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Tropical Storm Greg Discussion Number   7
NWS National Hurricane Center Miami FL       EP072017
900 PM MDT Tue Jul 18 2017

The convective organization of Greg has improved this evening.  A
band consisting of cloud tops colder than -70C wrapping about 3/4 of
the way around the circulation is apparent in IR imagery.  Dvorak
current intensity estimates from TAFB and SAB have increased to 45
knots, so the initial intensity has been increased to that value.

Despite the presence of a large upper-level low to the north east
and outflow from nearby Fernanda and TD Eight-E, the analyzed shear
over the center of Greg remains low.  The low shear, combined with
warm SSTs and a fairly moist environment should allow for at least
modest intensification for the next 24-48 hours.  After that time, a
combination of factors including a drier and more stable
environment, interaction with TD Eight-E, and increased shear
associated with the upper-level low should cause weakening and the
eventual dissipation of the tropical storm. The current forecast
is close to the model consensus and assumes that Greg will be the
dominant cyclone in any interaction with TD Eight-E.  Weakening
would probably occur sooner if the opposite scenario occurred.

A 2316 UTC SSMIS overpass was very helpful in locating the center of
Greg, which was a little farther west than previously estimated.
The initial motion is now 275/9 kt.  Although no significant changes
have been made to the track forecast, confidence remains fairly
low, especially beyond 72 hours.  For the next 24 hours, there is
good agreement that Greg will move generally westward, along the
southern periphery of a mid-level ridge.  Although the extent of
the interaction varies, the ECMWF, GFS, and UKMET all show some
degree of a Fujiwhara-type interaction with TD Eight-E.  This
interaction, coupled with a slight weakening of the ridge, should
cause Greg to move a little farther north and more quickly toward
the west during days 2 and 3.  A turn back toward the west-southwest
is shown at the end of the forecast, assuming at least some
continued interaction with the remnants of TD Eight-E.  The forecast
is closest to a blend of the GFS and ECMWF models.


FORECAST POSITIONS AND MAX WINDS

INIT  19/0300Z 14.6N 110.1W   45 KT  50 MPH
 12H  19/1200Z 14.8N 111.5W   50 KT  60 MPH
 24H  20/0000Z 15.2N 113.4W   55 KT  65 MPH
 36H  20/1200Z 15.8N 115.6W   55 KT  65 MPH
 48H  21/0000Z 16.5N 118.1W   55 KT  65 MPH
 72H  22/0000Z 17.4N 123.7W   45 KT  50 MPH
 96H  23/0000Z 16.8N 128.1W   30 KT  35 MPH
120H  24/0000Z 15.5N 132.0W   25 KT  30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Zelinsky

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