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CNRFC Hydrometeorological Discussion

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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
830 AM PST Tue Dec 7 2021

...TODAY/S SYSTEM EXITING WITH NEXT ARRIVING LATER WED INTO THU...
...MORE PRECIP POSSIBLE THIS WEEKEND INTO NEXT WEEK...

.METEOROLOGICAL CONDITIONS (SHORT TERM: TUE AM - FRI AM)...

Weak s/wv trof continue to dive south-southeast just off the CA 
coast...while an associated sfc cold front is traversing southern 
portions of the area this morning. Latest radar imagery shows a 
narrow line of echoes ahead of the cold front streaming across 
southern CA from eastern San Bernardino county back west-southwest 
to Orange county. Amounts falling in this line of precip are 
generally light with amounts less than 0.10-inch...and this should 
continue as the precip continues to shift southward across the rest 
of the area before shifting south of the CA/MX border.

As this initial s/wv trof continue to drop south-southeast along the 
coast...it will exit the area later today with precip tapering off 
tonight into Wednesday morning. However...the next s/wv trof will 
quickly arrive into the picture with moist onshore flow increasing 
by the afternoon and evening hours with light precip ramping up over 
portions of northern CA and northern NV. While the s/wv trof reaches 
just off the CA coast...a moisture plume advecting northward from 
near Hawaii will arch eastward between 30N and 40N on the northern 
fringe of the upr high pressure cell centered near 25N/145W. 
Generally light precip for areas north of I-80 (except for locally 
moderate for the northern Sierra) will increase in coverage and 
intensity over areas south of I-80 as the moisture plume reaches the 
central CA coast and interacts with the s/wv trof moving across 
northern CA on its way toward central CA.

Overall...best totals area expected over the Sierra from Lake Tahoe 
southward and then points eastward across the southern 3/4 of NV. 
Then as the precip shifts across southern CA...highest amounts will 
be on southwest and west facing slopes between southwest San 
Bernardino county down through San Diego county. In general...all 
these areas will range from 0.25- to 0.75-inch (locally 1.00-inch). 

Freezing levels in general will start from 5000-ft near the CA/OR 
border...and 7000-ft up to 9000-ft across the Sierra and the state 
of NV. By the time the s/wv trof passes...these will drop to 2500- 
to 3000-ft near the CA/OR border...4500- to 5500-feet across the 
Sierra and 3000- to 6000-ft across NV.


.METEOROLOGICAL CONDITIONS (LONG TERM: FRI AM - MON AM)...

Just a few showers are expected to linger Friday morning over far 
northern areas of CA and NV, but should quickly wrap up by the 
afternoon. A transitory ridge should keep conditions dry for the 
rest of Friday, but the next system quickly drops southward out of 
the Pac NW Saturday. Models are fairly decently aligned for the 
weekend system, showing a meager 1" moisture plume but favorable jet 
dynamics over the region. Expect precip to begin over the North 
Coast Saturday afternoon/evening, moving into the Shasta basin and 
Northern Sierra by Sunday morning. Expect the focus of precip during 
the day and evening hours on Sunday to remain along the coast from 
the Russian through the Santa Cruz Mtns, and inland through the Sac 
Valley and Northern/Central Sierra. Very slow southward motion 
finally occurs Monday, outside of the forecast window.

As mentioned above, jet dynamics are favorable enough to eek out as 
much moisture from the only moderate plume as possible, with the 
potential for 2-3" over the Smith Basin, and closer to 1-2" 
elsewhere along the coast and mountains through early Monday 
morning. Freezing levels are also on the lower side compared to 
other recent storms. Freezing levels start around 4000-5000-ft 
Saturday near the OR/CA border, then quickly lower to 3-4-kft 
Sunday. Freezing levels over the Sierra are expected to fall around 
4500-5500-ft for Shasta and the Northern Sierra, and 5500-6500-ft 
for the Central Sierra.