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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
740 AM PDT Fri May 9 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 INTO WEDS...

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

A strong ridge of high pressure currently sits overhead as a an 
upper low approaches from the Gulf of Alaska. Dry conditions today 
and tomorrow aside from a slight chance of isolated 
showers/thunderstorms over the Sierra in the afternoon/evening. High 
pressure will result in well above normal high temperatures through 
tomorrow with anomalies generally +10 to +20 deg F. A trough will 
reach the west coast on Saturday, but not soon enough to break the 
heat away from the immediate nrn CA coast. Troughing will start to 
head inland into Sunday as a low approaches from the northwest. 
Initial showers expected to reach the north coast Sunday morning out 
ahead of the main front. Precip to spread along the nrn CA coast 
overnight with showers likely remaining confined west of I-5 until 
Monday morning when the front arrives. Precip to then spread inland 
across nrn CA and into the nrn Sierra as the front and parent upper 
low head inland. There will be at least some chance of thunderstorms 
over nrn CA as the low moves through in the afternoon/evening. The 
low will then dig further south across CA into Tuesday as the 
eastern edge of the system enters NV. This will keep showers going 
across the region for Tuesday with continued thunderstorm chances. 
The best chances for showers/thunderstorms at this point will be 
over the Sierra and eastward into NV. Precip will diminish from west 
to east Tuesday as the low begins to exit into the Four Corners. 

Lingering showers for Wednesday, mainly over the Sierra and NV, as 
the back edge of the low keeps troughing over much of the region 
with drying along the coast as high pressure attempts to rebuild 
offshore. Both the GFS and ECMWF keep some troughing over the area 
through the rest of the period. The change in pattern will also cool 
things down pretty abruptly starting Sunday and lasting through the 
rest of the period. Afternoon temperature anomalies will go from +10 
to +20 deg F Saturday to -5 to -15 deg F Monday (locally to -20 deg 
F). There is still disagreement with the ensembles on the timing and 
positioning of the low with 24 hr QPF spread now on the order of 
about an inch. Though, the range has narrowed since yesterday from 0-
2" to about 0.50-2" for Monday at Arcata.

QPF for the period is expected Sunday to Wednesday with lighter 
lingering showers on Wednesday generally on the order of less than a 
tenth of an inch. Highest amounts along the north coast, Shasta, and 
the northern Sierra. Amounts have gone up over nrn CA since 
yesterday by about 0.10-0.50" while lowering over the central Sierra 
by 0.10-0.25". QPF was a blend of WPC guidance and the 13z NBM. 
Forecast 12z Sun-12z Weds: 0.50-1.50" north coast, 0.50-1" nrn 
Sierra/Shasta, 0.10-0.75" central Sierra/rest of nrn CA, a few 
hundredths to 0.25" southern Sierra/Bay Area, and less than a tenth 
for coastal srn CA.

Freezing levels will drop from 11-14 kft today to 5.5-10.5 kft north 
of I-80 Sunday afternoon. Lower freezing levels will spread across 
the region early next week down to 4.5-7.5 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 5-7.5 kft north of Point Conception before rebounding 
from sw to ne as the system exits into Wednesday. By late afternoon, 
expect most of the region to be back up to 8-10 kft.


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.