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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
115 PM PST Wed Jan 22 2025

...DRY CONDITIONS ACROSS THE REGION THROUGH THURSDAY...
...CHANCES OF LIGHT PRECIP OVER THE SIERRA, CNTRL/SRN CA, & NV LATE 
FRIDAY TO MONDAY AS A COLDER SYSTEM MOVES THROUGH...

.METEOROLOGICAL CONDITIONS (SHORT TERM: WED PM - SAT AM)...

Forecast remains on track with only minor updates to the afternoon 
package. A strong upper ridge remains over the eastern Pacific just 
off the west coast with an embedded upper low. A cold polar trough 
across much of central/eastern US in combination with the ridge 
continues to generate dry northerly flow across CA/NV. TPW satellite 
imagery shows a wide area of 0.10-0.20" PW over the region this 
afternoon. Such a pattern will persist the rest of today and most of 
tomorrow before a trough drops into the PacNW late Thursday digging 
into nrn CA/NV on Friday. This trough will push the ridge further 
offshore bringing in a colder airmass for Friday as the trough 
deepens and expands in scope. Still not any significant moisture but 
enough of a bump to potentially drop some light showers over the 
central/northern Sierra and eastward across northern NV late Friday 
night. QPF remains at a few hundredths to around a tenth of an inch 
for the aforementioned areas.

Freezing levels will lower from n to s on Friday from 8-11.5 kft 
down to 3.5-8 kft north of I-80 by the evening and 2.5-6 kft by the 
end of the period.


.METEOROLOGICAL CONDITIONS (LONG TERM: SAT AM - TUE AM)...

An upper level trough digs over the region with a low forming over 
central CA on Saturday and progressing S-SE on Sunday into Monday. 
As the low moves south the positively tilted high pressure ridge 
returns offshore of the Pac NW and nudges into Nrn portion of the 
region Sunday into Monday. The low will bring a chance of showers 
mainly over the Sierra and portions of Central CA and most of Srn CA 
and NV and colder temperatures expected over the weekend into Monday 
as a low moves through the region. The models continue to vary on 
the exact track, timing and strength of this low for lower 
confidence in precip amounts timing and locations. The 12Z GFS and 
12Z EC det are farther west and bring the low south along the 
Central CA coast on Sunday then the GFS moves low onshore into Srn 
CA on Monday and into AZ Monday night and the EC is slower and 
deoesn't move the low onshore into Srn CA until Monday night. This 
general track farther west would generally increase precip amounts 
over Srn CA. The main change for the afternoon forecast was to 
increase amounts over Srn CA (up to 0.1-0.5 over higher terrain) 
Saturday night into Sunday night and Srn NV (up to around a tenth of 
an inch) on Sunday. The forecast mainly uses a mix of the latest 
NBM, WPC and previous forecast.  Current forecast precip amounts for 
the weekend through Monday night are generally nothing to a few 
hundredths for the Central Valley and north Bay area and around 0.25-
0.75 for the Srn Sierra and around 0.1-0.4 over the higher terrain 
of NV and SLO to Santa Barbara County and SE CA and 0.25-0.75 for LA 
and 0.4-0.9 along Orange and San Diego County coast and 0.75-1.6 
inches over the San Gabriel and San Bernardino mtns down to the San 
Diego County mtns.  

Cold air will also come down with this system dropping freezing 
levels to around 1500-3000 ft near the ORCA border and around 3000 
to 5000 ft over the Northern and Central Sierra and around 4500-6500 
ft over the Srn Sierra and around 6000-8000ft over Srn CA Saturday 
morning and dropping to around 2000 to 4500 ft for most of the 
region Sunday morning. Freezing levels rise over Srn OR and Nrn Ca 
on Sunday to around 5000-9000 ft (lowest along the Sierra and 
highest along NW CA coast) Sunday night as high pressure noses into 
the area and 7000-10000 ft over Nrn CA then 4500-7000 ft over 
Central CA and around 4000-5000 ft for Srn CA and most of NV by 
Monday night. High temperatures generally drop to near normal to up 
to 20 degrees below normal on Saturday and Sunday then warm up to 
near normal over Srn Or and Nrn CA and into parts of Central CA as 
the ridge builds into the north  and remain below normal to around 
10 degrees for Srn CA and NV on Monday. Minimum temperatures 
generally around normal except up to 10 degrees below normal over 
the far north on Saturday morning drop to below normal up to 15 
degrees across most of the region Sunday morning then warm a little 
to near normal to 10 degrees below normal for most of CA and remain 
below normal up to 15 degrees over NV Monday morning.