Cool and unsettled weather is expected through much of the week with shower and thunderstorms chances. The higher elevations will see some late-season accumulating snow.
Satellite imagery shows the trough centered offshore continuing to drop south off the coast. The upper level front and associated band of mid and high clouds has pushed through the Central Valley and is now over northeast California and northwest Nevada. Behind the front, IR difference imagery shows areas of low clouds developing that will become more extensive early this morning as the boundary layer cools and marine layer cloudiness spreads inland. Onshore surface pressure gradients continue, though they have slackened since Sunday evening, and local southerly gusts of 15-30 mph are still present.
Current temperatures are considerably cooler compared to 24 hours ago, especially inland, and are running around 5-15 degrees cooler. Readings are in the mid 40s to mid 50s through much of the Central Valley.
The upper-level ridge that brought the heat last week has shifted east over the Rockies. The trough along the west coast evolve into a cut-off low and will become the primary driver for this week’s cooler and more unsettled weather.
The trough will continue to deepen along the coast today with additional short-wave energy forecast to drop south to around the Bay Area by late today, and we may see a few showers develop along the Coast Range and over the northern Sierra. By this evening, more widespread shower activity is forecast to develop across much of the region and continue into Tuesday.
Travel impacts are expected to be pretty limited tonight and Tuesday morning with perhaps an inch or two of wet slushy snow at the pass levels in the Sierra.
More extensive showers, and perhaps a few thunderstorms, will be possible Tuesday afternoon and Tuesday night which could lead to greater impacts over the higher northern Sierra passes with accumulations of 8 to 12 inches possible along the crest. Snow levels will be around 5-6k ft.
The trough lingers over the region through the end of the week, though most shower activity is expected to occur over the mountains after mid-week.
Temperatures will also begin to moderate somewhat, but will still be well below average for early May.
Extended Discussion (Friday through Monday)
Clusters and ensembles show good confidence in a cool and unsettled pattern continuing to persist into the weekend, with a reinforcing low dropping down from the Gulf of Alaska late in the week.
High temperatures will remain 5 to 12 degrees below normal with many Valley locations forecast to remain in the upper 60s to low 70s, though a very gradual increase is expected into the weekend.
Current cluster solutions vary in depth of the trough for weekend but along with the National Blend of Models (NBM) continue to maintain a slight chance (15-25%) to chance (25-40%) of precipitation, mainly for the northern mountains.
Overall moisture looks quite light, so impacts should be limited. The Valley is forecast to see little to no precipitation, except maybe for the northern Sacramento Valley.