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Tropical Storm Hilary Discussion Number 19
NWS National Hurricane Center Miami FL EP092023
800 PM PDT Sun Aug 20 2023
Radar images, satellite data, and surface observations indicate the
center of Hilary is becoming more diffuse while moving near the
coast of southern California. Rainbands associated with the storm
continue to spread northward across the southwestern United States.
The wind field of Hilary has become very asymmetric this evening,
with tropical-storm-force winds now confined to the eastern portion
of the circulation well away from the center. Based on the latest
observations and assuming some continued decay over land, the
initial intensity is set at 40 kt.
Hilary remains embedded within strong southerly flow between a ridge
over the south-central U.S. and a mid- to upper-level low near the
central California coast. Thus, a fast north-northwestward motion is
expected to continue until dissipation occurs. The forecast track
with this update has shifted slightly left of the previous one based
on the adjusted initial position of the storm. Ultimately, this is
of little consequence as the rainfall and wind impacts extend far
from the center. Continued land interaction and strong upper-level
winds will result in additional degradation of the storm's
structure, and Hilary is forecast to become post-tropical by early
Monday. Regardless, gusty winds are expected to continue well to the
east of Hilary's center, especially in areas of higher terrain.
KEY MESSAGES:
1. Across the Southwestern United States, the ongoing and
potentially historic amount of rainfall is expected to cause
life-threatening to locally catastrophic flash, urban, and arroyo
flooding including landslides, mudslides, and debris flows through
early Monday morning. Localized flooding impacts, some significant,
are also expected across northern portions of the Intermountain
West.
2. Tropical storm conditions are expected in portions of the
southwestern U.S. within the Tropical Storm Warning area through
tonight. 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.
3. Large swells generated by Hilary will affect portions of the Baja
California Peninsula and southern California through early Monday.
These swells are likely to cause life-threatening surf and rip
current conditions.
FORECAST POSITIONS AND MAX WINDS
INIT 21/0300Z 33.9N 118.2W 40 KT 45 MPH
12H 21/1200Z 39.8N 118.3W 30 KT 35 MPH...POST-TROP/INLAND
24H 22/0000Z...DISSIPATED
$$
Forecaster Reinhart
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