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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
130 PM PDT Mon Mar 20 2023

...SCATTERED SHOWERS MAINLY OVER THE HIGHER TERRAIN AND OVER SRN CA 
TAPER OFF THIS AFTERNOON AND EVENING...
...COOLER SYSTEM TONIGHT INTO WEDNESDAY WITH THE BULK OF PRECIP 
ALONG THE I-80 CORRIDOR AND SOUTH...
...ANOTHER SYSTEM BRINGS PRECIP MAINLY TO SRN OR AND NRN CA AND NV 
AND DOWN ALONG THE SIERRA THURSDAY INTO SATURDAY...

.METEOROLOGICAL CONDITIONS (SHORT TERM: MON PM - THU AM)...

No major changes to the afternoon forecast with the biggest changes 
increasing amounts along the transverse range around 0.25-5 inches. 
Isolate to Scattered showers mainly over Srn CA and along the Sierra 
and over NV this afternoon and evening as one system exits to the 
east an a cooler low pressure system approaches the west coast. The 
colder upper level low and surface low along with associated trough 
offshore of the CA coast (around 130W 37N) early Tuesday morning. 
This system taps into a main subtropical moisture plume (about 1.3 
inch) and streams it into Srn CA  tonight into Tuesday morning. 
Weaker moisture plume (Around an inch) moves along the Srn CA coast 
Tuesday afternoon into the evening. The deterministic models in 
better agreement with the low center just off the coast of the Bay 
Area by noon Tuesday and then weakens and moves to the SE and inland 
over the S Bay Area and into Central CA Wednesday and into NV 
Wednesday night. Precip will increase along the Central and Srn CA 
coast later tonight and continue into Tuesday. Northern CA, 
especially the Russian and Napa basins and the east slope of the 
coastal range will increase precip with the low along the Bay Area 
coast Tuesday afternoon and evening. Precip will become more showery 
Tuesday night into Wednesday night with upper level trough moving 
through the region. The majority of the precip with this colder 
system is expected around the I-80 corridor and south. Freezing 
levels around 2500-3500ft near ORCA border and 3500-5500ft over the 
Sierra and 5000-8000ft over Srn CA Tuesday morning and remain around 
3000-5000ft Tuesday afternoon-Wednesday.  Precip amounts Monday 
night-Wednesday night generally are 2-6 inches across the transverse 
Mtns (except up to 7.5 inches along the San Gabriel into San 
Bernardino and into San Diego Cty Mtns and and 2-4 inches for the 
Srn Sierra and Santa Cruz Mtns and Big Sur coast and 1-2 inches for 
the Central Sierra and 0.75-1.5 inch for the Russian and Napa Basins 
and east side  of Coastal Range/west side Sac Valley and Nrn Sierra 
and 0.25-1.25 over Nevada and Central Valley. 

.METEOROLOGICAL CONDITIONS (LONG TERM: THU AM - SUN AM)...

Forecast remains on track with no major updates to the afternoon 
forecast package. Lingering showers across the region on Thursday 
due to the remnants of a surface/upper low impacting CA earlier this 
week. The next upper trough will then approach the PacNW arriving in 
the late afternoon/early evening. The southern end of the system 
will scrape the CA/OR and northern NV borders through early Friday 
morning bringing light precip along the CA/OR border and over nrn 
NV. There is some disagreement between the models later Friday. 
Though both show another burst of energy sliding down the backside 
of the trough, the ECMWF has a deeper system that penetrates into 
northern CA Friday night while the GFS is more shallow. This results 
in precip spreading further to the south into nrn CA/NV Friday night 
in the ECMWF solution compared to the GFS. The 12z ECMWF then 
deepens this system Saturday morning until it engulfs the entire 
region result in additional scattered light precip. The overall 
pattern in the GFS is still a trough, but the system that moved 
through the PacNW does not expand to the south as the ECMWF 
predicts. This is creating QPF differences between the models for 
Friday through Saturday with the ECMWF about 0.10-0.20" or so wetter 
than the GFS in general, and about 0.50-0.75" wetter along the north 
coast. Either way, additional light scattered showers are expected 
on Saturday while the next surface/upper low approaches from the 
Gulf of Alaska Saturday night into Sunday. Precip from this next 
system is not expected during the current forecast period. 

For the forecast, mainly stuck with WPC again resulting in only 
minor changes from this morning's QPF. QPF 12z Thurs to 12z Sun is 
as follows: 0.50-1.25" north coast (S to N), 0.75" to just over 1" 
for the soOR Cascades, 0.25-0.50" for the northern Sierra, 0.10-
0.25" central Sierra/Shasta, and generally 0.10" or less elsewhere 
across northern/central CA. Freezing levels to start out at roughly 
3-4.5 kft for most of the region early Thursday. Levels will rise 
over the se portion of the region and lower to the nw Thursday night 
reaching 1-3.5 kft north of I-80 and 3-7.5 kft to the south early 
Friday morning. Freezing levels will further decrease into Saturday 
at under 500 ft along the CA/OR border and up to 5 kft by Point 
Conception. These will fluctuate the rest of Saturday but lower back 
down overnight.