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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
730 AM PDT Wed May 7 2025

...LINGERING SHOWERS THIS MORNING OVER SE CA AS A LOW EXITS, A FRONT 
MOVING THROUGH TODAY MAY GENERATE A SHOWER OR TWO OVER NRN CA...
...DRY CONDITIONS THE REST OF THE WEEK WITH WELL ABOVE NORMAL 
TEMPS...
...LARGER SYSTEM MAY RETURN PRECIP CHANCES TO NRN CA SUN/MON...

.METEOROLOGICAL CONDITIONS (WED AM - TUE AM)...

An upper low is exiting SE CA into AZ this morning leaving about 
0.75" PW of moisture over far se CA and generating showers over AZ 
near the CA border. Some showers possible as the system continues to 
pull out of the region with chances diminishing throughout the 
morning. Behind the trough, a ridge is building in overhead while a 
trough approaches the PacNW along with a cold front. Can see the 
front on satellite imagery making its way inland over the WA/OR 
coast with the southern end of the front just offshore of the north 
coast. TPW imagery shows around 1" PW of moisture surrounding the 
front while water vapor imagery shows a stream of dry air just behind 
it. The ridge over CA/NV will help steer the upper shortwave to the 
north moving inland around WA later today while the surface front 
undercuts the ridge and moves through CA. The moisture it carries is 
shown quickly weakening when the front moves inland in the model 
forecasts. There may be enough lingering moisture to generate a 
shower or two later today over northern CA, but accumulations would 
be minimal (maybe a few hundredths). None of the GFS ensemble 
members show a drop of rain at Arcata while only 3 of the 50 ECMWF 
ensemble members show more than a trace. This front is not expected 
to be strong enough to prevent the start of a warming trend across 
interior northern CA which is forecast to see temperature anomalies 
today of +5 to +10 deg F. Coastal areas should remain relatively 
cool along with se CA as the low still exerts some influence.

For the rest of the week, high pressure will strengthen overhead 
keeping conditions dry and continuing to raise temperatures. Some 
lingering moisture from today's cold front may allow for a few light 
showers to form over the Sierra. Most of the region can expect to 
see temperature anomalies of +5 to +15 deg F for Thursday and +10 to 
+20 deg F for Friday and Saturday. Models still have a larger system 
approaching over the weekend beginning to kick the ridge off to the 
east on Saturday, but not soon enough to prevent another relatively 
warm day. This larger low pressure system is progged to send showers 
to nrn CA Sunday and Monday. There is still a good amount of 
uncertainty as models disagree on the timing and movement of this 
system. Expecting showers to reach the north coast Sunday morning 
spreading inland overnight into Monday as the system pushes onshore. 
The GFS/ECMWF show the system digging deeper into CA throughout 
Monday as it gradually swings eastward moving into NV at the end of 
the current period. Again, there are disagreements on the exact 
timing as well as the general overall pattern. The GFS/CMC are 
wetter than the ECMWF at the moment while also spreading lighter 
precip further to the south (across the Bay Area vs mostly north of 
Point Arena). There is a decent spread amongst the ensembles as well 
with 24 hr QPF at Arcata ranging from 0 to nearly 2". 

The official forecast was a combination of the latest NBM and 
morning WPC guidance. Less than a tenth of an inch is expected today 
from stray showers over se CA and maybe a few hundredths over parts 
of the Sierra Thursday/Friday. The large bulk of the 6 day QPF will 
fall Sunday and Monday. Highest amounts over the Smith Basin with 
little to no precip expected to make it south of the Golden Gate. 
QPF 12z Sun-12z Tues: 0.50-1.50" north coast, 0.50-1" Shasta, 0.10-
0.75" northern Sierra, and a few hundredths to 0.25" central Sierra 
and the rest of nrn CA.

In addition to precip, this next system will begin to lower 
temperatures from nw to se on Sunday with much of nrn/cnt CA 
expected to see near to below normal temperatures. For Monday, 
anomalies are forecast at -10 to -25 deg F. Freezing levels will 
drop from 10-13 kft or so this week to 5.5-11 kft north of I-80 
Sunday afternoon. Lower freezing levels will spread across the 
region early next week down to 4.5-9.5 kft north of I-80 mid Monday 
morning and down to 4-6.5 kft in the evening. By the end of the 
period, these lower levels will push as far south as Monterey County.


Please Note: This product may NOT be routinely updated this
summer. Please refer to the following product issued by the CNRFC
www.cnrfc.noaa.gov/Daily-Briefing for a graphical summary of
weather and hydrologic conditions.