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Tropical Storm Irwin Forecast Discussion


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000
WTPZ45 KNHC 230853
TCDEP5

Tropical Storm Irwin Discussion Number   4
NWS National Hurricane Center Miami FL       EP102017
200 AM PDT Sun Jul 23 2017

An ASCAT-B overpass at 0516 UTC showed several vectors of 32-33 kt
winds and a max value of 34 kt in the northeast quadrant of the
tropical cyclone.  Based on that data, the system has been named
Tropical Storm Irwin.  Irwin is the fifth system to be named in the
eastern North Pacific so far in July.

Although the system has been upgraded based on the ASCAT data, the
structure has not improved.  Shortwave-IR imagery shows that the
low-level center is clearly exposed to the northwest of the deep
convection, and Dvorak intensity estimates from TAFB and SAB have
not increased.  The global models suggest that the shear should
decrease quite a bit after about 12 to 24 hours, which should allow
for some intensification.  The regional dynamical models HWRF and
COAMPS-TC depict Irwin quickly recovering from its current sheared
state and intensifying to a category 2 or 3 hurricane.  On the other
hand, the statistical models still only show modest strengthening,
and keep Irwin below hurricane strength.  As a course of least
regret, the intensity forecast splits these two scenarios, but
slightly favors the lower statistical guidance since the dynamical
models may show the structure of Irwin recovering from the current
shear too quickly.  Near the end of the forecast period, Irwin is
expected to encounter a higher shear environment, due in part to
outflow from Hilary to the east, so the forecast depicts gradual
weakening.

The initial motion is 280/6 kt.  Irwin continues to be steered by a
low- to mid-level ridge to the northwest.  Although no significant
changes have been made to the track forecast, confidence is still
somewhat low since there is a fair amount of speed-spread in the
models.  The GFS remains an outlier in being the only model that
shows substantial binary interaction with Hilary, and is thus the
farthest east, though the 00 UTC ECMWF is also a little slower
than before.  The official forecast is a little slower at the end of
forecast period than the previous advisory, but still assumes that
any significant track deviation due to interactions with Hilary
will occur beyond 120 h.

FORECAST POSITIONS AND MAX WINDS

INIT  23/0900Z 14.7N 115.2W   35 KT  40 MPH
 12H  23/1800Z 14.8N 116.0W   40 KT  45 MPH
 24H  24/0600Z 14.9N 117.0W   45 KT  50 MPH
 36H  24/1800Z 15.0N 118.0W   50 KT  60 MPH
 48H  25/0600Z 15.0N 118.8W   60 KT  70 MPH
 72H  26/0600Z 14.7N 121.0W   65 KT  75 MPH
 96H  27/0600Z 14.5N 123.1W   60 KT  70 MPH
120H  28/0600Z 14.0N 125.5W   55 KT  65 MPH

$$
Forecaster Zelinsky