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Hurricane Hilary Discussion Number   9
NWS National Hurricane Center Miami FL       EP092023
900 AM MDT Fri Aug 18 2023
 
After an impressive 70-kt increase in winds over the past 24 hours,
Hilary's intensity has leveled off.  Satellite data indicate that
Hilary remains an impressive major hurricane with a 15-20 n mi
diameter eye and symmetric convective cloud pattern surrounding that
feature.  A blend of the latest satellite intensity estimates
support holding the initial intensity steady at 125 kt.  Microwave
data is showing some signs of a double eyewall structure, which is
quite common in strong hurricanes like Hilary.  Hilary is quite
large with its tropical-storm-force winds extending up to 250 n mi
from the center. The Air Force Hurricane Hunters are scheduled to
investigate the hurricane later today, and the data they collect
will provide a better assessment of the system's strength and
structure.
 
Hilary has turned to the northwest with the initial motion estimated 
to be 305/9 kt.  A general north-northwest to north motion with a 
steady increase in forward speed is expected later today and through 
the weekend as the system is steered by the flow between a strong 
subtropical ridge over the south-central U.S. and a mid- to 
upper-level low off the central California coast.  This pattern 
should take the core of Hilary near the central Baja California 
Peninsula coast Saturday night and then inland over southern 
California Sunday night.  The models are in fairly good agreement, 
and little change was made to the previous track forecast.  It 
should be noted that strong winds and heavy rains will occur well 
ahead of the center.
 
Fluctuations in the hurricane's strength are expected today, but
Hilary is expected to begin weakening late tonight as it moves over
progressively cooler SSTs and into an environment of increasing
shear and drier air.  The NHC intensity forecast follows the HCCA
and IVCN models and is largely an update of the previous one.
 
Based on the forecast, a Tropical Storm Watch has been issued for
portions of southern California.  This is the first time NHC has
issued a watch for that region.  Supplemental soundings (weather
balloons) are being launched by much of the National Weather Service
offices across the western U.S.  This effort is highly appreciated
as the data should help provide a better assessment of the
environment and steering pattern ahead of Hilary.
 
 
KEY MESSAGES:
 
1. Heavy rainfall associated with Hilary could produce areas of
flash flooding and result in landslides over portions of the Baja
California Peninsula late tonight through late Sunday.  Rainfall
impacts from Hilary within the Southwestern United States are
expected to peak this weekend into Monday. Flash, urban, and arroyo
flooding is expected, with the potential for rare and dangerous
impacts.
 
2. Hurricane conditions are expected within portions of the
hurricane warning area along the west-central coast of the Baja
California Peninsula on Sunday and are possible in the Hurricane
Watch area.
 
3. Tropical storm conditions are possible on Sunday in portions of
southern California where a Tropical Storm Watch has been issued.
 
4. Large swells from Hilary will spread northward along the coast of
southwestern Mexico and the Baja California Peninsula.  These swells
will reach the Gulf of California and northern portions of the Baja
California Peninsula later this weekend.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  18/1500Z 17.9N 111.4W  125 KT 145 MPH
 12H  19/0000Z 19.2N 112.5W  130 KT 150 MPH
 24H  19/1200Z 21.3N 113.6W  115 KT 130 MPH
 36H  20/0000Z 23.7N 114.4W  100 KT 115 MPH
 48H  20/1200Z 26.8N 115.2W   85 KT 100 MPH
 60H  21/0000Z 30.4N 116.4W   65 KT  75 MPH
 72H  21/1200Z 35.2N 117.4W   45 KT  50 MPH...INLAND
 96H  22/1200Z...DISSIPATED
 
$$
Forecaster Cangialosi
 
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