December 10, 2020 – Gradual cooling into the weekend. A pattern change late week into the weekend with rain and snow chances returning to the forecast.
Discussion
A short wave trough is pushing into the northern Rockies with upper level ridging building into the PacNW. There is also an upper level low located over SoCal pushing into AZ. This is helping to create an increased pressure gradient over the Sierra and the northern portions of the area resulting in breezy winds. Currently we are seeing gusts up to 25 mph across the northern Sac Vally and generally gusts to 30 mph over the Sierra although there are some locally higher gusts in the typical wind prone areas. These winds have likely peaked and they will diminish this morning as the gradient across the area weakens. In the areas that are seeing the breezy winds you can expect only moderate humidity recoveries.
A trough will then dig out of the Gulf of Alaska and push into the PacNW and far northern CA during the day today. This will also push a weak cold front through the area. With little to no moisture in place no precip is expected as the front and trough move through.
Quiet and dry weather is expected for much of Friday but a deeper trough over the Pacific will be able to push moisture back into NorCal by Friday night. This trough will also bring warm air advection to the area starting Friday night and continuing into Saturday morning. Showers will develop over the Coastal Range after 0z Saturday and spread across the reminder of the area Friday night. The best shower chances still look to remain over the higher elevations but valley locations will likely also see shower activity. Snow levels will be low Friday night in the 2500 to 3500 foot range and some light snow down to 3000 feet will be possible over Shasta County and down to 4000 feet over the Sierra but accumulations will remain light. Snow level will quickly rise Saturday morning and will rise above pass level during the afternoon as short wave ridging builds in. This will also limit shower activity with just some light showers over higher elevations into the Saturday evening.
The trough that will bring WAA Friday into Saturday will dig out the Gulf of Alaska on Sunday and push over NorCal. This will also push a cold front into the region likely reaching northern areas early Sunday morning and pushing through the rest of the area during the day Sunday. Widespread showers can be expected along and ahead of the front. Snow levels ahead of the front will be hovering around pass level and will fall to 4000-5500 feet Sunday night. Most shower activity will diminish Sunday evening but showers will linger in the Sierra into Monday as the trough axis works through. Storm total QPF right now is looking to be 0.25-0.50″ over much of the Valley with 0.75-1.25″ in the northern Sac Valley and 1-2.50″ over the higher elevations, locally higher amounts possible over the Shasta County Mountains. Storm total snow is looking to be about 4-8″ over the Sierra passes with maybe a few inches over I-5 Friday night.
Highs will be similar to yesterday to a few degrees cooler in some locations with much cooler highs by Friday with them returning to near average. Highs will remain near average in to the weekend. We will see chilly overnight lows tonight and frost will be likely Friday morning across the area. Warmer overnight lows expected after tonight.
Extended discussion (Monday through Thursday)
Unsettled weather likely into next week, as moist west to northwest flow keeps an active pattern in place. It will be hard to nail down the details of the numerous quick moving systems expected in the extended, until they progress more in the short term. It does appear confidence is increasing for an above normal precipitation period from mid-December progressing towards the Christmas holiday. Will stay in line with NBM guidance at this time, given timing challenges