December 22, 2018 – Dry today 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.
High pressure slipping over the west coast bringing dry conditions and mainly fair skies this morning although some high clouds are spilling over the ridge. Light northerly winds have brought drying most areas and the central valley is seeing little in the way of fog or low clouds this morning. Exception is the northern San Joaquin valley and delta where light fog or low clouds are still being mixed out by the northerly winds.
Drier air and clearer skies are allowing for cooler temperatures this morning with many valley morning lows likely to drop into the 30s. RH progs indicate high cloud cover will spread in over the north state this afternoon but daytime highs are still likely to come in a few degrees above normal. A weak Pacific frontal band is forecast to push into the Pacific Northwest tonight spreading precipitation into the northern zones north of about Chico by morning.
Precipitation chances will sink southward to about Sacramento by Sunday afternoon but models tending to keep this system farther north so may see actual precipitation not reaching this far south. Either way, this appears to be a fairly dry system so only light precipitation is expected and even with below pass level snow levels, mountain travel impacts will be minor.
Next Pacific low pressure system follows quickly on the heals of the Sunday system pushing precipitation across the forecast area by Monday morning. By Monday afternoon, upper level diffluence ahead of the low and good orographics will bring increased precipitation over the northern Sierra just in time for holiday travel.
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Snow levels below pass levels are likely to bring significant travel impacts over the major Sierra passes even through precipitation amounts have trended down slightly from previous runs. Winter storm watch still looks like a good bet. Upper low shifts east of the state by Christmas morning Tuesday so a significant drop off in precipitation should occur by Tuesday afternoon.
Northerly flow will bring drying and clearing Tuesday night with another cool morning expected Wednesday.
Extended Discussion (Wednesday through Saturday)
Early Wednesday morning an upper-level closed low will be located over the Four Corners region with upper ridging over the west coast. A shortwave behind this ridge will move across the Pacific Northwest Wednesday into Thursday bringing small chances for precipitation to the northern portion of California and mountain regions. Minimal impacts are expected at this time for this mid- week system.
Winds in the Valley and ridges Thursday and Friday may be breezy at times as the pressure gradient across the Sierra increases as another positively tilted trough digs into the southwest US. Dry conditions seem likely into the early weekend next week.