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000
WTPZ42 KNHC 210236
TCDEP2

Tropical Storm Greg Discussion Number  15
NWS National Hurricane Center Miami FL       EP072017
800 PM PDT Thu Jul 20 2017

Greg`s center is embedded between a large and persistent band of
convection to the south and newly developing convection to the
northeast, but the banding continues to have a tough time wrapping
around the northwestern side of the circulation.  Since Dvorak
estimates from TAFB and SAB have not changed all day, and final-T
ADT numbers have actually decreased, the initial intensity will
remain 45 kt.

Greg is located to the south of anomalously strong low- to
mid-level ridging, which is pushing the cyclone westward with an
initial motion of 275/9 kt.  Greg is forecast to move on a westward
or west-northwestward course for the rest of the forecast period,
and there is good model agreement through 48 hours.  After 48
hours, however, the spread in the model guidance is larger than
normal.  The GFS and HWRF, for example, show a stronger Greg moving
more slowly toward the northwest by day 5.  On the other hand, the
ECMWF and UKMET have a weaker Greg moving faster and staying
farther south, with some interaction with the remnants of T.D.
Eight-E still possible.  As a compromise between these two
scenarios, the updated official forecast is just a little slower
and south of the previous forecast on days 3 through 5, and it is
not too far from the various model consensus aids.

The environment seems as though it should be conducive for
strengthening.  However, Greg has defied that thought for the past
day or so, as it seems that something about its structure has
prevented it from benefiting from relatively low shear and warm
SSTs.  However, microwave data suggests that the cyclone has
recently developed some improved low-level banding around the
center, which could be a harbinger of a gradual intensification
trend.  The new NHC intensity forecast is basically the same as the
previous one, showing Greg becoming a hurricane in 36 hours and then
weakening on days 3 through 5 as it moves over progressively cooler
waters.  This forecast generally lies between the less aggressive
statistical-dynamical models and the stronger HWRF and HCCA models.

FORECAST POSITIONS AND MAX WINDS

INIT  21/0300Z 14.3N 117.4W   45 KT  50 MPH
 12H  21/1200Z 14.6N 118.8W   50 KT  60 MPH
 24H  22/0000Z 15.1N 120.9W   60 KT  70 MPH
 36H  22/1200Z 15.5N 123.0W   65 KT  75 MPH
 48H  23/0000Z 15.7N 125.3W   70 KT  80 MPH
 72H  24/0000Z 15.8N 129.9W   65 KT  75 MPH
 96H  25/0000Z 16.1N 133.4W   55 KT  65 MPH
120H  26/0000Z 17.0N 136.0W   50 KT  60 MPH

$$
Forecaster Berg




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