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Tropical Storm Harvey Forecast Discussion

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WTNT44 KNHC 180842

Tropical Storm Harvey Discussion Number   4
NWS National Hurricane Center Miami FL       AL092017
500 AM AST Fri Aug 18 2017

Harvey's cloud pattern has changed very little in organization
during the past several hours. The low-level center is difficult to
find even using the 1-min images from GOES 16, but it appears to
be located on the eastern edge of the convection due to the
prevailing easterly shear. Dvorak estimates from both TAFB
and SAB support keeping the intensity at 35 kt. Another Air
Force plane will be investigating Harvey at sunrise.

The moderate easterly shear affecting the cyclone is expected to
increase a little during the next day or so, and this factor should
not allow significant strengthening. Once the cyclone reaches the
western Caribbean Sea in 3 or 4 days, an environment of lower shear
and high moisture is forecast to prevail, and Harvey should then
gather some strength. The cyclone could be near hurricane strength
by the time it is approaching Central America or the Yucatan
peninsula. The NHC forecast is very similar to the previous one and
is very close to the intensity consensus. The GFS and the ECMWF
global models are just a little more enthusiastic in keeping the
cyclone from dissipating in this last run, but who knows what they
might forecast the next time.

Harvey has not changed in track or speed, and it is still moving
toward the west or 270 degrees at 16 kt. The cyclone is well
embedded within the easterlies south of a persistent subtropical
ridge. This steering pattern will keep Harvey trapped in the
Caribbean Sea while moving westward for the next few days. The
track forecast is similar to the one issued by my predecessor and it
follows closely the multi-model consensus. The guidance envelope is
quite tight and is bounded by the northernmost ECMWF and the GFS to
the south.


INIT  18/0900Z 13.1N  59.1W   35 KT  40 MPH
 12H  18/1800Z 13.2N  61.7W   35 KT  40 MPH
 24H  19/0600Z 13.5N  65.2W   40 KT  45 MPH
 36H  19/1800Z 13.9N  68.8W   40 KT  45 MPH
 48H  20/0600Z 14.1N  72.7W   50 KT  60 MPH
 72H  21/0600Z 15.0N  80.2W   60 KT  70 MPH
 96H  22/0600Z 16.5N  86.5W   60 KT  70 MPH
120H  23/0600Z 17.5N  90.0W   35 KT  40 MPH...INLAND

Forecaster Avila