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Tropical Storm Irwin Discussion Number  19
NWS National Hurricane Center Miami FL       EP102017
800 PM PDT Wed Jul 26 2017

Irwin is still displaying the cloud pattern of a sheared tropical
cyclone, with the strongest convection displaced to the west and
northwest of the low-level center due to moderate southeasterly
vertical wind shear. Upper-level outflow is restricted in the
southeastern semicircle, but is good to the northwest. Satellite
intensity estimates are a consensus T3.5/55 kt from TAFB and SAB
using a shear pattern. However, a recent SSMI microwave image
indicates that just a narrow band of convection exists under the
cirrus canopy near and to the north-through-west of the center, so
the intensity has only been increased slightly to 50 kt.

The initial motion estimate is 255/06 kt. Hurricane Hilary continues
to steadily draw closer and is now within about 500 nmi to the east
of Irwin. Hilary's larger circulation should gradually begin to
influence Irwin's track by imparting an eastward motion, resulting
in Irwin slowing down significantly and possibly even stalling in
the 48-72 hour period when Hilary passes about 300 nmi to the north.
On days 4 and 5, Irwin is forecast to accelerate northward and then
northwestward up the eastern and northeastern portion of Hilary's
outer circulation. The global and regional models are in decent
agreement on this developing track scenario, and the new official
track forecast is a little slower than and east of the previous
track, and lies close to the consensus model TVCN.

Hilary is currently experiencing moderate southeasterly shear of
about 15 kt, caused by the outflow on the west side of Hurricane
Hilary. As the two cyclones draw closer to each other during the
next three days, the outflow-induced shear from Hilary is expected
to gradually back around to the east and northeast, and steadily
increase to more than 20 kt. This unfavorable upper-level flow
regime should prevent any significant strengthening from occuring
for the next 24 hours or so, and cause some slight weakening
thereafter. The official intensity forecast follows the intensity
consensus model IVCN through 36 h, and is lower than the consensus
for the remainder of the forecast period due to the aforementioned
increasing shear, and also due to Irwin moving sub-26C SSTs by 96 h.

FORECAST POSITIONS AND MAX WINDS

INIT  27/0300Z 14.9N 123.1W   50 KT  60 MPH
 12H  27/1200Z 14.8N 123.8W   50 KT  60 MPH
 24H  28/0000Z 14.6N 124.5W   45 KT  50 MPH
 36H  28/1200Z 14.6N 125.0W   45 KT  50 MPH
 48H  29/0000Z 14.7N 125.3W   45 KT  50 MPH
 72H  30/0000Z 16.2N 124.7W   45 KT  50 MPH
 96H  31/0000Z 19.7N 125.6W   40 KT  45 MPH
120H  01/0000Z 24.0N 130.0W   40 KT  45 MPH

$$
Forecaster Stewart

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