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Tropical Storm Greg Discussion Number  19
NWS National Hurricane Center Miami FL       EP072017
800 PM PDT Fri Jul 21 2017

Greg is continuing to experience significant vertical shear, as the
deep convection is only observed in the northeastern semicircle and
its center is on the edge of the convective overcast.  A blend of
subjective Dvorak estimates from SAB/TAFB, objective Dvorak from
CIMSS, and an ATMS analysis all suggest about 45 kt maximum
sustained winds, unchanged from the previous advisory.

Vertical shear analysis from CIMSS suggests substantially stronger
tropospheric vertical shear than analyzed by either the GFS or
ECMWF global models.  The southwesterly shear appears to due to a
vigorous upper-level trough north-northwest of Greg.  While the
shear should subside some as the upper trough and Greg move away
from each other, the atmosphere that Greg will move through should
become progressively drier and more stable.  Around day 5, the
hostile thermodynamics are expected to cause Greg to lose deep
convection completely and thus become a remnant low.  The official
intensity forecast  - showing little to no intensification before a
gradual decay starting around day 3 - is based upon the HCCA
corrected consensus technique and is slightly lower than that of the
previous advisory.

Greg is moving toward the west at about 12 kt, as it is moving
along the southern periphery of a subtropical ridge.  A gradual
bend toward the west-northwest is anticipated around day 4 and 5,
as a weakening Greg should be advected along by the low-level
trades.  The model guidance is in good agreement for this scenario -
except for the GFS which has a sharp turn toward the northwest.
This appears to be due to a stronger, deeper Greg being more
affected by an approaching upper-level trough.  This GFS solution is
not, however, mirrored by most of the GEFS ensemble members which
show more of a west to west-northwest track.  The official track
prediction is based upon the TVCN multi-model consensus with
the GFS replaced by the GEFS ensemble mean and is nudged slightly
southwest from the previous advisory.

The initial wind radii was contracted inward some based upon the
ATMS analysis from 2129Z.  The official wind radii forecast is
closest to the RVCN consensus approach.

FORECAST POSITIONS AND MAX WINDS

INIT  22/0300Z 15.1N 121.8W   45 KT  50 MPH
 12H  22/1200Z 15.3N 123.5W   45 KT  50 MPH
 24H  23/0000Z 15.5N 125.7W   45 KT  50 MPH
 36H  23/1200Z 15.5N 128.0W   50 KT  60 MPH
 48H  24/0000Z 15.5N 130.4W   50 KT  60 MPH
 72H  25/0000Z 15.8N 133.6W   45 KT  50 MPH
 96H  26/0000Z 17.1N 136.5W   35 KT  40 MPH
120H  27/0000Z 18.0N 139.0W   25 KT  30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Landsea/Ramos

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