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Tropical Storm Irwin Discussion Number  20
NWS National Hurricane Center Miami FL       EP102017
200 AM PDT Thu Jul 27 2017

Overall, Irwin's cloud pattern has change little during the past
several hours.  An earlier 0218 UTC SSMIS microwave pass shows a
considerable structural vertical tilt toward the northwest due to
the impinging southeasterly shear produced by Hilary to the east.
Experimental GOES-16 channel 9 mid-level water vapor imagery
indicates drier air intruding in the northeast quadrant of Irwin
with subsequent convective erosion in that portion of the cloud
canopy.  The Dvorak satellite intensity estimates from SAB and TAFB
support maintaining the initial intensity at 50 kt for this
advisory.

The dynamical intensity models indicate that the moderate
southeasterly shear will back more toward the northeast during the
next 12-24 hours as Hilary invades from the northeast.  Beyond the
24 hour period, the intensity forecast becomes a bit problematic.
The hurricane models, the HFIP Corrected Consensus, the Florida
State Superensemble, and the GFS/ECMWF global models all show
Irwin strengthening back into a hurricane under much improved
diffluent flow aloft.  The statistical-dynamical intensity models,
on the other hand, show only slight strengthening.  As a compromise
for this forecast package, the official forecast shows modest
strengthening, then gradual weakening after 48 hours.

The initial motion is estimated to be west-southwestward, or 255/05
kt.  As Hilary continues to moves closer and passes to the
north of Irwin, a significant reduction in Irwin's forward speed,
essentially drifting in a cyclonic fashion, is expected during the
next 24-36 hours.  After that time, the cyclone is forecast to
accelerate northward and then northwestward up the eastern and
northern portion of Hilary's peripheral circulation. The
deterministic guidance remains in fair agreement with this synoptic
pattern scenario, and the NHC forecast is similar to the previous
one and is based on a blend of the TVCX and HCCA consensus models.


FORECAST POSITIONS AND MAX WINDS

INIT  27/0900Z 14.7N 123.8W   50 KT  60 MPH
 12H  27/1800Z 14.6N 124.4W   50 KT  60 MPH
 24H  28/0600Z 14.5N 124.8W   55 KT  65 MPH
 36H  28/1800Z 14.3N 124.7W   55 KT  65 MPH
 48H  29/0600Z 14.7N 124.3W   55 KT  65 MPH
 72H  30/0600Z 17.8N 124.4W   50 KT  60 MPH
 96H  31/0600Z 22.0N 128.0W   40 KT  45 MPH
120H  01/0600Z 24.5N 133.5W   35 KT  40 MPH

$$
Forecaster Roberts

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