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Tropical Storm Irwin Forecast Discussion


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000
WTPZ45 KNHC 272039
TCDEP5

Tropical Storm Irwin Discussion Number  22
NWS National Hurricane Center Miami FL       EP102017
200 PM PDT Thu Jul 27 2017

There has been no change in the structure of Irwin. The cyclone
continues to be sheared with the center intermittently located in
and out of the convection. Dvorak numbers remain unchanged,
consequently, the initial intensity is kept at 50 kt in this
advisory. The environment is neither hostile nor favorable for a
significant change in strength for the next 48 hours, so only small
fluctuations in intensity are anticipated during that period.
Thereafter, steady weakening should begin as Irwin's circulation
encounters cooler waters. By day four, Irwin should have merged with
Hilary. This continues to be the solution provided by the ECMWF and
GFS global models, and it is indicated in the NHC forecast.

Steering currents are very weak and Irwin is basically meandering
westward. No significant motion is forecast until Hilary passes to
the north in about 2 days. By then, Irwin is expected to begin
moving northward a little bit faster steered by the southerly flow
in the wake of Hilary. Guidance is consistent with the very slow
motion during the next day or so, and most of the models clearly
capture the faster northward motion around Hilary later in the
period. The NHC forecast, primarily beyond 2 days, follows the
multi-model consensus, and is in between the faster ECMWF and the
slower GFS.

FORECAST POSITIONS AND MAX WINDS

INIT  27/2100Z 14.9N 124.3W   50 KT  60 MPH
 12H  28/0600Z 14.8N 124.4W   50 KT  60 MPH
 24H  28/1800Z 14.7N 124.7W   55 KT  65 MPH
 36H  29/0600Z 14.6N 125.0W   55 KT  65 MPH
 48H  29/1800Z 15.0N 125.0W   55 KT  65 MPH
 72H  30/1800Z 19.0N 125.5W   45 KT  50 MPH
 96H  31/1800Z...Merged with Hilary

$$
Forecaster Avila