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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
740 AM PDT Thu May 8 2025

...MOSTLY DRY CONDITIONS AND ABOVE NORMAL TEMPERATURES THROUGH 
SATURDAY AS HIGH PRESSURE BUILDS OVERHEAD...
...A LARGE COLDER SYSTEM IS EXPECTED TO BRING PRECIP TO THE 
REGION LATE SUN THRU EARLY NEXT WEEK...

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

Dry conditions across the region this morning as an upper ridge 
builds overhead. Behind this ridge, a large upper low is traveling 
across the Gulf of Alaska. This low will eventually make its way to 
CA to bring the next round of precip. High pressure will continue 
our warming trend today with above normal temperatures in the 
forecast by about 5 to 15 deg F. TPW imagery shows some lingering 
moisture over parts of CA (0.50-0.75") as a front continues to 
traverse the area. This may generate a shower/thunderstorm or two 
over the Sierra later today, but nothing significant is expected. 
The ridge will continue to build in overhead the rest of the week 
keeping dry conditions in place and allowing temperature anomalies 
to rise to +10 to +20 deg F for Friday and Saturday. 

A trough will develop off the south side of the Gulf low on Saturday 
headed towards the west coast. This will allow some initial cooling 
over the north coast for Saturday while as mentioned before the rest 
of CA/NV will see above normal temperatures. The core of the low 
will then shift to the south throughout Sunday sending showers into 
CA. The timing of precip has slowed down since yesterday, and models 
continue to disagree on the details of this system. Ensembles of 500 
mb height patterns are divided as well on the timing and positioning 
of the low into early next week. For now, the majority of the precip 
for Sunday is forecast to stay north of Cape Mendocino aside from 
lighter showers reaching Sonoma County. Both the GFS and ECMWF show 
the low moving inland into the PacNW on Monday while troughing digs 
into nrn CA. Precip to spread into central CA throughout the day. 
Troughing will dig deeper into the region into Tuesday with showers 
possible over parts of srn CA. The trough should begin to exit into 
the Four Corners by the afternoon with showers lingering into early 
Wednesday (mainly over the Sierra and eastward). As the low/trough 
moves overhead, thunderstorms will also be possible. Ensemble spread 
regarding the QPF remains similar to yesterday with 24 hr QPF values 
at Arcata anywhere from 0" to nearly 2" between the GFS/ECMWF. There 
is decent agreement on a system impacting the area late this weekend 
into early next week, but low confidence on the timing and amounts 
of precip.

All the QPF in the forecast, aside from a couple hundredths over the 
Sierra, is for the extended with the best day for precip likely to 
be Monday. Highest amounts over the northern/central Sierra and the 
Smith Basin. QPF 12z Sun-12z Weds: 0.50-1" north coast/nrn Sierra, 
0.25-0.75" Shasta, 0.10-0.75" central Sierra, a few hundredths to 
0.25" southern Sierra and the rest of nrn CA, and less than a tenth 
for coastal srn CA.

In addition to precipitation, the system will lower temperatures 
from nw to se starting Sunday when much of the CA coast will drop to 
below normal. This trend will continue into Monday with widespread 
anomalies of -5 to -15 deg F. Cooler temperatures will persist for 
Tuesday as well. Freezing levels will drop from 10-13.5 kft or so 
this week to 5.5-10 kft north of I-80 Sunday afternoon. Lower 
freezing levels will spread across the region early next week down 
to 4.5-8 kft north of I-80 mid Monday morning, these lower levels 
will push as far south as Monterey County by the evening. Freezing 
levels to bottom out early Tuesday at 4.5-7.5 kft north of Point 
Conception before rebounding from sw to ne as the system exits into 
Wednesday.


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.