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Tropical Storm IRWIN Forecast Discussion


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000
WTPZ45 KNHC 280839
TCDEP5

Tropical Storm Irwin Discussion Number  24
NWS National Hurricane Center Miami FL       EP102017
200 AM PDT Fri Jul 28 2017

Although conventional satellite imagery shows recent improvement in
the banding feature wrapping around the eastern half of the
cyclone, an ambiguity solution of a 0514 UTC ASCAT-B overpass
indicates that Irwin has weakened.  Based on the scatterometer data
and a blend of the subjective Dvorak satellite estimates from TAFB
and SAB, the initial intensity is lowered to 45 kt for this
advisory.  The CIMSS wind shear analysis and the SHIPS model both
indicate that moderate northeasterly shear, produced by an
anticyclone to the north, will be the primary inhibitor for any
significant strengthening prior to Irwin's demise in 4 days.
Little change in the cyclone's intensity is forecast before the
merging of Irwin with Tropical Storm Hilary occurs around the 96
hour period.

The initial motion estimate continues as a drift toward the
west, or 270/02 kt.  The large-scale models indicate this slow
motion, or meander, continuing through the 36 hour period.
Afterward, Irwin is expected to accelerate northward around
the eastern peripheral flow of Hilary.  The global and
regional models also generally agree with the merging of the two
tropical cyclones to occur in 4 days, or less.  The official
forecast has been adjusted to the right of the previous track
forecast beyond 36 hours to align more closely to the TVCX and HCCA
consensus models.

FORECAST POSITIONS AND MAX WINDS

INIT  28/0900Z 14.9N 124.8W   45 KT  50 MPH
 12H  28/1800Z 14.9N 124.9W   50 KT  60 MPH
 24H  29/0600Z 15.0N 124.8W   50 KT  60 MPH
 36H  29/1800Z 15.7N 124.5W   50 KT  60 MPH
 48H  30/0600Z 17.4N 124.5W   45 KT  50 MPH
 72H  31/0600Z 22.0N 126.3W   45 KT  50 MPH
 96H  01/0600Z...DISSIPATED

$$
Forecaster Roberts