000
WTNT42 KNHC 060840
TCDAT2

TROPICAL STORM GRACE DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072015
500 AM AST SUN SEP 06 2015

Deep convection has become better organized during the past several
hours, with a more pronounced curved band developing to the west
and south of the center.  Recent microwave imagery, especially a
0331 UTC GCOM pass, showed a surprisingly well-organized inner core,
with the low-level center located a little farther south than
previous estimates.  Subjective Dvorak estimates are T3.0/45 kt from
TAFB and T2.5/35 kt from SAB, and the initial intensity is therefore
raised to 40 kt.  If Grace is as well organized as suggested by the
microwave data, this intensity could be conservative.

With the slight southward adjustment of the initial position, the
estimated motion is now 270/11 kt.  Grace is located due south of a
mid-tropospheric anticyclone, and a ridge axis extends westward
across the Atlantic to north of the Lesser Antilles.  This pattern
is expected to change little during the forecast period, which
should keep Grace on a general westward path through day 5.  The
guidance is in generally good agreement, and the updated NHC track
is shifted a little bit south of the previous forecast, mainly to
account for the adjusted initial position.  This solution is a
bit south of the TVCN model consensus and leans closest to the
GFS, HWRF, and the Florida State Superensemble.

Grace will be moving through a low-shear environment for the next
36 to 48 hours, which is likely to support additional strengthening
while it moves westward over the tropical eastern Atlantic.
Between days 3 through 5, while Grace is moving over the central
tropical Atlantic, vertical shear is forecast to increase to over
20 kt and veer from southwesterly to westerly by the end of the
forecast period.  Such a hostile environment should induce weakening
beginning on day 3.  The new NHC intensity forecast indicates
continued gradual strengthening through 36 hours while the shear is
low and shows a slightly higher peak intensity than the previous
forecast.  Weakening could be fast once the shear sets in, and the
day 4 and 5 intensities are unchanged from before.

FORECAST POSITIONS AND MAX WINDS

INIT  06/0900Z 12.4N  28.5W   40 KT  45 MPH
 12H  06/1800Z 12.6N  30.8W   45 KT  50 MPH
 24H  07/0600Z 12.8N  33.6W   50 KT  60 MPH
 36H  07/1800Z 13.1N  36.3W   55 KT  65 MPH
 48H  08/0600Z 13.4N  39.1W   55 KT  65 MPH
 72H  09/0600Z 14.0N  44.7W   50 KT  60 MPH
 96H  10/0600Z 14.5N  50.5W   40 KT  45 MPH
120H  11/0600Z 15.5N  55.5W   35 KT  40 MPH

$$
Forecaster Berg