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Hurricane Hilary Forecast Discussion

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Hurricane Hilary Discussion Number  20
NWS National Hurricane Center Miami FL       EP092017
300 AM MDT Wed Jul 26 2017

The structure of Hilary has changed little during the past several
hours.  Conventional satellite imagery shows a central dense
overcast that is slightly elongated north-to-south, most likely due
to the impact of some northerly shear.  An eye is occasionally
appearing in infrared imagery, and microwave data continues to
indicate an eye present under the overcast.  However, the eye has
not yet become well-developed enough to justify an increase in the
initial intensity, which remains 90 kt in agreement with a blend of
the various satellite estimates.

The initial motion is 285/11.  The track forecast is essentially
unchanged for the next 72 h, while Hilary is expected to be steered
by the mid-level subtropical ridge to the north.  After that time,
the guidance has become increasingly divergent due to disagreements
on how much a mid- to upper-level trough just west of California
affects the ridge.  The GFS uses the trough to break the ridge and
allow Hilary to move almost northward by the end of the period,
which is a major shift to the right since the previous advisory.
The UKMET and Canadian models keep the strongest ridge and forecast
a more westward motion.  The ECMWF is between these extremes, but
has shifted northward since the previous advisory.  An additional
complication is the possibility of interaction with Hurricane
Irwin.  The new GFS forecast calls for little interaction, while the
UKMET and Canadian merge the two cyclones in 3-4 days and the ECMWF
has interaction at days 4-5.  The latter part of the new forecast
track will be moved northward based on the northward shift of the
GFS, ECMWF, and the various consensus models.  However, it lies to
the south of the consensus models and the center of the guidance
envelope in deference to the UKMET and Canadian forecasts.  This
part of the forecast is low confidence and additional adjustments
may be needed later.

Some slight strengthening is possible today even though Hilary is
in marginal shear conditions.  After that, the cyclone should move
over progressively cooler sea surface temperatures and gradually
weaken.  The new intensity forecast has changed little through 72
hours, and then shows a faster rate of decay based on a track over
colder water than previously forecast.  It should be noted that the
uncertainty in the track forecast also affects the intensity
forecast, as either a more northward motion or interaction with
Irwin would probably weaken the cyclone faster than currently


INIT  26/0900Z 16.3N 111.0W   90 KT 105 MPH
 12H  26/1800Z 16.7N 112.7W   95 KT 110 MPH
 24H  27/0600Z 17.1N 114.7W   90 KT 105 MPH
 36H  27/1800Z 17.6N 116.5W   85 KT 100 MPH
 48H  28/0600Z 18.1N 118.2W   80 KT  90 MPH
 72H  29/0600Z 19.5N 121.5W   75 KT  85 MPH
 96H  30/0600Z 21.0N 125.0W   55 KT  65 MPH
120H  31/0600Z 22.5N 128.5W   35 KT  40 MPH

Forecaster Beven