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December 18, 2018 – Light rain north of Marysville today and tonight. Dry Wednesday then light rain spreading across the region late Thursday and Friday. A period of heavier precipitation early next week.

Discussion

Dry over the CWA this morning under high pressure ridging. Some morning fog has formed from about Sacramento southward, but clouds spilling over the ridge ahead of the next weak frontal band precluding much fog development farther north. Some light showers are already showing up over the north coast with light precipitation expected to spread over the northern CWA later this morning. Building upper ridge will bring clearing by Wednesday morning so patchy morning valley fog may pop up again from about Sacramento southward.

Otherwise, fair skies are expected Wednesday afternoon with daytime highs climbing to around 10 degrees above normal. Another fair night Wednesday will bring more patchy valley fog early Thursday. Another Pacific frontal band pushes through late Thursday and Friday. Precipitation amounts are expected to be light again with this system but snow levels will be below pass levels so minor mountain travel impacts are likely.

Extended Discussion (Saturday through Tuesday)

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An active Pacific storm track should keep additional chances for precipitation into the weekend as well as early next week. Beginning Saturday, the start of the day is forecast to remain dry ahead of the next shortwave trough nearing the Northern California coast by the evening. Model guidance continue to be at odds with one another as the 00Z ECMWF favors a much wetter scenario across the region. This would mainly impact the second half of the weekend if this were to verify. On the contrary, the 06Z and 00Z GFS runs support a storm track farther north. Unsettled conditions can be expected but specific impacts still remain nebulous given model differences at hand.

Looking into next week, Christmas Eve could prove to be quite wet across the region. The mentioned runs of the GFS and ECMWF each show a much more impressive trough sweeping through Northern California by Monday. However, ensemble forecast solutions show a great deal of uncertainty with the timing and strength of this system. Stay tuned to future updates as this feature becomes better resolved. Given the current guidance available, it could be a very cold system which would lower snow levels over the Sierra and Southern Cascade ranges. Some lingering showers could extend into Christmas morning while a slightly cooler air mass enters the picture.