December 21, 2018 – Light precipitation today mainly morning hours. Light showers over the Sierra this afternoon. Dry Saturday with another round of light precipitation across the north state Sunday. Heavier precipitation Monday (Christmas Eve) with moderate to heavy snow bringing mountain travel impacts over the Sierra and mountains of western Plumas County into Tuesday (Christmas). Light mountain showers Wednesday and Thursday.
A Pacific frontal band now dropping southeast through Northern California bringing light showers across the forecast area. Snow levels are somewhere around Pass level over the Sierra so even light precipitation will bring minor mountain travel impacts today.
Precipitation will be shifting into the Sierra this afternoon with drying elsewhere today. Some high cloud cover is likely tonight but valley fog still probable Saturday morning. Remainder of the day Saturday looks dry under brief upper level ridging. Increasing cloud cover Saturday will keep valley temperatures from climbing much but daytime highs should still come in a little above season normals.
Another weak Pacific storm system moving into the Pacific northwest and NorCal Saturday night will bring down the upper ridge a bit as the ridge axis shifts east of the state. Light precipitation will accompany this system as well with light precipitation expected across the northern mountains Saturday night then slipping southeast across most of the forecast area during the day on Sunday. Snow levels below pass levels will bring mountain travel impacts again but low precipitation amounts will keep impacts minor. A stronger and moister Pacific low will be dropping into Northern California on Monday, Christmas eve. The heaviest precipitation over the Sierra Cascade range will be Monday night with significant snowfall over the major Sierra passes.
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Extended Discussion (Tuesday through Friday)
By Christmas morning, a mid-level trough will be located over Southern California moving east and out of the area. The GFS is quick to clear precipitation by Tuesday morning, while the ECMWF holds on through the mid-afternoon. The Winter Storm Watch continues through Tuesday afternoon, but future products may need to be refined as timing becomes more clear.
Ridging builds back in Wednesday with predominately dry conditions expected across California this day. A shortwave may move across the Pacific Northwest on Wednesday night, bringing chances for rain to the northern portion of the state and mountains before conditions clear again on Thursday. Model consistency is still low for the ladder half of the week, so stay tuned for future updates.