Another strong winter storm will bring multiple feet of snow with major impacts expected once again over the foothills and mountains Saturday into early next week. A warmer weather system is possible late week and into next weekend, but confidence is low on details and impacts.
Discussion
Building upper ridging into Alaska will help stall upper level low pressure off the Pac NW coast today through the middle of next week with multiple waves of mid-level vorticity rounding this gyre and pushing into northern CA with the first shortwave trough axis crossing the far northern CA coast this morning. At the same time, a pronounced and intensifying Pacific jet streak, progged to reach 150+kt by tonight, will overlap with the mid-level confluence to surge moisture across northern/central CA.
The overlap of height falls, PVA, and left jet exit diffluence along the upper jet will provide widespread deep layer synoptic lift to support rounds of moderate to heavy snowfall over the Sierra through Sunday night before rates decrease with the very occluded system.
Snow levels will rise up to 3000 ft today over the Sierra ahead of this first wave, but then lower this evening following cold front passage after 5pm, remaining below 2000ft through the event with significant snow down to many foothill communities.
2 to 5 feet of snow are forecast over much of the Sierra Nevada through Sunday night with 5 to 20 inches down into the foothills. During the heaviest snow, rates may approach as high as 2 to 3 inches per hour.
Southerly winds will increase tomorrow and be gusty through the afternoon and overnight into early Sunday.
Mountain and foothill travel will become extraordinarily difficult to impossible with extended delays and closures highly probable. Conditions will deteriorate after midnight tonight. Always check with Caltrans for the latest road conditions, restrictions, and closure information. And remember: NEVER drive beyond any road closed signs/barriers.
Total rainfall is only expected to bring minor impacts such as slick roadways and ponding of water in low-lying areas. There is around a 10 to 20 percent chance of afternoon thunderstorms today and Sunday afternoons, which may produce briefly heavy rainfall, small hail, and gusty/erratic winds. Total rainfall will generally range between 0.5 to 1.25 inches.
The very stalled pattern and focus of the Pacific jet farther east allows the heavier snow over the Sierra Nevada to become much more intermittent Monday/Tuesday, but the onshore flow does last through midweek which will likely hinder efforts to help Sierra Nevada communities buried in recent snows, as several inches of snow are possible each day.
Extended Discussion (Wednesday through Saturday)
Ensembles and clusters keep a large scale upper level trough over the West Coast States through the end of the week. This will maintain cool and unsettled weather conditions over NorCal through the week.
Weak disturbances rotating through the trough will provide periods of light snow through Friday. Relatively low snow levels will bring snow down into the upper foothills at times.
Current forecast snow amounts would suggest 2 to 3 inch per day rates, which could bring 4 day totals to near a foot in some areas. Temperatures will be about 10 degrees below normal with highs in the 50’s and lows in the 30’s at Valley locations.
Uncertainty increases in the Friday/Saturday timeframe, as ensembles deviate drastically on strength and timing of potential system next weekend.