Active weather continues through the end of the week, with periods of moderate to heavy snow today. A brief early weekend lull will then be followed by another system Sunday into Monday with another potential system on the horizon later next week. Periods of moderate to occasional major travel impacts expected.
Discussion
A robust, but progressive shortwave is tracking toward interior NorCal early this morning. Radar returns are rapidly increasing across the region with the arrival of this system and periods of moderate to heavy precipitation are expected throughout the day. Heaviest precipitation is anticipated at higher elevations, generally above 2000 feet, although a low 5 to 15 percent chance of isolated thunderstorms will be possible later today from Interstate 80 southward. Additionally, a a window of gusty south- southeast winds is expected this morning through the early afternoon as a surface front progresses. Strongest Valley gusts up to 45 mph are expected from around Chico southward at this time, with a Wind Advisory in effect from 5am PDT this morning through 5pm PDT this evening.
Outside of thunderstorms, mostly moderate precipitation is expected, with totals across the lower elevations remaining around 0.25 to 0.5 inches. Across the northern Sacramento Valley, Northeast foothills, and Sierra/southern Cascades, precipitation totals of 0.5 to 1.5 inches are anticipated. Snow levels are expected to remain relatively steady around 2000 to 3000 feet today for much of the region today, although some lingering uncertainty surrounding northern Sacramento Valley snow levels this morning does exist. This uncertainty is primarily tied to the strength of mid level frontogenesis with the approaching shortwave and the conditional impact of resultant localized cooling. While most accumulations are expected above 2000 feet, there is some nonzero potential for a brief period of accumulating snowfall as low as 1000 feet in the northern Sacramento Valley this morning.
Otherwise, snowfall totals of 6 to 15 inches are expected above 2000 feet and up to 2 feet over peaks for the Coast Range and portions of the northern Sacramento Valley, including the northeast foothills, Burney Basin, and Shasta County mountains. Along the Sierra/southern Cascades, snowfall totals of 10 to 18 inches are expected above 4000 feet with up to 2 feet over peaks, while accumulations of 5 to 10 inches remain possible between 2000 and 4000 feet. Periods of moderate to major travel impacts are expected, with hazardous conditions along both the Interstate 80 and Interstate 5 (in Shasta County) corridors anticipated today. As the shortwave remains progressive, a brief period of ridging aloft looks to build in through most of the weekend. Some showers may linger across far northern Shasta County during this time, but predominantly dry weather is anticipated for the remainder of interior NorCal, although temperatures will remain near to slightly below normal.
Extended Discussion (Monday through Friday)
Some uncertainty continues with the next expected trough late weekend into early next week, but additional periods of at least moderate precipitation are anticipated as it moves through. Ensemble guidance generally agrees that the overall trajectory of the system would favor potential for slightly higher precipitation totals relative to the Friday system, but some uncertainty regarding the magnitude of the attendant atmospheric river plume keeps forecast confidence in exact details low at this time. Current probabilities moving into next week are as follows:
Probability of Rain > 1″ Late Sunday-Tuesday AM: 20-40% Valley/foothills
Probability of Snow > 12″ Late Sunday-Tuesday AM: 60-95% above 4500`
Probability of Wind Gusts > 40 mph Sunday: 60-95% north of I-80
Probability of Thunderstorms Monday: 15-25%
It continues to look like most precipitation is expected to be coming to an end by Tuesday morning, with primarily lingering mountain showers anticipated at this time. Ensemble guidance is trending toward continued active weather moving into late next week, but consensus indicates a stretch of dry weather much of Tuesday through Wednesday. Regardless, be sure to monitor the latest forecast at weather.gov/sto and plan ahead for continued cool, wet and windy weather into early next week!