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Hurricane Irwin Forecast Discussion


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000
WTPZ45 KNHC 260256
TCDEP5

Hurricane Irwin Discussion Number  15
NWS National Hurricane Center Miami FL       EP102017
800 PM PDT Tue Jul 25 2017

Irwin has strengthened a little more during the past several hours.
The eye of the hurricane has been occasionally evident in
satellite images and cloud tops in the eyewall are quite cold,
close to -80 C.  The convective structure is asymmetric, however,
with the majority of the deepest convection located to the west of
the center.  The latest Dvorak classifications have increased
slightly, and support nudging the initial intensity upward to 80 kt.

Irwin is moving westward at 7 kt as it is still being steered by a
mid-level ridge to its northeast.  A turn to the west-southwest is
expected tonight and Wednesday as Hurricane Hilary approaches from
the east.  The forward motion of Irwin will likely come to a halt on
Thursday and Friday as Hilary passes by to the north.  After that
time, a general northward motion is expected as Irwin gets caught in
the flow on the east side of Hilary's circulation.  The models are
in better agreement compared to previous cycles, but there is still
a fair amount of spread in where and when Irwin makes the northward
turn.  The NHC track forecast is largely an update of the previous
one, and lies closest to the multi-model consensus TVCE.

The hurricane could be nearing its peak intensity, and weakening
should commence soon due to moderate wind shear and the potential
for cold water upwelling due to the expected slow motion of Irwin.
By the end of the forecast period, Irwin will be moving over much
cooler SSTs and into a drier air mass, which should continue the
weakening trend.  The models are in good agreement, and the NHC
intensity forecast lies near the middle of the guidance envelope.

Irwin is a very compact hurricane.  ASCAT data from around 1800 UTC
indicated that the tropical storm force winds extend no more than 60
n mi from the center.

FORECAST POSITIONS AND MAX WINDS

INIT  26/0300Z 16.1N 120.9W   80 KT  90 MPH
 12H  26/1200Z 15.7N 121.9W   75 KT  85 MPH
 24H  27/0000Z 15.2N 123.0W   70 KT  80 MPH
 36H  27/1200Z 14.9N 123.8W   60 KT  70 MPH
 48H  28/0000Z 14.7N 124.0W   55 KT  65 MPH
 72H  29/0000Z 15.5N 123.5W   55 KT  65 MPH
 96H  30/0000Z 18.2N 123.5W   55 KT  65 MPH
120H  31/0000Z 22.0N 126.0W   45 KT  50 MPH

$$
Forecaster Cangialosi