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Hurricane Hilary Discussion Number  12
NWS National Hurricane Center Miami FL       EP092023
300 AM MDT Sat Aug 19 2023
 
Hilary continues to display a large and well-defined eye, surrounded 
by very cold convective cloud tops of -75 degrees Celsius.  There 
has been some noticeable thinning of the convection on the western 
side of the circulation, which could indicate that the hurricane is 
beginning to pull down cooler, more stable air from the north.  That 
said, an Air Force Reserve Hurricane Hunter aircraft measured peak 
700-mb flight-level winds of 128 kt, which still supports an initial 
intensity of 115 kt.  The plane also found that the central pressure 
has dropped slightly to 943 mb.
 
Hilary continues to move toward the north-northwest, or 335/11 kt.  
Southerly flow between a large mid-level high over the Central 
Plains and a cut-off low near the central California coast is 
expected to accelerate Hilary north-northwestward and then 
northward during the next 48 hours, with its forward motion likely 
reaching 25-30 kt by the time the system reaches southern 
California on Sunday.  In fact, the track guidance has sped up 
again on this cycle, and the updated NHC track forecast is a little 
faster than the previous forecast after 24 hours.  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 will extend far from the 
center.  These hazards will also begin well in advance of the 
arrival of the center.
 
The degradation of the convection on Hilary's western side probably 
suggests that weakening will begin very soon.  Hilary's circulation 
will begin moving over sea surface temperatures colder than 26 
degrees Celsius later this morning, and the cooler waters and 
corresponding more stable air mass are likely to cause fast 
weakening over the next couple of days.  Hilary is expected to pass 
near the west-central coast of the Baja California peninsula still 
as a hurricane tonight, but then weaken to a tropical storm by 
Sunday while it's moving near the northwestern coast of the 
peninsula.  Hilary will still be producing tropical-storm-force 
winds when it reaches southern California on Sunday.  The 
circulation is likely to dissipate around or soon after 48 hours, 
but a 60-hour forecast point is still provided for track continuity.

 
KEY MESSAGES:
 
1. Heavy rainfall associated with Hilary will begin well in advance 
of the center, from the Baja California Peninsula into the 
Southwestern United States. Preparations for the impacts of flooding 
from rainfall should be completed as soon as possible, as heavy rain 
will increase ahead of the center today. In the Southwestern United 
States, flash, urban, and arroyo flooding is expected, with 
dangerous and locally catastrophic impacts likely from tonight into 
Monday.
 
2. Hurricane conditions are expected along the west-central coast of 
the Baja California Peninsula within the hurricane warning area 
tonight and Sunday morning, and are possible in the Hurricane Watch 
area on Sunday.
 
3. Tropical storm conditions are expected to begin on Sunday in 
southern California within the Tropical Storm Warning area.  Winds 
could be particularly strong and gusty in and near areas of higher 
terrain.
 
4. Large swells generated by Hilary will affect portions of the
coast of southwestern Mexico, the Baja California peninsula, and
southern California over the next few days. These swells are likely
to cause life-threatening surf and rip current conditions.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  19/0900Z 20.9N 113.3W  115 KT 130 MPH
 12H  19/1800Z 22.8N 113.9W  105 KT 120 MPH
 24H  20/0600Z 26.2N 114.8W   85 KT 100 MPH
 36H  20/1800Z 30.6N 116.0W   60 KT  70 MPH...W COAST OF BAJA CALIF
 48H  21/0600Z 36.4N 117.4W   40 KT  45 MPH...INLAND
 60H  21/1800Z 42.6N 117.5W   25 KT  30 MPH...POST-TROP/INLAND
 72H  22/0600Z...DISSIPATED
 
$$
Forecaster Berg
 
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