Seasonably warm and mostly dry weather persists through the end of the week. A pattern shift toward cooler and more active weather is then anticipated by the weekend. Confidence is increasing for periods of rain and isolated thunderstorms across the Valley/foothills along with mountain snow impacts from Sunday into next week.
Discussion
In stark contrast to yesterday morning, GOES-West satellite imagery depicts clear skies across interior NorCal this morning as the ridge axis becomes centered over the region into today. While some Valley fog potential will be possible through the early morning, it will likely be more patchy in nature and confined to generally Interstate 80 southward where more appreciable overnight humidity recovery is expected. Current probabilities of visibility less than 1 mile sit around 10 to 20 percent at this time. Otherwise, given the overall stagnating pattern aloft and generally clear skies through the middle of the week, above normal high temperatures are expected to prevail. For today, widespread high temperatures in the 70s are expected throughout the Delta, Valley, and foothills, with low to mid 60s at higher elevations. Thursday then looks to see high temperatures jump an additional 3 to 5 degrees as ridging aloft amplifies, with even some 60 to 80 percent probabilities to reach 80F in the northern San Joaquin Valley.
Ensemble guidance is then indicating some disagreement in the evolution of a cutoff low approaching the West Coast Friday into the weekend. The majority of guidance is trending toward a southward progression of this trough, bringing attendant impacts primarily to SoCal, although roughly 33 percent of ensemble membership is hinting at a trajectory toward the Bay Area before transitioning southwestward. In this latter scenario, the introduction of some mid level moisture aloft by Friday could result in some scattered shower and isolated thunderstorm potential from the Interstate 80 corridor southward, with current probabilities around 5 to 10 percent. Regardless of the exact track the system takes, a cooling trend is expected to take hold Friday into the weekend. High temperatures in the lower to mid 70s for low elevations and 60s at higher elevations are expected for Friday, falling to low to mid 60s across the lower elevations and 50s at higher elevations by Saturday afternoon.
Extended Discussion (Sunday through Wednesday)
Overall trends in the upper level pattern remain largely consistent to prior runs within ensemble guidance, with confidence beginning to increase for precipitation impacts through the end of the weekend and into early next week. Despite this, ensembles do show some minor differences in precipitation onset timing late Saturday into Sunday. Regardless, precipitation impacts are expected by Sunday at least, with some isolated thunderstorm potential around 10 to 20 percent across much of the region Sunday afternoon as the trough moves through.
Snow levels beginning 5000 to 6000 feet on Sunday are expected to fall to 4000 to 5000 feet by Monday as the trough progresses. While exact precipitation details remain a bit uncertain at this time, probabilities of up to 0.5 inches of precipitation through Monday sit around 10 to 30 percent throughout the Valley and 40 to 70 percent across the Shasta County mountains and Sierra/southern Cascades. Despite lowering snow levels into Monday, the majority of accumulating snowfall is expected above 6000 feet along the Sierra/southern Cascades, with 30 to 60 percent probabilities of snowfall greater than 8 inches through Monday for these locations.
Clusters indicate some minor differences in timing and magnitude, but consensus is trending toward an active pattern continuing into the following week as periodic shortwaves eject across the western CONUS. Exact details remain largely uncertain, but an extended period of wetter weather is looking likely into the beginning of March. With some ensemble guidance depicting limited time between progressing waves, the potential for exacerbating precipitation impacts will continue to be monitored. Otherwise, this general pattern shift will continue to favor near to below normal temperatures across the extended as well.