This should be a quick discussion, but given the complicated nature of the current weather situation, this may be difficult. A major winter storm is currently gathering strength over the open waters of the Pacific, taking aim at areas from far NorCal to British Columbia. With a deep tropical tap (PW plume of 1.7 inches), very heavy rain totals are likely in upslope locations along much of the West Coast. Very steep pressure gradients east of a deepening surface low will contribute to damaging winds across the OR and WA coastlines, with gusts in excess of 100 mph possible in normally windy areas. River flooding could occur, a result of heavy rainfall and melting snow in the mountains. The Bay Area will be on the far southern receiving end of this precipitation, possibly recieving as much as 1-2 inches in the far North Bay. By Thursday, however, another dramatic change in the pattern will occur. The jet stream will buckle substantially offshore and allow the storm track to briefly sag southward across all of CA. This southward jog will drag some very cold air aloft down from the Gulf of Alaska, and potentially produce some very interesting weather. The details of this upcoming cold event are still uncertain, but it does look like the potential for significant rain, thunderstorms, and lower elevation snowfall is a real one. 850 mb temps drop to nearly -5 degrees Celcius as far south as the Bay Area by Sunday, and drop below 0 degrees Celcius as far south as central Baja CA. Needless to say, snow levels will be at their lowest levels of the season thus far in CA, below 2000 feet in the north and below 3000 feet in the south. If the coldest GFS scenario pans out, snow could briefly fall to near sea level in heavy showers. Speaking of showers–the convective potential with this system looks to be quite substantial, as very cold air aloft, good moisture through the atmospheric column, decent surface heating if the clouds part, and the added effect of a strong jet in SoCal will be present. These ingredients could combine for some strong thunderstorm activity in all of CA, and possibly severe acivity in SoCal. The main threat would be hail, but some waterspouts and funnel clouds are also a good bet. A tornado cannot even be ruled out with a system this cold, esp. favored areas in SoCal. Lots of details still to be worked out, so stay tuned.