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Hurricane Hilary Forecast Discussion

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WTPZ44 KNHC 260249

Hurricane Hilary Discussion Number  19
NWS National Hurricane Center Miami FL       EP092017
900 PM MDT Tue Jul 25 2017

Hilary's convective cloud pattern has continued to fluctuate
between an occasional symmetric appearance containing a
cloud-covered eye to the current asymmetric pattern with little
or no outer banding features evident and the eye completely
obscured in conventional infrared imagery. However, recent SSMI,
SSMIS, and AMSU microwave images still indicate that Hilary has a
well-defined, 10-nmi-diameter eye and a thick eyewall beneath the
cirrus canopy. Satellite intensity estimates range from T4.5/77 kt
and T5.0/90 kt from TAFB and SAB, respectively, to T5.5/102 kt from
the NHC Dvorak objective technique. Given the large range of
estimates, the initial intensity will remain at 90 kt for now, which
could be a little conservative.

The initial motion remains a steady 285/10 kt. Hilary is forecast to
continue on a general west-northwestward track for the next 72 hours
or so. Thereafter, a slight bend toward the west with a decrease in
forward speed is expected due to binary/Fujiwhara interaction with
Hurricane Irwin. The forward speed difference between Hilary and
Irwin is about 3-4 kt, with Hilary steadily getting closer to Irwin.
That trend should continue on days 4 and 5 with the distance between
the two cyclones decreasing to about 300 nmi, resulting in the two
systems orbiting cyclonically about each other. Since Hilary is
forecast to be the larger and more dominant circulation, Irwin's
effect on Hilary's track is expected to be minimal. The latest NHC
model guidance remains tightly clustered around the previous
advisory track, so the new track forecast is just an extension of
the previous advisory.

Although SHIPS and UW-CIMMS shear analyses indicate north to
northwesterly vertical wind shear of about 15 kt, there is no
evidence of this shear based on water vapor imagery showing nearly
symmetrical cirrus outflow and the eye being embedded in the center
or south side of the CDO. The main inhibiting factor that has kept
Hilary from strengthening appears to have been intermittent dry air
intrusions into the inner-core region causing a brief erosion of
the eyewall convection. Microwave data suggests that dry air remains
nearby, so little if any intensification is forecast for the next
24 hours as a result. By 36 h and beyond, Hilary will be moving
over sub-26C SSTs and upper-ocean heat content values near zero, and
reaching 24C water on days 4 and 5. The decreasing thermodynamics
should result in gradual weakening from 36-72 hours, with faster
weakening thereafter. The new NHC intensity forecast closely follows
the previous advisory and the IVCN intensity consensus model.


INIT  26/0300Z 16.0N 109.8W   90 KT 105 MPH
 12H  26/1200Z 16.4N 111.5W   95 KT 110 MPH
 24H  27/0000Z 16.9N 113.6W   90 KT 105 MPH
 36H  27/1200Z 17.4N 115.7W   85 KT 100 MPH
 48H  28/0000Z 18.0N 117.5W   80 KT  90 MPH
 72H  29/0000Z 19.2N 121.1W   75 KT  85 MPH
 96H  30/0000Z 20.0N 124.8W   60 KT  70 MPH
120H  31/0000Z 21.0N 128.3W   45 KT  50 MPH

Forecaster Stewart