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Tropical Storm Irwin Discussion Number   5
NWS National Hurricane Center Miami FL       EP102017
800 AM PDT Sun Jul 23 2017

Irwin's center is exposed to the north and west of its associated
deep convection due to about 15 kt of northerly shear.  Subjective
and objective Dvorak intensity estimates are a consensus T2.5, so
35 kt will remain the initial intensity for this advisory.

The initial motion remains westward at 280/6 kt, with Irwin located
south of a weak low-level ridge and almost midway between Tropical
Storms Greg and Hilary.  This orientation should cause Irwin to
maintain a westward, albeit much slower, motion during the next
several days.  However, once Greg degenerates into a remnant low in
about 4 days, a binary interaction between a stronger Irwin and
Hilary is more likely to occur, which could force Irwin to turn
west-southwestward at the end of the forecast period.  There is
some notable spread in the guidance, highlighted by the GFS and its
ensemble mean on the southern end of the guidance envelope, and the
ECMWF and its ensemble mean on the northern edge.  The HWRF is also
north of most models by the end of the forecast period, presumably
because it is not capturing any interaction between Irwin and
Hilary.  The updated NHC forecast is nudged a little south on this
advisory, and it is south of the model consensus after day 3 to
account for an Irwin-Hilary track interaction.

The shear over Irwin is expected to decrease within the next 12-24
hours, which should allow the cyclone to begin a sustained period
of strengthening.  Sea surface temperatures will be between 27-28
degrees Celsius, and the ambient environment appears sufficiently
moist to foster intensification.  The NHC intensity forecast favors
a blend of the ICON intensity consensus, HCCA, and the Florida
State Superensemble, showing Irwin reaching hurricane intensity in
about 48 hours.  This forecast is not too different from the
previous advisory.

FORECAST POSITIONS AND MAX WINDS

INIT  23/1500Z 15.0N 115.8W   35 KT  40 MPH
 12H  24/0000Z 15.0N 116.6W   40 KT  45 MPH
 24H  24/1200Z 15.0N 117.5W   50 KT  60 MPH
 36H  25/0000Z 15.0N 118.2W   60 KT  70 MPH
 48H  25/1200Z 15.0N 119.1W   65 KT  75 MPH
 72H  26/1200Z 14.8N 121.4W   65 KT  75 MPH
 96H  27/1200Z 14.2N 123.9W   60 KT  70 MPH
120H  28/1200Z 14.0N 125.4W   55 KT  65 MPH

$$
Forecaster Berg

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