Active weather continues through the end of the week, with a brief early weekend lull followed by another system late Sunday into Monday. Some low elevation snowfall possible Friday morning. Periods of moderate to occasional major travel impacts expected.
Discussion
Scattered convective precipitation lingers across interior NorCal early this morning as mostly cloudy skies and breezy winds prevail. Primary impacts from any stronger showers will be occasional heavier precipitation rates, although some low chances for isolated thunderstorms will persist through the morning as remnant instability persists. While some small hail cannot be completely ruled out this morning, threats from this elevated convection will be mostly tied to brief heavy rainfall, gusty winds, and lightning. Instability is then expected to increase this afternoon as low elevation clearing builds in behind morning precipitation.
Some 25 to 35 percent probabilities of thunderstorm development then exist this afternoon, with highest probabilities across eastern portions of the Valley into the foothills. Overall precipitation totals through the remainder of the day will be largely tied to this scattered activity, with generally 0.1 to 0.25 inches of precipitation expected at lower elevations and higher totals of 0.5 to 1.5 inches across higher elevations.
Snow levels are expected to remain relatively steady around 2500 to 3500 feet today, with the majority of accumulating snowfall remaining above 3000 feet through this evening. While heaviest snowfall has generally concluded, periodic times of snowfall rates up to 1.5 inches per hour will be possible throughout the day with any stronger convective cells aided by orographic lift moving along the terrain. Additional snowfall totals of 8 to 12 inches will be possible above 4000 feet along the Sierra/southern Cascades through this evening, with some 30 to 60 percent probabilities of snowfall greater than 12 inches above 5000 feet.
Very little downtime in mountain precipitation impacts is expected between the exiting system and a rapidly arriving system overnight into Friday. Details around snow levels remain uncertain but nonzero probabilities of accumulating snowfall as low as 1000 feet in the northern Sacramento Valley and as low as 1500 feet along the Sierra/southern Cascades foothills do exist, particularly early Friday morning. Despite the quick moving nature of this system, periods of moderate to locally heavy snowfall will be possible at times on Friday.
For the Sierra/southern Cascades, probabilities of exceeding 8 inches of snowfall on Friday sit around 40 to 80 percent above 4000 feet. Across the Coast Range/northern Sacramento Valley, probabilities of exceeding 8 inches of snowfall on Friday sit around 40 to 80 percent above 2000 feet. As a result, additional travel impacts are expected along the Sierra/southern Cascades and along portions of the Interstate 5 corridor as well.
While the orientation of the passing shortwave on Friday will keep precipitation totals for much of the Valley under 0.25 inches, a period of gusty southerly winds is expected from mid morning through the afternoon. Current probabilities of wind gusts exceeding 40 mph on Friday sit around 40 to 70 percent across most of the Valley on Friday. 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 Thursday)
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 4000`
Probability of Wind Gusts > 40 mph Sunday: 60-95%
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 past the extended forecast period, 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!