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Tropical Storm IRWIN Forecast Discussion


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Tropical Storm Irwin Discussion Number   8
NWS National Hurricane Center Miami FL       EP102017
200 AM PDT Mon Jul 24 2017

The convective structure of Irwin has improved quite a bit this
evening.  A recent AMSU microwave pass at 0541 UTC suggests that
deep convection is wrapping nearly all the way around a developing
mid-level eye.  It isn't yet clear if this feature is aligned with
the low-level center.  Current intensity estimates from TAFB, SAB,
and the UW-CIMSS ADT at 0600 UTC were all 45 kt, so that value has
been set as the initial intensity.

Determining the initial motion of Irwin has been tricky, since
recent microwave fixes of the center between 2100 UTC yesterday and
0600 UTC today suggest that the center has barely moved during that
time.  It is assumed that at least some westward motion is still
occuring, and the initial motion is a rather uncertain 270/5 kt.
In the short term, Irwin should continue slowly westward to the
south of a low- to mid-level ridge.  At days 4 and 5, there remains
a fair amount of uncertainty in the degree to which Irwin's track
will be affected by Hurricane Hilary to the east, with the global
models still generally showing more and the regional models less
interaction.  Although the NHC track forecast has been adjusted
slightly to the north at day 5, it assumes that only a little
interaction will take place, and remains south of the GFS and
ECMWF consensus.

The environment in the short-term still appears favorable for
intensification and the guidance is a little higher for this
advisory.  The NHC forecast now calls for Irwin to become a
hurricane within 24 hours.  After about 2 days, outflow from Hilary
should begin to reach Irwin, resulting in an increase in vertical
shear.  The ECMWF model in particular shows a large increase in
shear at days 3-5, and the NHC forecast therefore begins to show
gradual weakening at that time.  The intensity forecast is near the
HFIP Corrected Consensus, HCCA.

FORECAST POSITIONS AND MAX WINDS

INIT  24/0900Z 14.8N 117.4W   45 KT  50 MPH
 12H  24/1800Z 14.8N 117.9W   55 KT  65 MPH
 24H  25/0600Z 14.8N 118.6W   65 KT  75 MPH
 36H  25/1800Z 14.8N 119.4W   70 KT  80 MPH
 48H  26/0600Z 14.8N 120.5W   70 KT  80 MPH
 72H  27/0600Z 14.4N 122.0W   65 KT  75 MPH
 96H  28/0600Z 14.5N 123.1W   60 KT  70 MPH
120H  29/0600Z 15.7N 124.1W   60 KT  70 MPH

$$
Forecaster Zelinsky

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