The forecast for the next few days has undergone a complete transformation over the past 48 hours. No longer are we looking at a moisture-starved, extremely cold inside-slider type system bringing some snow to sea level in CA, but instead a deep and powerful but significantly retrogressive cutoff low off the coast with excessive moisture of a subtropical origin. I’ll get to that in a minute…
Storm #1
Today, an entirely separate cutoff has generated a band of heavy rain and embedded thunderstorms that is currently progressing down the coast and will reach the San Diego area within a couple of hours. Quite a bit of rain fell with this band in some locations, especially in more northerly parts of the Central Coast. Showers and isolated thunderstorms will continue over SoCal overnight and into tomorrow, with the risk of some strong storms containing hail or waterspouts/brief weak tornados. Showers with the potential for isolated thunderstorms will redevelop over NorCal tomorrow as the center of the upper low moves overhead and 850 mb temps drop to -3 C or so. Snow will fall down to 1500-2000 feet once again, but accumulations are less certain. Rain will never really end over the state before storm #2 begins to drop in…
Storm #2
This is the storm that yesterday was thought to have the potential for sea-level snowfall in NorCal. This is no longer a consideration, as the models dig the low center well offshore and develop powerful southerly flow aloft on the east side of the circulation, entraining copious subtropical moisture as it does so and causing a dramatic spike in 850 mb temps (from -3C to +9 C over NorCal over 24 hours!). The evolution of this storm system is extremely uncertain at this point, but there is a significant potential for very heavy precipitation over much of CA Friday into the weekend. It’s too convoluted at this point to try to specify which locations will recieve the most rainfall or the exact magnitude of the precipitation, but suffice to say that the mountains of the Central Coast will likely see over 12 inches of liquid on top of the 4-7 inches they recieved today. Coastal areas in SoCal stand a decent chance of seeing some very heavy rainfall, as well. This is also the sort of pattern that can lead to embedded severe thunderstorms, esp. far SoCal, with very intense squall lines embedded in the deep subtropical plume. We’ll see. Believe it or not, there has been enough snow in the southern Bay Area mountains that a rain-on-snow flood event is a possibility–something that I cannot recall ever having taken place (mountains near Big Sur coast and the Santa Lucias have seen 20-24 INCHES of snowfall over the past 48 hours; the highest East Bay hills have seen 3-6 inches, and parts of the Santa Cruz Mountains and the Diablo Range have seen 4-10 inches!). This is a system that bears watching, but it could suprisie us either with its intensity or with its paucity. Certainly, all the ingredients necessary for a memorable event will be present…
Storm #3 and beyond
The remnants of the Friday storm will be lifted over NorCal late in the weekend, bringing showers and isolated thunderstorms with lowering snow levels once again. A pretty cold system drops in early next week, bringing back the potential for low snow levels once again. The longwave pattern remains favorable for a cold and unsettled pattern for quite some time, at least in NorCal. We’ll, folks, it IS a strong La Nina year…
