Heavy snow event in NorCal this week

The last few days have brought lots of mainly orographically-forced cloudiness and precipitation to CA. Snow totals in the Northern and Central Sierras have been rather significant over the past few days…measured in the 2-3 foot range in some places. This locally heavy precipitation along with relatively low snow levels (3000-4000 ft) have been very helpful in easing the snow-water content defecit that has developed during this dry winter season, and much more snow is on the way. The next 12-18 hours will see showers over NorCal…mainly near the coast and over higher terrain. The main story is the system that will move in later on Monday. This cold front will herald the arrival of very cold air aloft and a periods locally heavy convective precipitation. The cold front itself will be pretty strong and dynamic, with a nearly 100 kt jet and strong cold air advection driving a relatively consolidated frontal band south over the northern 2/3 of the state. There is a good chance that the front will contain some strong convective elements, including thunderstorms with gusty winds. Relatively brief heavy rain/snow will fall with the front. More important, however, will be the cold convective regime that moves in behind the front. There will be a brief break in the shower activity immediately following the main cold front as a dry slot and post-frontal subsidence moves overhead. As this happens, though, the air at 850 mb will cool dramatically. 850 mb temps will drop as low as -6 C in the Eureka area by later Tuesday, -5 C in the Bay Area, and -3 C in the Los Angeles area or even a bit further south. Needless to say, snow levels will be quite low across the region, especially considering the amount of precipitation that is expected to fall while the cold air is present. In far NorCal, snow will fall as low as 500-1000 ft, perhaps as low as sea level during heavy precipitation. In the Bay Area and over much of interior CA, snow will fall at elevations as low as 1000-1500 feet, perhaps even a bit low during the heaviest showers. SoCal will see fairly low snow levels, too–maybe 2000-3000 feet. With all this cold air aloft, the question becomes how much precipitation will fall. Very cold air aloft…combined with the higher angle of the late February sun and the associated increased insolation potential and occasional vorticity maxima rotating through the region…will lead to some rather intense convection over CA. Although the potential for severe weather is not particularly high with this event, some strong thunderstorms will probably develop over the Central Valley, and some fairly numerous storms could also impact coastal regions. The main threat with these storms…even along the coast…would be locally heavy accumulations of hail. In fact, small hail could fall from the taller cumulus clouds, even the ones without lightning, just about anywhere. As for snow accumulations: 2 to possibly 5 feet could fall on the highest peaks in the Sierras, but 1-2 feet of the white stuff could fall even at lower elevations in the Coastal Range. In fact, Bay Area peaks such as Mt. Hamilton, Mt. Diablo, Mt. St. Helena, Ben Lomond in the Santa Cruz Mountains, and perhaps even Mt. Tamalpais could see some very significant (for these parts) snowfall…perhaps on the order of 8-12 inches on the higher peaks and 3-7 inches on the lower ones. Heavy snow will also fall near the Eureka area, though not in the town itself. Many highways in that region briefly climb into the 1500 to 2500 foot range, which may become impassable at times throughout the coming week. The story for SoCal is not nearly so exciting…but the Southland probably will see some light rainfall from the initial front followed by some scattered showers and isolated thunderstorms with low snow levels. I know–not much to go on–but it is better than some past drought years (plus, your drinking water is safe for the year, it appears). It’s interesting to note that 1976 and 1977–years during which CA faced its most dire water shortage in modern history–were topped off by a weak to moderate El Nino event. These weakly positive ENSO years can produce some of the driest years on record. We’re not quite at that point this year, but in January the situation was looking awfully precarious. Until later…