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Tropical Storm Greg Discussion Number  13
NWS National Hurricane Center Miami FL       EP072017
800 AM PDT Thu Jul 20 2017

Greg has become notably better organized during the past several
hours with a growing curved band stretching around the eastern and
southern semicircles of the storm.  This organization trend is also
seen on a 1139Z SSMIS microwave pass, which shows a better-defined
central core.  Satellite intensity estimates are between 45-55 kt,
and the initial wind speed is set to 45 kt, on the low side since
the satellite signature has been above the ASCAT data for the past
day or so.

The northwesterly shear from yesterday has diminished, leading to a
symmetric outflow pattern near the storm.  While the SSTs are
expected to be warm enough for strengthening for the next few days,
the possible interaction of Greg and a mid/upper-level trough
near 130W is a complicating factor in the forecast, along with the
interaction of the remnants of Eight-E.  While both of these
features could temporarily cause less favorable conditions, they are
not expected to be enough to prevent at least gradual strengthening
of Greg.  Most of the model guidance is a bit higher than the
previous advisory, and the NHC forecast is bumped up from the
previous package.  This forecast could turn out to be conservative
if the favorable environments seen in the HWRF and HMON models
materialize.

The initial motion estimate is 275/9.  A westward to west-
northwestward motion is forecast for the next several days while
Greg is steered by a ridge to the north.  There is some possibility
that the aforementioned trough could weaken the ridge enough to
induce a more northwestward turn by the end of the forecast,
although that is dependent on exactly how far north Greg is at that
time.  Since the forecast is for a slightly stronger cyclone, which
would be more likely to feel the weakness, it makes some sense to
shift the track a little northward by day 5.  Thus the NHC track
forecast is adjusted poleward at long range, but remains south of
the model consensus.


FORECAST POSITIONS AND MAX WINDS

INIT  20/1500Z 14.1N 115.4W   45 KT  50 MPH
 12H  21/0000Z 14.3N 116.7W   50 KT  60 MPH
 24H  21/1200Z 14.7N 118.7W   55 KT  65 MPH
 36H  22/0000Z 15.1N 120.8W   60 KT  70 MPH
 48H  22/1200Z 15.4N 122.9W   65 KT  75 MPH
 72H  23/1200Z 16.0N 127.5W   65 KT  75 MPH
 96H  24/1200Z 16.3N 131.5W   55 KT  65 MPH
120H  25/1200Z 17.0N 135.0W   45 KT  50 MPH

$$
Forecaster Blake

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