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Morning Valley and foothill fog possible today and Wednesday. Another weather system could bring mountain snow, light Valley showers, and colder temperatures late Wednesday into Thursday. Cold overnight lows and frost Thursday night into the weekend. Unsettled weather late weekend into next week.

Discussion

Showers continue moving south through the Northern San Joaquin Valley and the Central Sierra early this morning, and should be out of the forecast area by daybreak. High clouds are also clearing north to south as the upper trough moves into Central California, and patchy fog could form in the Valley and foothills this morning in areas that get enough clearing. Dry and warmer conditions return today in between weather systems. Clear skies will result in better chances for patchy to dense Valley and foothill fog in the late evening hours tonight through tomorrow morning.

Another round of precipitation is slated for Northern California late Wednesday into Thursday. This is an interesting setup that is still bringing some uncertainty to the forecast. Ensemble guidance continues to show a trough tracking out of the Gulf of Alaska into British Columbia Wednesday with a shortwave digging south and strengthening along the backside of the trough into Northern California into Thursday. The track seems to be a bit quicker and producing less moisture than previous guidance.

Some light showers are possible ahead of the main frontal push in Shasta County and the Southern Cascades Wednesday afternoon/evening, but the best chances for widespread rain and mountain snow will be late Wednesday night through early Thursday.

The majority of precipitation will fall in the Sierra and foothills, mainly south of Plumas County, generally around 0.2-0.6 inch over the foothills and up to an inch over the Sierra. Rain totals will be much lighter over the Valley with only a few hundredths in most locations and up to 0.15-0.25 inch along the far eastern side.

This will be a colder system as it will bring cold air down from BC, so snow levels will begin around 6000-7000 feet Wednesday evening, quickly falling to 4500-5500 feet by early Thursday.

Snow amounts are forecast around 3-7 inches, locally up to 10 inches, for elevations above 5500 feet, with lighter amounts down to 4500 feet. The Winter Storm Watch remains in effect from 4 pm Wednesday through 4 pm Thursday. Those with Sierra travel plans should prepare for hazardous driving conditions, significant travel delays, and chain controls.

Cold air will funnel into California behind the trough passage Thursday into Friday. Overnight temperatures will be the coldest of the season so far with Valley and foothill lows in the upper 20s to mid 30s and teens to 20s in the mountains and foothills Friday and Saturday mornings. This will bring widespread frost to the area.

Daytime temperatures will also remain chilly and a few degrees below normal, with the Valley in the low 50s, and 20s to 40s in the mountains and foothills.

Extended Discussion (Saturday through Tuesday)

Fri and Sat morning expected to begin with frosty conditions in the Valley with min temps expected to be in the 30s, possibly near or a little below freezing especially in the N end of the Valley (i.e. RDD,RBL). Meteograms from the GFS show a tighter spread in the shorter term temps than the ECMWF, suggesting more confidence in the potential for the initial frost/freeze headlines for interior NorCal at the end of the week.

As the long wave trof progresses Ewd across NOAM Fri and through the weekend, a drier but cool NW flow pattern is likely to develop over NorCal on Fri before quickly transitioning to cyclonic Wly flow Sat and WSWly flow Sun as the GOA/NErn Pac upper low begins to dig Swd off the W Coast of NOAM.

Initial frontal precip band forecast to move into NorCal early Sun morning, with the front stalling over the Sierra into Tue morning due to aggressive digging of the trof off the coast and not much Ewd progress early Tue. Model differences become more apparent late Tue/early Wed while the GFS holds back a lot of energy similar to Clusters 2 and the multi-model ensemble.

Regardless, this will be a multi-day precip event for NorCal with at least a weak AR, if not moderate AR intensity especially for coastal locations early next week. With the origin of this low from the Bering Sea and Arctic influences as well, this will be a cold system with the potential for heavy Sierra snowfall as well. Higher snow ratios are expected for the Sierra as the colder air moves inland late Mon and Tue.

Timing may vary given the amplitude of this digging system, but as of now the 12th-14th looks to be main impact period for the first wave. After which, timing of successive systems is variable and uncertain.

BTW, on this “…date (12/7) which will live in infamy,” Four Score years ago, no precip was recorded at DTS, RDD and SCK, MOD or BLU according to the xmACIS2 data.