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December 23, 2018 – Light precipitation across NorCal today. Heavier precipitation Monday (Christmas Eve) with moderate to heavy snow bringing mountain travel impacts over the northern Sierra and mountains of western Plumas County into the early morning hours on Tuesday (Christmas Day). Light lingering mountain snow showers possible Wednesday and Thursday.

Discussion

Lots of cloudiness along with some areas of light precipitation across the region early this morning as fast zonal flow across the eastern Pacific impinges on NorCal and displaces the ridge eastward. Beneath the higher clouds, stratus remains banked up along the eastern edge of the Central Valley from about the eastern suburbs of Sacramento southward into the San Joaquin Valley. Temperatures are considerably milder across most of the region compared to 24 hours ago and range from the upper 20s to mid 30s in the mountain valleys to mainly the 40s elsewhere.

Light amounts of QPF are expected today, mainly over the mountains and to the north of Sacramento in the valley, as warm-advection continues ahead of the stronger system for tomorrow. Snow levels will be around 5500-6500 feet with around 1-3 inches of snowfall accumulation possible across the higher elevations.

Little change in the details for Monday`s stronger system other than raising snow levels slightly ahead of the cold front. Precipitation will increase over the mountains during the day, but the bulk of the rain in the valley will occur during the afternoon and evening as the frontal band sweeps southward. Strong forcing and increasing instability may allow for some embedded thunderstorms with enhanced rain rates with the front, but its rather quick movement should limit overall rain amounts at any one location. We`ll continue to monitor.

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Peak snowfall in the northern Sierra is expected Monday afternoon and evening with the likelihood for travel impacts across the passes with 8-12 inches of snowfall accumulation expected. A Winter Storm Watch remains in effect.

Upper low shifts east by early Christmas morning, so a significant drop off in precipitation should occur early, especially in the valley. Brisk northerly flow will bring drying and clearing Christmas Day into Wednesday except for some lingering showers possible across the highest elevations.

Extended Discussion (Thursday through Sunday)

Little in the way of sensible weather in the extended period. A shortwave trough over the Pacific Northwest may bring some light precipitation Thursday into Friday over the southern Cascades area. No impacts are expected at this time. An amplified positively tilted trough will dig into the southwest US by Friday and any precipitation will push south of Northern California leaving dry conditions by Friday afternoon.

Breezy, northerly winds still look likely for Thursday and Friday afternoons with an increased pressure gradient setting up over the Sierra during these times.

By Sunday, models vary considerably and have kept a dry forecast going through the end of the weekend.