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Hurricane Hilary Discussion Number  17
NWS National Hurricane Center Miami FL       EP092017
900 AM MDT Tue Jul 25 2017

Satellite imagery shows that deep, but somewhat asymmetric,
convection continues in the inner core of Hilary.  The 1-minute
visible imagery from GOES-16 indicates strong convective towers are
firing in the northeastern eyewall, then attempting to rotate
cyclonically around the center.  Still, the eyewall remains open in
the southwest quadrant, as suggested in the visible imagery and
shown in a recent 1340 UTC SSMIS microwave pass.  Intensity
estimates remain about the same as overnight, so the initial wind
speed will stay at 90 kt.

Hilary is forecast to remain in a conducive environment for
strengthening during the next day or so.  However, the northerly
wind shear is not far away, as evidenced by cirrus clouds moving
toward the center in the northern semicircle, which could cause the
hurricane to level off in intensity.  Similar to yesterday at this
time, the model guidance has really backed down off the peak
intensity, with few of the solutions even showing Hilary reaching
100 kt.  It was easier to ignore the models yesterday because they
seemed unrealistic; today the shear is approaching and should
impact the cyclone within 24 hours.  It seems best to only gradually
reduce the intensity forecast, so the latest NHC prediction is
adjusted toward the model consensus by day 3, when more significant
weakening is expected due to cooler waters, drier air, and
interaction with Hurricane Irwin.

The initial motion estimate is a little to the left and faster,
285/10 kt. A mid-tropospheric ridge extending westward from the
southwestern United States should cause Hilary to continue on a
west to west-northwestward track for the next several days.  In a
few days, however, Hilary is likely to pass close to Hurricane
Irwin.  While Irwin won't affect Hilary too much, the bulk of the
guidance is showing a slowdown and slight turn to the left of
Hilary as Irwin tugs on its circulation.  Overall, models are in
fairly good agreement considering the complexity of the situation,
with even most of the GFS-based guidance now acknowledging the
binary interaction.  The official forecast has been gradually
shifting westward at long range, and this advisory continues that
trend, near or just west of the model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  25/1500Z 15.3N 107.8W   90 KT 105 MPH
 12H  26/0000Z 15.8N 109.5W  100 KT 115 MPH
 24H  26/1200Z 16.3N 111.7W  100 KT 115 MPH
 36H  27/0000Z 16.8N 113.9W  100 KT 115 MPH
 48H  27/1200Z 17.4N 116.0W   95 KT 110 MPH
 72H  28/1200Z 18.5N 120.0W   80 KT  90 MPH
 96H  29/1200Z 19.5N 124.0W   70 KT  80 MPH
120H  30/1200Z 20.0N 127.5W   60 KT  70 MPH

$$
Forecaster Blake

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