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Tropical Storm Irwin Discussion Number  31
NWS National Hurricane Center Miami FL       EP102017
800 PM PDT Sat Jul 29 2017

Irwin had a partially cloud-free region near its center in visible
satellite imagery a few hours ago, but that feature has since
filled, and cloud-top temperatures have cooled in the central
region.  Satellite intensity estimates range widely from 35 kt to
55 kt, but an ASCAT pass from around 1800 UTC showed maximum winds
just over 40 kt in the southeastern quadrant.  Based on that data,
the initial intensity is lowered to 45 kt.  Although the shear over
the cyclone is low and should remain low for several days, Irwin
only has another 12 hours or so before it reaches waters colder than
26 deg C.  Therefore, additional weakening is anticipated after 12
hours, and Irwin could degenerate into a remnant low as early as 48
hours while it is over sea surface temperatures of 22 deg C.  Based
on a consensus of the global models, the remnant low should
dissipate by day 5.

Irwin is accelerating toward the north as anticipated, and the
initial motion is now 355/7 kt.  The cyclone should turn
north-northwestward soon and continue accelerating during the next
36 hours in the wake of Tropical Storm Hilary.  After 48 hours,
once the two remnant circulations get even closer together, Irwin
is likely to turn northwestward and slow down considerably.  There
is a normal amount of spread among the track guidance, and the
updated NHC track forecast was only shifted slightly northeast of
the previous forecast on days 3 and 4 to lean closer to the ECMWF,
HCCA, and the various multi-model consensus aids.

FORECAST POSITIONS AND MAX WINDS

INIT  30/0300Z 16.4N 124.6W   45 KT  50 MPH
 12H  30/1200Z 17.7N 125.0W   45 KT  50 MPH
 24H  31/0000Z 19.9N 126.0W   40 KT  45 MPH
 36H  31/1200Z 22.3N 127.4W   35 KT  40 MPH
 48H  01/0000Z 24.6N 128.9W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  02/0000Z 27.4N 132.1W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  03/0000Z 28.5N 133.5W   15 KT  15 MPH...POST-TROP/REMNT LOW
120H  04/0000Z...DISSIPATED

$$
Forecaster Berg

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