Christmas storm for CA, then a drying trend…

The weather across CA has remained unsettled but not overly wet over the past week. After the low snow event of last week, temperatures aloft and at the surface have moderated, and it is now seasonably cool across the state. A quick look at satellite imagery shows a rapidly-developing storm system about 700 miles west of San Francisco and moving in a roughly southeasterly direction. The models have struggled with the exact placement and intensity of this storm, and the location of the low center as it moves across the coast will be crucial in determining how much rain will fall or how strong winds will be. Currently, it looks like the system may be moving a little to the north of the consensus forecast, which would tend to bring stronger winds and heavier rains to NorCal. SoCal still stands to see quite a bit of rain out of this system, especially the Central Coast near Santa Baraba. The Transverse Range may squeeze as much as 6 inches of precip out the moist atmosphere, though most places will see much less, generally in the 1-2 inch range. Further north, in the Bay Area, I expect widespread 1-2 inches amounts with locally as much as 3-4 inches in the mountains if the system takes the more northerly track. This system is forecast to reach its maximum intensity and depth of the low center in about 12 hours, with the low gradually filling and lifting eastward after that. Therefore, the system will be weakening as it approaches the coast and it remains to be seen how strong the pressure gradients are at that point. I do expect to see wind gusts locally over 50 mph in the Bay Area, Central Valley, and higher elevation locations almost everywhere else. This storm does have the slight potential to surprise us with significantly stronger winds if it deepens more than expected, however. There will be at least a small potential for thundestorms near the front, as dynamic forcing will be strong with this system (indeed, that will be the main method of precip generation with this storm). Once the main low center has moved up and east of the area by Christmas Day, strong cold air advection will bring unstable conditions and widespread convective showers to CA, especially in the north. Isolated thunderstorms and small hail will certainly be likely in the north, and snow levels will fall considerably once again. Brief flurries will be possible to sea level once again on the far North Coast though the general snow level up there will be around 500-1000 feet. In the Bay Area, snow may fall as low as 1000-2000 feet, though there is not expected to be as much accumulation as in the last storm. The Sierra foothills will see snow once again on Christmas Day, and even the Grapevine in SoCal may be a bit treacherous with some slushy/snowy conditions.

After Thursday, the pattern will quiet down considerably, and conditions will dry out across the state. In fact, after tomorrow’s storm, light warm-advection type rains in far NorCal wil be the only precipitation going well into January. This is not a good sign, as even with this upcoming system NorCal is significantly below the norm for this time of year. An extended dry period in early January will certainly not help this. Let’s hope that this is just part of our “annual 2-3 week midwinter dry period.” Anything longer and we’ll be in trouble.