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Tropical Storm Greg Discussion Number  16
NWS National Hurricane Center Miami FL       EP072017
200 AM PDT Fri Jul 21 2017

A band of deep convection remains wrapped around the eastern half
of the circulation, but little convection is present on the western
side of the cyclone.  Since the convective structure is more or
less the same as before, Dvorak estimates from TAFB and SAB are
unchanged, and the initial intensity is held at 45 kt.

Both the track and intensity forecasts are challenging.  The
intensity guidance spread is high, ranging from the COAMPS-TC
model, which makes Greg a major hurricane in 3 days, to the LGEM,
which peaks at 49 kt.  Based on the SHIPS diagnostics from both the
GFS and ECMWF forecast fields, the warm SST, low shear environment
would seem to support strengthening in the short term.  On the
forecast track, Greg should encounter a drier and more stable
environment that should bring a halt to any intensification after
about 72 hours.  The NHC forecast is close to the HWRF model for the
first 48 hours, and still shows Greg reaching hurricane strength.
After that, the HWRF and COAMPS-TC models show continued
intensification, but that seems unlikely in what is expected to be
an unfavorable environment.  The NHC forecast instead follows the
trend of the statistical models late in the period and shows steady
weakening.

The center has been difficult to locate tonight, and the initial
motion is an uncertain 280/9 kt.  For the first 48 hours or so, Greg
should continue on a west or west-northwest heading, below a low to
mid-level ridge. Beyond that, there is tremendous spread in the
model guidance.  The GFS and UKMET positions vary by over 450 n mi
at 120 h, with the ECMWF lying in between.  The GFS and the
GFS-based regional models all depict a relatively stronger Greg
making a right turn around 48 h and heading into the mid-level
ridge.  On the other hand, the ECMWF and UKMET continue a weaker
Greg westward for another couple of days.  Given that only modest
strengthening is forecast, the latter scenario seems somewhat more
likely at this point.  The official track forecast is very close to
the ECMWF at days 4 and 5. Given the high spread in the guidance,
confidence in the track forecast beyond 48 h is low.

FORECAST POSITIONS AND MAX WINDS

INIT  21/0900Z 14.5N 118.3W   45 KT  50 MPH
 12H  21/1800Z 14.8N 119.7W   50 KT  60 MPH
 24H  22/0600Z 15.3N 121.8W   55 KT  65 MPH
 36H  22/1800Z 15.6N 124.0W   60 KT  70 MPH
 48H  23/0600Z 15.7N 126.1W   65 KT  75 MPH
 72H  24/0600Z 16.0N 130.0W   65 KT  75 MPH
 96H  25/0600Z 16.4N 133.3W   55 KT  65 MPH
120H  26/0600Z 18.0N 136.0W   45 KT  50 MPH

$$
Forecaster Zelinsky

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