ZCZC MIATCDEP5 ALL
TTAA00 KNHC DDHHMM

Tropical Storm Irwin Discussion Number   9
NWS National Hurricane Center Miami FL       EP102017
800 AM PDT Mon Jul 24 2017

Irwin's structure continues to improve, with the low-level center
embedded beneath a CDO feature that has persisted for the past
several hours.  In addition, recent microwave data have revealed
the formation of a mid-level eye.  Dvorak satellite intensity
estimates now range between T3.0/45 kt and T3.5/55 kt, so the
initial intensity is raised accordingly to 50 kt.

For once, the track forecast for a tropical cyclone is more
challenging and more uncertain that its intensity forecast.  Irwin
is expected to have some binary interaction with Hurricane Hilary
beginning in about 3 days, but the amount of interaction is still
highly uncertain.  The ECMWF, UKMET, and Canadian models all show a
strong Fujiwhara interaction, with Irwin swinging to the east and
northeast of Hilary by day 5.  The GFS shows much less interaction,
with Irwin staying well to the southwest of Hilary on day 5, and
the regional HWRF and HMON models don't even seem to know about
Hilary's existence to allow an interaction to occur.  Needless to
say, there is significant, higher-than-normal spread in the track
guidance beginning in about 72 hours.  The updated NHC track
forecast continues to show a slow westward motion through day 3,
followed by an acceleration toward the northwest and north-northwest
on days 4 and 5.  At the longer ranges, this forecast is closest to
HCCA, but future adjustments are likely due to the significant
spread among the models and their ensemble members.

Since it appears that Irwin is developing a well-defined inner
core, it is likely well on its way to becoming a hurricane.
Vertical shear is forecast to be low for the next 48 hours or so,
while sea surface temperatures are very warm between 28-29 deg
Celsius.  The NHC intensity forecast is generally a blend of HCCA
and the ICON intensity consensus, bringing Irwin to hurricane
status by 24 hours.  Significant strengthening beyond that
threshold is not anticipated at this time due to the possibility
of increased shear from Hilary's outflow, and the NHC forecast
shows little change in intensity between days 3 and 5.

FORECAST POSITIONS AND MAX WINDS

INIT  24/1500Z 14.8N 117.5W   50 KT  60 MPH
 12H  25/0000Z 14.8N 117.9W   60 KT  70 MPH
 24H  25/1200Z 15.0N 118.6W   70 KT  80 MPH
 36H  26/0000Z 15.0N 119.6W   70 KT  80 MPH
 48H  26/1200Z 14.9N 120.6W   70 KT  80 MPH
 72H  27/1200Z 14.4N 122.2W   65 KT  75 MPH
 96H  28/1200Z 15.0N 123.0W   65 KT  75 MPH
120H  29/1200Z 17.0N 124.0W   60 KT  70 MPH

$$
Forecaster Berg

NNNN