Very active weather in California this week

March, despite being a rather quiescent month weather-wise in the state of California, will end on a very active note. A major pattern change is already beginning to impact far northern California this evening, with light rain beginning to spread south from the North Coast. Mostly light rain will continue to spread as far south as about the Bay Area by tomorrow afternoon. By Tuesday, though, a major cold frontal boundary will push south across the northern half of the state, bringing increased rainfall and much colder temperatures (along with some windy conditions). Temperatures aloft will drop rapidly behind this main frontal boundary, below 0 C at 850 over all of NorCal by late Tuesday afternoon. As a result, lapse rates will begin to increase and isolated thunderstorms will be possible from around San Francisco northward on Tuesday and Tuesday night. The main event, though, will wait for Wednesday/Thursday.

Water vapor imagery depicting active Pacific storm track (NOAA)

On Wednesday, strong cold air advection will continue over the northern two thirds of the state and begin across the southern third, and 850 mb temperature will further drop to as low as -4 C near Sacramento and -5 C near Eureka. 500 mb temps will drop to around -34 C or even a bit lower. These are very cold temperatures for late March/early April in Calif0rnia and in conjunction with solar surface heating during the day on Wednesday will lead to extremely steep lapse rates. Thunderstorms are almost certain on Wednesday in some part of NorCal, though the aerial coverage of deep convection is hard to pinpoint this far out. If the main impulse remains offshore until late in the day or overnight, convection will be more of the “popcorn” variety with scattered to numerous showers and isolated to scattered thunderstorms with lots of small hail.


NAM depiction of main "impulse" moving ashore in NorCal on Wednesday night (NCEP)

When the aforementioned impulse does move ashore, however, convection will likely become much more widespread in coverage and potentially more organized. Recent model runs indicate that this impulse may not traverse the region at peak heating, which would tend to mitigate the severe weather potential. Even so, however, hail and heavy rain would be expected in virtually all stronger cells, and some severe storms are certainly not out of the question. NorCal will also be in the jet exit region at this time, with a strong cyclonically curved jet just to our south, so the severe potential is not negligible. Southern California will begin to see increasing lapse rates and convective chances by late Wednesday, as well, but especially on Thursday. Thursday may be another convectively active day in NorCal, depending on how quickly the trough axis moves to the east. Southern California stands a decent chance of some thunderstorm activity in this time frame, as well.

Low snow levels will also be a concern with this system, as well. Unseasonably cold air aloft will bring snow levels and/or wet bulb zero levels to 2000 feet or lower throughout NorCal. I would not be surprised to hear of some snow reports from Redding or the far northern Sacramento Valley between 1000-1500 feet overnight on Wednesday. In fact, it’s possible that snow levels could even drop as low as 1500-2000 feet in Southern California, so snowfall is possible in some of the higher inland valleys. The Sierra foothills and the higher Bay Area peaks could see rather significant early April snowfall accumulations, as well. Accumulations of small hail in intense showers or thunderstorms are likely in isolated locations throughout the state, irrespective of elevation.

It does not appear that the active pattern will end on Friday (though there may be a break on that day). Model progs are currently indicating the potential for another cold/cutoff low in the 7-10 day time frame. All in all, a pretty active spring pattern is shaping up for the entire state, with some highly beneficial late-season snowfall in the mountains.

UPDATE: 3/29

Convective chances for Wednesday still on track. Below is a rather impressive visible satellite loop depicting the vast field of cold air cellular cloudiness (cumulus) over the Pacific headed for CA…

Visible satellite imagery showing extensive cold air convective cloudiness over Pacific (NRL)


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