December 17, 2019 – Dry weather will continue today. Precipitation chances return tonight and continue into the weekend.
Discussion
High clouds are increasing across the area as the upper ridge shifts east and the offshore trough approaches. Temperatures are once again on the chilly side and we’ll likely see patchy frost again through much of the valley early this morning. Current temps range from the single digits in the mountain valleys to the 30s to lower 40s across the Central Valley.
Water vapor imagery shows the upstream system splitting and closing off as it approaches 130W. The system will swing onshore tonight spreading showers across most of the region by early Wednesday. QPF is expected to generally range from 0.10″ – 0.25″. With the cold, relatively dry, air in place and showers moving in Wednesday morning, initial low elevation snow isn’t out of the question. Elevations below 1000 feet including the city of Redding could see a few snow flakes Wednesday morning with any showers. Little to no accumulation is expected at low elevations. Snow level rise after sunrise, rising to 3000 to 4000 feet. Snow accumulation is looking to be 1-4″ with most of the accumulation above 4000 feet.
Shower chances diminish Wednesday evening but the dry period of weather will be short-lived. Troughing over the PacNW will bring warm air advection over the northern part of the area north of I-80 late Wednesday night and continue into early Friday before retreating further to the northwest during the day on Friday. The system will be able to pull more moisture with it with the highest totals expected over the northern mountains and Coast Range. Snow levels Thursday morning start out around 4000 feet but rise to 5500 to 6000 feet by Thursday afternoon.
Extended Discussion (Saturday through Tuesday)
Widespread precipitation returns this weekend as a storm system is forecast to impact interior NorCal. Model differences are still evident regarding timing/QPF amounts, so specific storm details will likely change as the event gets closer. NBM guidance indicates precipitation spreading over the Coastal Range, the northern mountains, and the northern Sacramento Valley by Saturday morning; then, spreading to the rest of the area in the afternoon/evening hours.
Ensemble guidance suggests moderate to elevated 24-hour precipitation probabilities for 0.50″ and 1″ over the Coastal Range and western Shasta County. Snow levels from 5000-6000 feet Saturday, then falling to 4000-5000 feet Sunday. This system could lead to travel difficulties over the mountains, especially since it is the pre-holiday weekend. Precipitation chances could linger into early next week, though uncertainty remains in terms of coverage.