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Tropical Storm Irwin Forecast Discussion


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000
WTPZ45 KNHC 280254
TCDEP5

Tropical Storm Irwin Discussion Number  23
NWS National Hurricane Center Miami FL       EP102017
800 PM PDT Thu Jul 27 2017

Irwin's convective pattern has changed little since the previous
advisory. A long curved but fragmented band of convection wraps
almost 75 percent of the way around the circulation center, yielding
a consensus Dvorak intensity estimate of T3.5/55 kt from both TAFB
and SAB. However, objective estimates from UW-CIMSS ADT and NHC are
T2.9/43 kt and T3.0/45 kt, respectively, which are supported by an
earlier UW-CIMSS AMSU estimate of 45 kt. Based on average of these
estimates, the initial intensity has been held at 50 kt.

The initial motion estimate is a slow drift toward the west, or
270/02 kt. For such weak steering currents, the latest NHC model
guidance is in very good agreement on Irwin moving little for the
next 36 hours or so, followed by a faster motion toward the north at
48 and 72 hours as the cyclone moves up the eastern and the northern
sides of Tropical Storm Hilary. Irwin is then forecast to merge with
Hilary by 96 hours. The new NHC track forecast is similar to the
previous advisory, and lies close to the multi-model consensus TVCN.

The overall environment surrounding Irwin is expected to change
little before the cyclone merges with Tropical Storm Hilary, so only
minor fluctuations in intensity are forecast, based primarily on
slight changes in the vertical wind shear. The new forecast follows
the previous advisory and the consensus model IVCN.

FORECAST POSITIONS AND MAX WINDS

INIT  28/0300Z 14.9N 124.6W   50 KT  60 MPH
 12H  28/1200Z 14.8N 124.8W   50 KT  60 MPH
 24H  29/0000Z 14.8N 125.0W   55 KT  65 MPH
 36H  29/1200Z 15.0N 125.1W   55 KT  65 MPH
 48H  30/0000Z 16.4N 124.8W   50 KT  60 MPH
 72H  31/0000Z 20.0N 126.0W   45 KT  50 MPH
 96H  01/0000Z...MERGED WITH TROPICAL STORM HILARY

$$
Forecaster Stewart