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Tropical Storm Irwin Forecast Discussion


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000
WTPZ45 KNHC 250235
TCDEP5

Tropical Storm Irwin Discussion Number  11
NWS National Hurricane Center Miami FL       EP102017
800 PM PDT Mon Jul 24 2017

Irwin has been gradually strengthening throughout the day. Satellite
images show that deep convection has been persisting over the center
and in fragmented curved bands, especially over the south and west
portions of the circulation.  Microwave images indicate that the
system is vertically titled from south to north, likely due to
southerly shear.  The latest Dvorak CI numbers are 3.5/55 kt from
TAFB and 4.0/65 kt from SAB.  These estimates and higher ADT values
from CIMSS at the University of Wisconsin support increasing the
initial intensity to 60 kt.

Irwin is crawling west-northwestward, with the initial motion
estimated to be 285/3 kt.  A slow westward to west-northwestward
motion is forecast through early Tuesday as a weak mid-level ridge
remains in place to the north of the storm.  A west-southwestward
motion is expected between 24 and 72 h as Hurricane Hilary
approaches from the east.  Beyond that time, the track forecast is
very challenging as the models remain quite divergent with the
spread being more than 700 n mi at day 5.  The UKMET, ECMWF, and GFS
global models show Irwin rotating around the east side of the
circulation of Hilary and ultimately merging with that hurricane.
The CMC model shows some binary interaction between the tropical
cyclones, but keeps the systems separate, and the regional hurricane
models HWRF and HMON show no interaction between the systems and
continue to move Irwin westward.  The NHC track forecast favors the
global models and shows a slow northward motion on days 4 and 5.
Needless to say, the long range track forecast is of low confidence.

The environmental conditions should support some additional
strengthening during the next day or so, and Irwin is expected to
become a hurricane tonight or on Tuesday.  Thereafter, an increase
in vertical wind shear, associated with the outflow of Hilary, and
slightly cooler waters along the forecast track should cause a
gradual weakening trend.  The NHC intensity forecast is similar to
the previous one and in fair agreement with the consensus models.

FORECAST POSITIONS AND MAX WINDS

INIT  25/0300Z 15.2N 118.2W   60 KT  70 MPH
 12H  25/1200Z 15.3N 118.8W   65 KT  75 MPH
 24H  26/0000Z 15.1N 119.9W   70 KT  80 MPH
 36H  26/1200Z 14.8N 120.9W   65 KT  75 MPH
 48H  27/0000Z 14.5N 121.7W   60 KT  70 MPH
 72H  28/0000Z 14.2N 122.8W   55 KT  65 MPH
 96H  29/0000Z 15.5N 123.0W   55 KT  65 MPH
120H  30/0000Z 18.0N 123.0W   50 KT  60 MPH

$$
Forecaster Cangialosi