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Tropical Storm Irwin Discussion Number  27
NWS National Hurricane Center Miami FL       EP102017
800 PM PDT Fri Jul 28 2017

Microwave data indicate that inner-core convection has recently
developed near Irwin's center; however, the convection is not all
that deep, with cloud top temperatures no colder than about -65C.
Even though Dvorak intensity estimates from TAFB and SAB are
T3.5/55 kt, the UW-CIMSS ADT and SATCON estimates are significantly
lower, so the initial intensity will remain 50 kt.  Vertical shear
does not appear that it will be too prohibitive for the next 24
hours, and in fact, some models like the HWRF and HCCA suggest that
Irwin could gain a little strength during that time.  However, Irwin
should reach waters colder than 26C in about 36 hours, which will
induce a marked weakening trend.  The NHC intensity forecast
closely follows a blend of HCCA and the ICON intensity consensus,
and it now shows Irwin becoming a tropical depression by day 3.

The global models no longer show Irwin being absorbed by Hilary,
probably because Hilary will reach colder waters sooner and will not
be energetic enough to be the dominant system.  The guidance shows
one of two options:  Irwin absorbing Hilary or the two staying as
separate system.  Either way, these scenarios required keeping
Irwin through day 4, but as a remnant low since ocean waters will
be quite cold.

Center fixes off of recent microwave images indicate that Irwin had
been drifting southward or meandering for much of the day.  As
Hilary moves northwestward, Irwin is expected to get pulled slowly
northward during the next 24-36 hours, and then accelerate
north-northwestward on the east side of Hilary on days 2-4.  There
is a fair amount of spread among the track models, especially after
48 hours, but the NHC track forecast is close to the tight
clustering consisting of the GFS, ECMWF, HCCA, TVCN.

FORECAST POSITIONS AND MAX WINDS

INIT  29/0300Z 14.8N 124.7W   50 KT  60 MPH
 12H  29/1200Z 15.0N 124.7W   55 KT  65 MPH
 24H  30/0000Z 16.0N 124.8W   55 KT  65 MPH
 36H  30/1200Z 17.7N 125.1W   50 KT  60 MPH
 48H  31/0000Z 20.0N 125.9W   45 KT  50 MPH
 72H  01/0000Z 24.9N 128.8W   30 KT  35 MPH
 96H  02/0000Z 27.5N 131.5W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  03/0000Z...DISSIPATED

$$
Forecaster Berg

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