Storm ISAAC


Tropical Storm Isaac Discussion Number  15
NWS National Hurricane Center Miami FL       AL092018
500 AM AST Tue Sep 11 2018

Isaac has not changed much overnight.  Satellite images indicate
that the strong tropical storm still has a central dense overcast
pattern and a limited amount of outer bands.  Earlier microwave
data indicated that the center was not located in the middle of the
convection, however, likely due to some westerly shear.  Since
the cyclone has changed little during the past several hours, the
initial intensity is held at 60 kt, which is an average of the TAFB
and SAB Dvorak final T- and CI-numbers.  A nearby NOAA buoy shows
that the wind field of Isaac is very compact, with tropical-storm-
force winds only extending up to 40 n mi from the center.

Satellite fixes suggest that Isaac continues to move westward at 13
kt.  Although there remains a fair amount of spread in the models,
the usually more reliable GFS and ECMWF models continue to show a
westward motion during the next several days as the storm moves on
the south side of a strengthening mid-level ridge.  The NHC track
forecast continues to follow that theme, and this forecast lies
near a blend of the GFS and ECMWF aids.  This forecast is a little
slower than the previous one, and takes Isaac across the Lesser
Antilles and into the eastern Caribbean in 2 to 3 days.

Predicting Isaac's intensity has been challenging.  Isaac appeared
to have been in relatively favorable conditions during the past day
or so, but it did not strengthen.  The upper-level pattern should
remain relatively favorable for another day, so modest strengthening
back to a hurricane is possible during that time.  Thereafter,
however, an increase in shear first from a trough in the central
Atlantic and then from the outflow of Florence should cause some
weakening when the storm moves into the Caribbean.  The models
are in a little better agreement this cycle as the HWRF is no
longer showing Isaac becoming a major hurricane.  The NHC intensity
forecast is very similar to the previous one and is fairly close to
the consensus aids.

Key Messages:

1. Isaac is expected to be at or near hurricane intensity when it
approaches the Lesser Antilles.  However, confidence in the forecast
is lower than normal.

2. Interests in the Lesser Antilles should continue to monitor
Isaac during the next couple of days. Watches will likely be
required for portions of the Lesser Antilles later today.


INIT  11/0900Z 14.6N  48.1W   60 KT  70 MPH
 12H  11/1800Z 14.6N  50.1W   60 KT  70 MPH
 24H  12/0600Z 14.6N  52.6W   65 KT  75 MPH
 36H  12/1800Z 14.8N  55.0W   65 KT  75 MPH
 48H  13/0600Z 15.1N  57.6W   60 KT  70 MPH
 72H  14/0600Z 15.5N  62.8W   55 KT  65 MPH
 96H  15/0600Z 15.6N  67.4W   55 KT  65 MPH
120H  16/0600Z 15.8N  72.0W   50 KT  60 MPH

Forecaster Cangialosi