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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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