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Tropical Storm GREG


Tropical Storm Greg Discussion Number   6
NWS National Hurricane Center Miami FL       EP072017
300 PM MDT Tue Jul 18 2017

Satellite imagery suggests that the circulation of Greg has become
better defined today.  However, the earlier dense convective
overcast has dissipated, leaving a small area of convection near
the center and some thin outer banding.  Subjective satellite
intensity estimates from TAFB and SAB are 35 kt, while estimates
based on microwave sounder data are 40-45 kt.  Based on the current
convective trends, the initial intensity remains 35 kt, and it is
possible this is conservative.  Water vapor imagery indicates that
upper-level westerlies produced by the outflow of Hurricane
Fernanda and Tropical Depression Eight-E are impinging on the
western side of Greg, although the analyzed vertical shear is
currently low.

The initial motion is 275/8.  For the first 48 h, Greg should move
generally westward to the south of a mid-level ridge.  After that,
the forecast becomes low confidence due to interaction, and
possible merger, with Tropical Depression Eight-E.  The
large-scale models are showing run-to-run inconsistency on which of
the cyclones will be dominant during this process and which might
get destroyed.  The current forecast scenario is that both systems
will survive, with Greg moving quickly around the north semicircle
of the circulation of Eight-E.  The new forecast track is near the
model consensus through 72 h and north of the model consensus after
that time.

A combination of light/moderate vertical wind shear and warm sea
surface temperatures should allow Greg to slowly strengthen during
the first 36-48 h.  After that, the intensity forecast also becomes
low confidence due to the interaction with Tropical Depression
Eight-E.  The forecast track takes Greg over decreasing sea surface
temperatures after 48-60 h, and on this basis the intensity
forecast shows a weakening trend.  However, an alternate forecast
scenario is for Greg to weaken faster than currently forecast during
the cyclone interaction.  The new forecast follows the trend of the
guidance and is a little above the intensity consensus.


INIT  18/2100Z 14.6N 109.0W   35 KT  40 MPH
 12H  19/0600Z 14.8N 110.3W   40 KT  45 MPH
 24H  19/1800Z 15.2N 112.4W   45 KT  50 MPH
 36H  20/0600Z 15.6N 114.5W   50 KT  60 MPH
 48H  20/1800Z 16.2N 117.0W   50 KT  60 MPH
 72H  21/1800Z 17.5N 122.5W   40 KT  45 MPH
 96H  22/1800Z 17.5N 127.5W   30 KT  35 MPH
120H  23/1800Z 16.5N 131.5W   25 KT  30 MPH...POST-TROP/REMNT LOW

Forecaster Beven