The pattern over the next 48 hours in NorCal will be rather exciting. Some pre-frontal thunderstorms this AM may have caused some winds damage not too far from Sacramento…and may have even been caused by a tornado. Check back in later for more info. A very cold upper low is spinning off the WA/OR coastline this evening, circulating bands of intense convective showers and isolated to scattered thunderstorms into WA, OR, and the northern half of CA. These squally lines of showers and thunderstorms are expected to become scattered during the overnight hours but increase in intensity as the day goes on tomorrow as another organized imoulse moves ashore. Also…daytime heating will more than likely lead to scattered thunderstorms over the Sacramento Valley as mid-tropospheric temperatures will be extremely cold for these latitudes (-25 C at 500 mb). The biggest concern of all, however, are snow levels. 850 mb temps are dropping rather rapidly this evening and will fall to nearly -5 C over the Bay Area by tomorrow afternoon. 850 mb temps near the OR/CA border will be as cold as -6.5 C. With such cold numbers, most places in the country would be seeing substantial snowfall even at the lowest elevations. The marine influence around CA will likely prevent snow from falling at sea level, but snow levels will be quite low for the next 48 hours. In the far north…snow will likely fall on most valley floors away from the immediate coast, with general snow levels of 500-1000 feet (locally lower). This includes parts of the far northern Sacramento Valley (from about Redding northward), where a bit of snow could fall in towns below 1000 feet in elevation. In the Bay Area snow levels will eventually fall to 1000-1500 feet (north) to 1500-2000 feet (south). This includes every major peak in the Coastal Range, and even some of the local hilltops. It is possible that some major roadways in the Bay Are could be impacted by significant winter weather conditions, including Highway 17 in the Santa Cruz Mountains, Pacheco Pass, and Highway 29 in Napa County. If snow really does fall at the 1000-1500 foot level, many more area roadways will be adversely affected. Significant precipitation is expected to continue until well after the coldest air aloft has moved in, so some truly impressive snowfall totals are possible where it does indeed snow. Peaks such as Mt. Hamilton and Mt. Diablo, in the immediate vicinity of the Bay Area, could see accums. in excess of a foot. Stay tuned. SoCal will see some rain out of this, but not much. Snow will fall at pretty low elevations, probably shutting down the Grapevine for a while at some point during the next 48 hours. Aside from the outside chance of isolated thunderstorms south of Santa Baraba, however, nothing too exciting to report at the moment. This is likely to be the “event of the winter” for NorCal, though…