An active weather pattern then arrives late Friday, lasting into early next week. Periods of moderate to heavy Valley and foothills rain and high elevation (snow levels generally 6000 feet or greater) mountain snow are expected with these periodic systems. There is also a slight (10 to 25%) chance for isolated Valley thunderstorms on Monday as well.
Discussion
As of early this afternoon, mostly cloudy skies are evident across much of interior NorCal. This will work to keep temperatures slightly cooler than yesterday, but still generally above normal by the end of the day. Given the abundant cloud cover present today, the chances for fog development will be a bit lower for Friday morning, but still nonzero. Latest HREF probabilities of seeing visibility drop below 1/2 mile are in the 30 to 60% range from the southern Sacramento Valley into the northern San Joaquin Valley, with highest probabilities along the latter. With the combination of cloud cover and potential fog development, overnight low temperatures into Friday should remain rather mild, which will lead into another day of above normal high temperatures on Friday as well.
Despite this quiet period, a more active weather pattern is still on track to begin late Friday as an upper leveltrough currently over the eastern Pacific continues to trek eastward. This upper trough looks to gradually move toward the Great Basin and Desert Southwest throughout the weekend while a second wave begins to arrive late weekend into early next week.
As a result, precipitation chances will begin to overspread the region from west to east Friday night, becoming more widespread overnight into Saturday and tapering off slightly by Sunday morning with the initial system. Lingering showers will likely persist throughout the day on Sunday, but the precipitation impacts from the second wave then look to arrive late Sunday and continue through Monday.
Circling back to the initial wave, generally late Friday through Sunday, compared to previous runs, the system is overall trending wetter. Probabilities of exceeding 1″ of QPF are in the 15 to 30% range for the northern San Joaquin Valley, 50 to 80% range across the Sacramento Valley (highest in the northern Valley), and 70 to 90% range over the foothills and mountains.
Overall, this system looks to be on the warmer side with snow levels from late Friday through Sunday in the 6000 to 6500 foot range. This equates to a 65 to 95% probability of snow exceeding 8″ for areas generally above 6500 feet. Given the warm, moist nature of this system, the anticipation is for heavy, wet mountain snow.
Furthermore, gusty southerly winds also look to accompany this system on Saturday as the surface low tracks through the area. Highest winds are expected across the central Sacramento Valley, generally along a line from Yuba City toward Red Bluff, where a 60 to 95% chance of gusts exceeding 35 mph exists.
As the upper trough begins to trail off toward the Great Basin and Desert Southwest, the periods of heavier precipitation will briefly cease during the day on Sunday. Some showers will continue to linger throughout the day on Sunday though ahead of the next arriving wave.
The approaching second wave will usher in periods of moderate to heavy precipitation once again beginning Sunday evening and persisting into the early week ahead. Otherwise, given the expected cloud cover and persistent southerly winds, temperatures will remain rather stagnant with Valley high temperatures in the mid to upper 50s and low temperatures in the upper 40s to low 50s.
Extended Discussion (Monday through Thursday)
Ensembles and clusters indicate that active pattern continues Monday. The back end of the strongest storm will be moving though early Monday morning. This is a period with an increase in the potential for ponding of water on roadways, flooding of low lying areas, increased stream levels and flows, mud and rock slides.
Snow levels continue to be high between 6500 and 7000 feet. The storm will gradually taper off to showers by the afternoon. Zonal flow is forecast over the area Tuesday night and Wednesday when a weak system is indicated by the various models and ensembles to move through the region.
Snow levels continue to be high at around 6000 feet. This system looks to bring generally around a half inch for the valley, less in the San Joaquin valley to an inch or more around Redding. Probabilities of at least half an inch of rain range from 5-10% over the San Joaquin Valley to 25-35% Sacramento area to 40-75% central and Northern Sac Valley. Mountains range from 25% in the south to 60-80% north of I-80.
A ridge will build over the region on Thursday to bring dry weather. Mild high and low temperatures are expected though the expected period.