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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
725 AM PST Sun Dec 22 2024

...ROUNDS OF PRECIP EXPECTED THROUGH THE PERIOD...
...BEST TOTALS EXPECTED FOR NORTHERN CA TAPERING OFF SOUTHWARD...

.METEOROLOGICAL CONDITIONS (SUN AM - SAT AM)...

Moist onshore flow along with a weak frontal boundary and isentropic 
lift is generating mainly light precip across portions of the 
central CA coast inland toward the southern Sierra this morning. 
Automated gauges since 22/12Z show amounts remaining fairly low with 
less than 0.10-inch over most areas...except the west slope of the 
southern Sierra where weak orographics are enhancing amounts 
slightly in the 0.10- to 0.25-inch range.

Radar imagery already shows signs of the next disturbance making its 
way toward the northern CA coast with precip expected to shift 
inland after 22/18Z. This s/wv trof is rotating through the cyclonic 
flow over the northeast Pacific...anchored by an upr low situated in 
the vicinity of the Alaska Peninsula...and will reach the Pacific 
Northwest coast and clip the northern CA coast this evening as the 
associated sfc cold front and entrained moisture makes its way south 
toward the I-80 corridor. As was the case with the system from 
yesterday...with the better mid-level forcing remaining to the 
north...precip will taper off as it makes its way toward central CA. 
Overall amounts will range from 0.25- to 0.75-inch (local 1.00-inch) 
over the favorable higher terrain of northern CA...while the 
Sacramento Valley will range from around 0.10-inch south to about 
0.33-inch. Freezing levels will remain elevated with little colder 
air filtering across the region...from 6000- to 7500-feet near the 
CA/OR border...and 7500- to 9000-feet along the I-80 corridor.

As this s/wv trof makes its way inland toward the northern Rockies 
on Monday...the next more impressive system will make its way toward 
the coastal waters with precip increasing later Monday into early 
Tuesday across northern CA and then shifting southward and farther 
inland through later Tuesday morning into the afternoon hours as the 
s/wv trof axis crosses the coast and then exits to the east by early 
Wednesday. This system also has a reasonable tap into lower latitude 
moisture with PW values just over 1.00-inch intersecting the coast. 
Best totals will once again be focused over the higher terrain of 
northern CA with 1.00- to 3.00-inches...and 0.75- to 1.50-inches 
across the Sacramento Valley. With a sharper more elongated s/wv 
trof reaching the coast...precip will hold together better across 
central CA with 0.75- to 1.50-inches over the higher terrain and 
0.25- to 0.75-inch for the San Joaquin Valley. Also...it continues 
to look like at least light precip will make its way past Point 
Conception and bring a few hundredths to 0.10-inch or so across 
coastal southern CA into southern NV. Freezing levels will start 
elevated in the warm sector of this system...from 8000-feet north to 
10000-feet along I-80 and between 11000- and 13000-feet farther 
south. However...there will be a decent drop in freezing levels as 
colder air does filter in across the area with the core of the 
system moving overhead...down to 3500- to 4500-feet for northern 
areas...5500- to 7500-feet for central areas...and above 8000-feet 
for southern areas.

Much of Christmas Day continues to look dry across the region as a 
brief s/wv ridge slides across the area. However...the next 
disturbance is still expected on Thursday with subsequent rounds of 
precip for the end of the week. Models are struggling a bit with 
timing and exact positioning of the best precip at this point...for 
lowered confidence...but overall areas near and north of I-80 
continue to look wet for the end of the week.