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December 20, 2018 – Patchy valley fog this morning. Light rain spreading across NorCal late today through Friday morning. Showers ending Friday afternoon. Dry Saturday with another round of light precipitation across the north state Sunday. Heavier precipitation Monday (Christmas Eve) with mountain travel impacts likely over the Sierra.

Discussion

Upper level ridge over the west coast bringing dry conditions this morning over the forecast area. Upper ridge axis has shifted east of the state allowing some high clouds to spill over the north state this morning. Even with the high clouds, areas of fog and low clouds have developed throughout the central valley but hopefully high cloud cover will keep any from becoming dense. Cloud cover will be on the increase today as a weak Pacific frontal band approaches the coast. Even with increased cloud cover, daytime highs should come in a little above normal.

The weak frontal band is forecast to move across the forecast area late this afternoon through Friday morning. Only light precipitation is expected with this system so even with snow levels beneath pass levels, mountain travel impacts should be minimal. Showers should end over the Sierra by Friday evening as high pressure starts to rebuild over the west coast. Decreasing cloud cover will likely allow for at least some Valley fog Saturday morning. Saturday should turn out to be a dry and fairly nice day with daytime highs still a bit above normal.

A shortwave through moving through the Pacific northwest Saturday night will bring a chance of rain and higher elevation snow Saturday evening through Sunday with best chances north of about Sacramento. Again, precipitation amounts are expected to be light so mountain snow impacts will again be minimal.

Extended Discussion (Monday through Thursday)

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The start to the week should be a wet one as a well advertised upper trough drives through Northern California on Christmas Eve. Models continue to depict a more northwest to southeast motion which typically is not the biggest rainfall producer for Valley locations. Based on the latest forecast guidance, a quarter to half inch of rain can be expected over such regions. Farther inland, enhanced topographic lift will augment amounts across the foothills and western slopes of the Sierra and Southern Cascades. Moderate to locally heavy snow is likely at pass level along main thoroughfares including Interstate 80 and Highway 50. While these numbers are subject to change, 12 to 18 inches could fall with this system with snow picking up in intensity Monday morning into the afternoon hours. Exact timing will likely change in future model cycles given spread among the reliable forecast guidance. In addition to the mentioned precipitation chances, gusty winds may be possible with the passage of the cold front. Please check back for future updates on this system given potential holiday travel impacts.

Looking ahead to Christmas Day, the timing differences between the 06Z GFS and 00Z ECMWF models do leave a window for shower activity leaking into the holiday. The slower ECMWF brings an upper low through during the morning to afternoon hours which would foster the development of scattered showers. The pattern beyond Christmas Day looks to be a little more uncertain with numerous model differences noted over the Central/Eastern Pacific. An upper ridge building between 150-140W longitude will allow another system to dive southeastward through the state by mid-week. While precipitation impacts are nebulous, the associated cold frontal passage should keep temperatures close to slightly below normal for much of next week.