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Hurricane Hilary Discussion Number  16
NWS National Hurricane Center Miami FL       EP092023
200 AM PDT Sun Aug 20 2023
 
The coverage and intensity of deep convection associated with
Hilary has gradually diminished.  However, numerous convective
bands over the eastern semicircle of the circulation continue to
spread northward over the Baja California peninsula and the Gulf of
California, and into the extreme southwestern United States.  Air
Force Hurricane Hunter aircraft observations indicate that Hilary
is slowly weakening as evidenced by flight-level winds and
dropsonde measurements of the central pressure.  The advisory
intensity estimate is set at 75 kt, although this might be a little
generous.
 
Hilary is beginning to accelerate toward the north-northwest and the
initial motion estimate is now 345/18 kt.  Over the next day or two,
the cyclone should continue to accelerate within the flow between a
strong mid-level ridge over the south-central United States and a
mid- to upper-level low near the central California coast.  This
motion will bring the center of Hilary near the northern Baja
California peninsula and then into southern California later today.
 The official track forecast is similar to the previous one, albeit
a little faster in accord with the latest consensus model guidance.
 Users are reminded that the exact details of the track forecast,
including where Hilary might make landfall, are of little overall
importance since strong winds and heavy rainfall extend far from the
center.  These hazards are already spreading northward well in
advance of the arrival of the center.
 
Hilary should continue to weaken due to cooler waters, the intrusion
of drier air into the circulation, and increasing vertical shear.
However, confidence remains high that the system will still be of
tropical storm intensity when it moves into southern California.
 
 
KEY MESSAGES:
 
1. Preparations for flooding impacts associated with Hilary should
be completed as soon as possible, as heavy rainfall is about to
begin.  In the Southwestern United States, the potentially historic
amount of rainfall is expected to cause flash, urban, and arroyo
flooding including landslides, mudslides, and debris flows.
Dangerous to locally catastrophic flooding impacts are expected
through Monday morning.
 
2. Hurricane conditions are expected along the west-central coast of
the Baja California Peninsula within the hurricane warning area
during the next few hours, and are possible in the Hurricane Watch
area a little later today.
 
3. Tropical storm conditions are expected to begin later today in
portions of the southwestern U.S. within the Tropical Storm Warning
area.  Winds could be particularly strong and gusty in and near
areas of higher terrain.  Gusty winds are expected to spread
well inland across the western United States.
 
4. Large swells generated by Hilary will affect portions of the
Baja California Peninsula and southern California over the next
couple of days. These swells are likely to cause life-threatening
surf and rip current conditions.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  20/0900Z 27.4N 115.0W   75 KT  85 MPH
 12H  20/1800Z 30.8N 116.3W   60 KT  70 MPH
 24H  21/0600Z 36.5N 117.6W   30 KT  35 MPH...INLAND
 36H  21/1800Z 42.3N 118.5W   20 KT  25 MPH...POST-TROP/INLAND
 48H  22/0600Z...DISSIPATED
 
$$
Forecaster Pasch
 
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