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Tropical Storm Irwin Forecast Discussion

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WTPZ45 KNHC 271435

Tropical Storm Irwin Discussion Number  21
NWS National Hurricane Center Miami FL       EP102017
800 AM PDT Thu Jul 27 2017

Satellite images indicate that Irwin's cloud pattern has not
changed very much since the last advisory. The low-level center
appears to be located on the eastern edge of the deep convection
due to the shear caused by Hilary's outflow.  The Dvorak estimates
still yield an initial intensity of 50 kt.  The environment is
neither hostile nor favorable for a significant change in strength
for the next 48 hours. Thereafter, a steady weakening should begin
as the circulation encounters cooler waters.  By day four, a weaker
Irwin should be very close to Hilary, and by day five, the two
cyclones should have merged, with Hilary becoming the dominant
feature.  This is the solution provided by the ECMWF and GFS global
models, and it reflected in the NHC forecast.

Currently, Irwin is forecast to remain embedded within very light
steering currents and only a small westward drift is anticipated
during the next two days.  After that time, Hilary should have
already passed by the north of Irwin.  The cyclone should then begin
to move northward and northwestward around Hilary's circulation
until the the merge occurrs.  The NHC forecast is in the middle of
the guidance envelope formed by the northernmost GFS and
southernmost ECMWF.


INIT  27/1500Z 15.0N 124.2W   50 KT  60 MPH
 12H  28/0000Z 15.0N 124.8W   50 KT  60 MPH
 24H  28/1200Z 15.0N 125.0W   55 KT  65 MPH
 36H  29/0000Z 15.0N 125.0W   55 KT  65 MPH
 48H  29/1200Z 15.5N 125.0W   55 KT  65 MPH
 72H  30/1200Z 19.0N 125.5W   45 KT  50 MPH
 96H  31/1200Z 23.0N 129.5W   35 KT  40 MPH MERGING

Forecaster Avila