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December 22, 2019 – Rain and snow bring travel impacts above 4000 feet into tonight. Unsettled conditions continue into the Holiday week but no major systems expected.

Discussion

Our winter system is taking its time to move into CA but we are starting to see light precip push into the Coastal Range. Currently the trough axis is near 30N and 125W or so. The trough axis will slowly push east this morning pushing through NorCal around 18z/10am. This will help to push a cold front through the area during the morning reaching the mountains by the early afternoon. Widespread precipitation is expected out ahead of the front this morning into the afternoon. There is quite a bit of moisture with this system and we could see moderate rain and snow for a time as the front pushes through.

We are also going to see breezy southerly winds ahead of the front. The strongest winds are expected in the southern Sac Valley into the northern San Joaquin Valley and adjacent foothills. The strong winds will likely only last a few hours and will start to diminish quickly with the frontal passage. These strong winds should pick up over the next few hours and will help to mix out some of the patchy dense fog we have been seeing in the Sacramento to Marysville area.

Snow levels this morning are around 4000 to 4500 feet and are expected to rise to around 5000 feet this afternoon. Sierra travel conditions will likely deteriorate quickly after 15z/7am as snow becomes widespread.

Activity will become more showery by the mid-afternoon as we see energy on the back side of the trough dig towards SoCal. This will prevent favorable lapse rates for afternoon convection. Have decided to take t-storms out of the forecast. The majority of shower activity will diminish overnight but showers will linger into Monday and Monday night south of Highway 50 as a portion of the trough pushes through SoCal. Mainly dry and cold conditions are forecasted for Monday night with patchy frost likely in the valley.

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Another short wave trough will approach the region late Tuesday into Wednesday. The better forcing is expected to stay to our west but we should see enough lift for some scattered showers across the area. Moisture will be low with this system and anything we see will remain light.

We will see low snow levels with this system though with them as low as 2000 feet Wednesday morning and only rising to 3000 to 3500 feet Wednesday afternoon. The majority of the accumulating snow is expected above 3500 feet and should remain light under 4″ but some light accumulation will be possible below 3500 feet.

Extended Discussion (Thursday through Sunday)

Ensembles in good agreement indicating a return to dry weather and northerly flow on Thursday into the weekend as ridging from the eastern Pacific shifts over NorCal in the wake of the mid-week trough. Uncertainty increases late in the weekend as models diverge significantly on the strength of the next upstream trough. For now, will lean toward the NBM mentioning a return of precip chances across the far northern portion of the state.