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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
1230 PM PST Wed Dec 31 2025

...INCREASING SHOWERS TODAY BECOMING HEAVY OVER SRN CA OVERNIGHT TO 
MID THURS AM AS A LOW APPROACHES FROM THE SW, T-STORMS POSSIBLE...
...WET PATTERN PERSISTS INTO EARLY NEXT WEEK AS ADDITIONAL SYSTEMS 
DROP IN FROM THE GULF OF ALASKA... 

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

An offshore upper low continues to draw tropical moisture into 
central/srn CA this afternoon as TPW imagery shows 1+" PW from north 
of Sonoma County inland to the foothills and down to Baja. Radar 
showing more widespread reflectivity than this morning as that 
moisture travels northward. The layer has finally moistened up and 
gages are reporting accumulations of about 0.10-0.50" over the past 
few hours along the coast. Not much has changed regarding the 
overall pattern and the model differences in the extended. Heavy 
precip overnight through mid Thursday morning over the Transverse 
with afternoon thunderstorms, particularly for coastal srn CA. A 
pair of additional lows is expected to end the week and late this 
weekend. The det GFS/ECMWF have not changed much in their 
disagreement in the fate of the low into early next week while 
ensemble spread remains high. Models have trended a little drier in 
the 12z runs across coastal srn CA causing a decrease in QPF of 
about 0.10-0.75" over the 6 days and wetter up over the Bay Area by 
0.25-1". 

The afternoon update was mainly a blend of the latest WPC guidance, 
NBM, and the previous forecast with a little 18z HRRR thrown in 
within the next 24 hours over smaller areas. QPF through 12z Tues: 
2.50-6" along the coast LA County northward (6-12" for the coastal 
mountains), 1.50-2.50" south of LA along the coast (up to 5.50" 
mountains), 6-17" northern Sierra, 4-9.50" central/southern Sierra, 
and 1-4" down the valleys.