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Tropical Storm Harvey Forecast Discussion


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WTNT44 KNHC 190842
TCDAT4

Tropical Storm Harvey Discussion Number   8
NWS National Hurricane Center Miami FL       AL092017
500 AM AST Sat Aug 19 2017

GOES-16 shortwave infrared imagery shows the partially exposed
center of Harvey on the eastern edge of a large area of deep
convection. While the center is closer to the convection than a few
hours ago, cirrus clouds be clearly seen moving from northeast to
southwest across the system; an indication of the persistent shear.
Satellite estimates suggest the maximum winds of Harvey remain 35
kt.

The storm has picked up some forward speed with a recent motion
estimate of 275/18.  A low- to mid-level ridge extending across the
western Atlantic should keep Harvey on a fast westward course across
the Caribbean Sea for the next 48 hours.  Thereafter, the ridge
weakens across the Gulf of Mexico due to a strong mid/upper-level
low currently seen there on water vapor images.  This steering flow
change should cause Harvey to slow down and turn a little more to
the west-northwest in the northwestern Caribbean Sea, with an even
slower motion forecast for Harvey in the Bay of Campeche.  Model
guidance is in fairly good agreement on this scenario, and no
significant changes were made to the previous forecast.

Harvey should be moving into a more conducive environment for
strengthening early next week since the strong northeasterly shear
that has been affecting the cyclone is forecast by almost all of the
guidance to weaken within about 24 hours.  In a few days, most of
the global models show a favorable upper-level environment for
intensification, and Harvey could be near hurricane strength between
the 72 hour forecast period and landfall.  There has been little
change to the guidance so the new intensity forecast is similar to
the previous one, close to the model consensus.  However, with
a weak storm moving so quickly across the central Caribbean, one
always has to be careful of the system opening up into a wave. This
is a plausible alternative scenario still suggested by the GFS and
ECMWF models.


FORECAST POSITIONS AND MAX WINDS

INIT  19/0900Z 13.8N  65.9W   35 KT  40 MPH
 12H  19/1800Z 14.0N  68.7W   35 KT  40 MPH
 24H  20/0600Z 14.3N  72.7W   40 KT  45 MPH
 36H  20/1800Z 14.6N  76.7W   45 KT  50 MPH
 48H  21/0600Z 15.2N  80.1W   50 KT  60 MPH
 72H  22/0600Z 17.0N  86.5W   55 KT  65 MPH
 96H  23/0600Z 18.3N  90.5W   35 KT  40 MPH...INLAND
120H  24/0600Z 19.3N  93.0W   40 KT  45 MPH...OVER WATER

$$
Forecaster Blake