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Hurricane Irwin Discussion Number  13
NWS National Hurricane Center Miami FL       EP102017
800 AM PDT Tue Jul 25 2017

Recent microwave imagery confirms that Irwin has a well-defined
mid-level eye, which has also been apparent in shortwave infrared
satellite imagery.  On the whole, satellite intensity estimates
have risen a little, so Irwin's initial intensity is now set at 70
kt.  Various shear analyses indicate that about 15 kt of southerly
shear has developed over Irwin, which isn't surprising since
microwave data has hinted that the low-level center may be located
just a bit south of the mid-level eye.  This shear is expected to
continue for another 24 hours or so, which will likely prevent
Irwin from any additional significant strengthening.  After 24
hours, Irwin's proximity to Hurricane Hilary is likely to induce
some weakening, as well as the potential for upwelling of colder
water due to Irwin's slow motion through 72 hours.  More
definitive weakening is likely by day 5 as Irwin reaches higher
latitudes and much colder waters.  The intensity models are fairly
stable at the moment the latest runs did not suggest that any
changes were needed to the previous forecast.  It should be noted
that while most of the global models maintain Irwin as a separate
and distinct cyclone from Hilary through day 5, the ECMWF model
indicates that Irwin could be absorbed by that time.

Irwin appears to be moving slowly westward, or 270/5 kt.  The
cyclone's future track will largely be dictated its the binary
interaction with Hilary.  First, as Hilary approaches from the
east, Irwin is expected to dip west-southwestward and southwestward
during the first 48 hours.  It will likely stall by day 3, but then
get pulled northward and northwestward around the eastern side of
Hilary's circulation.  Although there is still considerable spread
in the track models after 72 hours, the regional HWRF and HMON
models are now on board with the binary interaction.  Therefore,
the NHC track forecast is much closer to the TVCN multi-model
consensus than it has been during the past few days.

FORECAST POSITIONS AND MAX WINDS

INIT  25/1500Z 15.6N 119.4W   70 KT  80 MPH
 12H  26/0000Z 15.2N 120.2W   70 KT  80 MPH
 24H  26/1200Z 14.7N 121.4W   65 KT  75 MPH
 36H  27/0000Z 14.2N 122.2W   60 KT  70 MPH
 48H  27/1200Z 13.9N 122.8W   55 KT  65 MPH
 72H  28/1200Z 14.4N 122.7W   55 KT  65 MPH
 96H  29/1200Z 18.0N 122.0W   55 KT  65 MPH
120H  30/1200Z 21.5N 125.0W   50 KT  60 MPH

$$
Forecaster Berg

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