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Tropical Storm GREG Forecast Discussion


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000
WTPZ42 KNHC 200845
TCDEP2

Tropical Storm Greg Discussion Number  12
NWS National Hurricane Center Miami FL       EP072017
300 AM MDT Thu Jul 20 2017

An ASCAT pass at 0458 UTC revealed that Greg is a little weaker than
previously estimated.  The maximum winds were 31 kt, limited to a
small area 50 n mi due north of the center of circulation. Since the
time of the ASCAT pass, the cloud pattern of Greg has improved
considerably, and a band of deep convection now wraps nearly all the
way around the circulation.  The initial intensity is set at 35 kt,
assuming at least a little undersampling in the ASCAT pass, and
allowing for some increase in the winds due to the increase in
convection.

It is not yet clear if the recent increase in the organization of
Greg is temporary or the beginning of a longer-term trend.  It is
worth noting that around 0000 UTC yesterday, the tropical storm
featured a similar band that weakened after about 6 hours.  There is
a fair amount of spread in the intensity guidance.  Notably, the
LGEM model shows a peak intensity of only 43 kt while the HWRF peaks
at 77 kt.  Given that the environment appears at least somewhat
conducive for strengthening, the official forecast still shows Greg
reaching hurricane strength in a few days and is very close to the
intensity consensus IVCN.

The initial motion is again 270/9 kt.  No significant changes were
made to the first 72 hours of the track forecast, and Greg is still
expected to be steered generally westward by a strong ridge to the
north.  At days 4 and 5 there is more spread in the guidance, with
some models turning the cyclone toward the northwest and others
keeping it on a westward heading.  For now, the forecast splits the
difference between the GFS and ECMWF models, both of which show a
slowdown and slight turn toward toward the northwest at the end of
the forecast period.

FORECAST POSITIONS AND MAX WINDS

INIT  20/0900Z 14.1N 114.7W   35 KT  40 MPH
 12H  20/1800Z 14.3N 116.1W   40 KT  45 MPH
 24H  21/0600Z 14.7N 118.1W   50 KT  60 MPH
 36H  21/1800Z 15.1N 120.3W   55 KT  65 MPH
 48H  22/0600Z 15.4N 122.4W   60 KT  70 MPH
 72H  23/0600Z 15.6N 127.1W   65 KT  75 MPH
 96H  24/0600Z 15.8N 131.5W   55 KT  65 MPH
120H  25/0600Z 16.5N 135.0W   45 KT  50 MPH

$$
Forecaster Zelinsky