Satellite imagery shows high clouds beginning to spread across the region ahead of the offshore trough. Current temperatures remain very mild and range from the upper 20s to lower 30s in the mountain valleys to the mid 40s to mid 50s elsewhere.
Despite the increasing cloud cover, today is expected to be another dry and mild one for the most part. Temperatures will cool a bit due mainly to the increase in clouds as synoptic cooling is not forecast to really take hold until this evening. A few showers may occur late in the day across the higher elevations of the Coast Range and northern Sierra Nevada.
The strong short-wave that`s currently crossing 30N/130W will sweep northeastward into NorCal bringing an onset of showers and possibly a few thunderstorms by this evening.
Main impacts expected over the northern Sierra where accumulating snow will likely lead to travel delays beginning late tonight. Snow will initially be light and at pass levels (around 7k ft), then become moderate in heavier convective showers after midnight, with snow levels dropping to around 5500 feet Wednesday. Total snow accumulations of 4 to 8 inches, with localized amounts up to 12 inches are possible.
Valley total rainfall will less than half an inch with locally heavier amounts near thunderstorms. Thunderstorms will be possible again Wednesday afternoon and early evening as the core of the upper approaches.
A relative break in showers is expected on Thursday under short wave ridging ahead of another system that moves through Friday and Friday night.
Extended Discussion (Saturday through Tuesday)
Guidance for the weekend continues to show a trough passing the region on Saturday with weak ridging building behind it. Rain and snow showers will likely linger over the foothills and the ridges with drier weather expected by Sunday morning. Model and ensemble guidance continue to hint at wet weather for early next week, with a broad low pressure system sitting off the west coast. Rain could make into northern California as early as Monday morning, but the timing is likely to change as we`re fairly far out in the extended forecast.