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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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