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Hurricane Irwin Discussion Number  12
NWS National Hurricane Center Miami FL       EP102017
200 AM PDT Tue Jul 25 2017

Deep convection associated with Irwin now wraps almost all of the
way around the center, and there have been occasional hints of a
eye in microwave and conventional satellite imagery.  Subjective
satellite estimates from TAFB and SAB are just below hurricane
strength, while several objective estimates, including the CIMSS
satellite consensus, are above hurricane strength. With the attempts
at eye formation, the initial intensity will lean toward the higher
intensity estimates, and thus Irwin is upgraded to a hurricane.

The initial motion is a little more northward than before at 300/4.
A slow westward motion is forecast today as a weak mid-level ridge
remains in place to the north of the storm.  A west-southwestward
motion is expected between 24-72 h as Hurricane Hilary approaches
from the east.  After that, Irwin is expected to undergo an binary
interaction with Hilary, with the most likely result for Irwin
being a generally northward motion around the eastern semicircle of
Hilary.  The details of this are still uncertain, with the ECMWF
and Canadian models merging the two cyclones before 120 h, while
the GFS and UKMET keep them separate until after 120 h.  The new
track forecast is similar to the previous forecast through 72 h.
After that, it leans towards the GFS/UKMET in keeping the cyclone
separate and accelerating Irwin around the eastern side of Hilary.

Little change in strength is likely for the next 24 h.  After that,
increasing shear associated with the outflow of Hilary is likely to
cause Irwin to weaken, although there is considerable spread in the
guidance as to how much shear and how much weakening.  By the end
of the forecast period, cooler water along the forecast track and
proximity to Hilary should cause additional weakening.  The new
forecast track is a slight adjustment of the previous track.  An
alternative intensity scenario is that Irwin dissipates as it is
absorbed by Hilary before 120 h.

FORECAST POSITIONS AND MAX WINDS

INIT  25/0900Z 15.5N 118.7W   65 KT  75 MPH
 12H  25/1800Z 15.3N 119.4W   65 KT  75 MPH
 24H  26/0600Z 15.0N 120.5W   65 KT  75 MPH
 36H  26/1800Z 14.6N 121.4W   60 KT  70 MPH
 48H  27/0600Z 14.3N 122.2W   55 KT  65 MPH
 72H  28/0600Z 14.0N 123.0W   55 KT  65 MPH
 96H  29/0600Z 16.0N 122.5W   55 KT  65 MPH
120H  30/0600Z 20.0N 123.0W   50 KT  60 MPH

$$
Forecaster Beven

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