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Tropical Storm ISAAC


Tropical Storm Isaac Discussion Number  29
NWS National Hurricane Center Miami FL       AL092018
500 PM AST Fri Sep 14 2018

It has been a very interesting flight of the NOAA Hurricane Hunter
aircraft this afternoon.  Dropsonde and SFMR data indicate that the
maximum winds are around 35 kt again, and the pressure has fallen
to about 1002 mb.  However, the system is tilted southward with
vertical height due to northerly shear, and scatterometer and
Hurricane Hunter tail-Doppler radar data indicate the system is
elongated from northeast to southwest, albeit closed.  Note that the
plane was unable to sample the southwestern quadrant well because of
strong convection and intense lightning, so the flight-level winds
showing no circulation were unrepresentative.

The intensity forecast hasn't gotten any easier with Isaac.
Vertical shear is expected to decrease a bit more, which might
diminish the effects of dry air seen on water vapor images around
the cyclone. However, the GFS and ECMWF continue to insist that
Isaac will degenerate into a wave within the next 2 or 3 days, which
is almost unanimously supported by their respective ensemble
members.  This is the solution that the official forecast follows.
It should be noted that none of the global models show pressures
this low now with Isaac, except the UKMET, and that model is the one
that holds onto the circulation the longest.  The reality of the
situation is that the long-term future of Isaac is cloudy, and the
best advice is just to follow NHC advisories every 6 hours to see
if we gain any confidence.

Isaac is moving westward at about 12 kt.  A ridge over the
southwestern Atlantic should steer the cyclone westward over the
weekend then west-northwestward early next week as Isaac reaches a
weaker portion of the ridge.  Just like yesterday, a stronger system
would likely turn more to the west-northwest earlier, moving
closer to Jamaica.  Since the system is forecast to be weaker, the
forecast lies on the westward and faster side of the guidance, and
should also be considered low confidence since it depends on the
intensity.  Interests in Jamaica should monitor the progress of the


INIT  14/2100Z 15.3N  68.2W   35 KT  40 MPH
 12H  15/0600Z 15.4N  70.1W   35 KT  40 MPH
 24H  15/1800Z 15.4N  72.2W   35 KT  40 MPH
 36H  16/0600Z 15.7N  74.2W   35 KT  40 MPH
 48H  16/1800Z 16.2N  76.3W   30 KT  35 MPH
 72H  17/1800Z 17.5N  80.0W   25 KT  30 MPH
 96H  18/1800Z...DISSIPATED

Forecaster Blake