November 13, 2020 – Rain, snow, and breezy winds return today with mainly dry conditions over the weekend. Above normal temperatures expected early next week, with chances for additional precipitation mid- week.
An upper level trough is digging out of the Gulf of Alaska and currently the trough axis is just off the coast of the PacNW. This trough will continue to dig and move over OR and far NE CA today bringing rain, snow and breezy winds for us. Weak warm air advection has developed over northern portions of the area and warm air advection will continue to strengthen this morning north of I-80 as the trough drops south.
This will spread showers over areas mainly north of I-80 during the morning with the first showers reaching the Coastal Range around 12z/4am. Showers will continue to overspread the remainder of the area late morning into the afternoon. Widespread showers are expected along the a cold front which will reach northern areas around 21z/1pm and continue to work south into the late afternoon and evening.
Snow levels this morning are generally between 3500-4500 feet, locally lower over Shasta County. As the warmer air builds into NorCal we will see snow levels rise up to 5000-6000 feet by the afternoon but they will remain locally lower over northern Shasta County and I-5 north of Shasta Lake. Showers will diminish pretty quick behind the cold front Friday evening but some light showers will linger in the Sierra into Friday night.
Total QPF expected with this system is looking to be around 0.25-0.50″ in the Redding area to less than a 0.10″ as you head down the Valley towards Modesto. For the mountains it’s looking like 0.75-1.00″ north of I-80 to under 0.75″ south of I-80. As for snow a few inches of snow is expected in northern Shasta County mainly above 3000 feet including I-5 north of Sims. The majority of the accumulating snow for the Sierra will be above 5500 feet with amounts generally 3-6 inches, locally higher but some light accumulations will be possible down to 4500 feet.
This system will also bring breezy onshore winds with Valley gusts from 20 to 35 mph. Winds will be stronger in the Sierra Nevada with gusts 40 to 45 mph possible.
Dry conditions with less wind are expected across the area Saturday morning with some clearing. This will lead to fog especially for valley locations and could be locally dense. A short wave trough will track into BC late in the day into Sunday and this will bring some warm air advection to northern areas and the return of some light showers mainly over higher elevations. QPF looks to remain light with this and snow levels will remain well above pass level. Upper level ridging builds in later Sunday into Monday with dry weather returning.
Highs today will be running about 10-15 degrees cooler than yesterday. Highs will gradually warm over the weekend and will push above average into early next week.
Extended discussion (Tuesday through Friday)
Cluster ensemble analysis is in good agreement ridging will build over the Western US for the start of next week keeping our area dry. Solutions diverge by mid-week with a large range of possible weather patterns. The clusters with the highest percentages of the total mean indicate a low pressure system impacting northern California by mid to late week. This will likely bring some unsettled weather to the region.