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FXUS66 KSTO 052147 AFDSTO

Area Forecast Discussion National Weather Service Sacramento CA 147 PM PST Tue Dec 5 2023


Synopsis
The lingering patchy fog in the San Joaquin valley has lifted out of the area this afternoon, but may reform later again tonight.
Next weather system moves in Wednesday and will bring showers and mountain snow through Thursday evening, mainly along and north of Highway 50. Drier conditions on Friday with locally breezy north winds.

Discussion
GOES-18 satellite imagery shows the lingering patchy fog has dissipated. Some high clouds are moving through the area, with breaks of sunshine for most of the area. Today will be the last day of dry weather as an upper-level weather system from the Pacific Northwest pushing into the region on Wednesday, bringing rain and snow showers through Thursday night.

Precipitation is expected to begin Wednesday morning starting over the Coastal Range and northern Sacramento Valley and spreading across the rest of the region throughout the rest of the day. The National Blend of Models (NBM) has a 60-95% probability of 1" inch or more of rainfall from Chico northward on Wednesday. From the San Joaquin Valley to Sacramento there is a 10-50% chance of receiving 0.50" inches of rainfall Wednesday. The general consensus is the further north you are, there will be higher rainfall totals. Additionally, the NBM has a 10-15% chance of some thunderstorms to develop, mainly for the northern Sacramento Valley and the Sierra on Wednesday. Rainfall totals are 0.10" - 0.50" inches from Sacramento to Red Bluff, with higher amounts of 0.75" - 1.50" inches north of Shasta Dam and in the Sierra.
Snowfall is also expected over the Northern Sierra with this system, with a 10-40% chance of 4" inches or more of snowfall at pass levels. Snow levels are currently forecast to drop to 6500 feet Wednesday afternoon. A general 2-6 inches of snowfall accumulation is expected at pass levels Wednesday.

Thursday another weather disturbance pushes into our area, with lower snow levels, although rainfall totals will be much lighter than Wednesday totals. Best chances for rainfall are in areas north of I-80. Areas from Red Bluff northward have a 30-90% chance of 0.50" inches of rainfall on Thursday with higher probabilities the further north you go. Generally, 0.20" inches or less of total rainfall is expected for the central Sacramento Valley. and areas southward and into the Delta. Latest NBM runs have snow totals for pass levels have trended up, with and additional 4-8 inches now forecast. Snow levels will be around 4000 feet Thursday night. With the increased snow amounts and lower snow levels, a winter weather advisory has been issued for Thursday from 10 am -10 pm for elevations above 4000 feet in the Sierra Nevada Mountains. This will impact traffic on I80 and Highway 50, so persons with travel plans should plan accordingly.

Friday the trough will exit the area to our east and upper-level ridging will develop which will help dry things out. Locally breezy northerly winds may develop, mainly for the western side of the Sacramento Valley with northerly wind gusts of 15-20 mph.
Temperatures Wednesday through Friday are expected to cool thanks to the influence of the upper level trough, with Valley highs in the mid 50s to low 60s and low 40s to low 50s for higher elevations.

.EXTENDED DISCUSSION (Saturday THROUGH Tuesday)...

Clusters continue to show ridging developing off the coast of SoCal and extending into the forecast area. Light precipitation is possible on Saturday and early morning Sunday as a subtle shortwave in the Pacific NW moves moisture on top and over the ridge. Chances will be mainly limited the Northern Sacramento Valley and mountainous areas with low confidence in rain extended far into the Valley. Chances decrease as you move southward down the Sierra.

An upper level trough will move into the Great Basin late Monday and Tuesday, giving the chance for more light precipitation over the Sierra and breezy conditions. Clusters still indicate uncertainty on the timing and position of the troughing.

AVIATION

One more night of quiet VFR to MVFR conditions for Northern California before a weather system moves in on Wednesday. There is still the potential for some IFR/LIFR in the southern Sacramento/northern San Joaquin airports between 09z-18z. Breezy south to southwest winds will also impact the northern Sacramento valley airports ahead of the weather system...mainly from 12z-00z Thursday.

STO WATCHES/WARNINGS/ADVISORIES
Winter Weather Advisory from 10 AM to 10 PM PST Thursday for elevations above 4000 feet Burney Basin / Eastern Shasta County- Shasta Lake Area / Northern Shasta County-West Slope Northern Sierra Nevada- Western Plumas County/Lassen Park.

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