The Sacramento valley of northern California — as well as the San Joaquin valley, act as a canvas for surface based convection, including supercells, under favorable setups much more often than most think. Tornadoes occur in the central valley on a near-seasonal basis, with generally a few occurring each stormy season (October – May), mostly from supercellular thunderstorms. The strongest tornado in the past several years was an EF-2, which comparably to the central and eastern U.S. isn't significant, but it defeats the idea that tornadoes don't occur in California. If you ignore the strength and look at numbers, California sports the highest average annual tornado count in the western U.S., an interesting stat that may have multiple causes. California's supercellular, tornado-producing thunderstorms also separate themselves developmentally from storms east of the Rockies in regards what part of a storm they most commonly form in. Another way them separate themselves is their size, something that will be comparatively examined below.
Reported/recorded tornadoes across northern and central California from April 2000 – 2014.
The Sacramento and San Joaquin valley's (mostly) flat surface allows unhindered surface winds to flow without being hindered by mountains, a problem storms elsewhere in the west suffer from. The one and only obstacle lies in the the middle of the central Sacramento valley, that being the Sutter Buttes. This small mountain range is small enough to limit negative effects for storms nearby, and actually may aid shear on a small scale to the north of it. Most setups in which supercells are able to form and produce tornadic activity occur within a trough with a upper low offshore, driving surface winds up from the south (to the north). This southerly fetch of surface wind impacts the Sutter Buttes on the south face, and likely does two things; 1). some wind blows upslope, and 2). wind blows around the Buttes. This can create a very small wind eddy, or perhaps create a tad bit of convergence just north of the Buttes. Either one can have interesting and typically non-destructive effects on a strong or already rotating thunderstorm traversing across the valley.
On the note of wind, the southerly surface winds associated with most notable severe weather setups in northern California occur beneath a westerly mid and upper-level jet, creating clockwise turning (veering) winds with height, or better known as directional shear. When you've got turning winds with height, this allows stronger thunderstorms that reach up vertically enough to put this shear to use. If a thunderstorm's updraft is strong enough, it can take that turning of wind with height (horizontal rotation/rolling) and tilt it near-vertically, getting the thunderstorm rotating and increasingly capable of tornadic activity the stronger the shear and updraft is. However, before you even have that thunderstorm, you've got to have some instability and a way to get them developing in the first place.
A hodograph I'd expect to find on a day with potential for tornadic storms in the central valley.
Most severe weather events in California occur not ahead of a cold front in a warm sector of a trough or upper-low, but behind this within the cold pool, a place not many would expect to find potential for severe weather. Cold pools, while offshore, feature vast pools of open-celled cumulus and associated convection. Cold pool convection is typically most robust near the vort-max of a trough or the upper-low, thus wherever this impacts inland is where the best shot at thunderstorms is. In order to have thunderstorms, instability is key. A perfect setup would involve a cold front and it's associated precipitation slide through and exit the valley by early to mid-morning, in time to allow some sunshine behind it within the cold pool. It doesn't take a whole lot of sunshine within a cold pool to sufficiently destabilize the environment, as typically temperatures cool quite rapidly with height already in these cold pools. Adding warmer surface temperatures invigorates surface-based instability, with most severe events in the valley boasting surface based CAPE values in the 400 – 800 j/kg range. To those in the eastern U.S., these values sound minuscule, but when you have such cold air aloft, it doesn't take much for convection to blow up and thrive. Eastern U.S. events are typically in the warm sector, where you have to climb much higher to find the cold air, thus more instability is needed to allow storms to explode vertically into it.
Ignition is a go. Lift essential for the development of convection, or any precipitation for that matter. Thankfully, inside the cold pool at the base of a trough — or even better, ahead of a low (whether it's surface to upper-level based), lift will be present. A problem we deal with in the summer during monsoon events is a "cap", a dry/warm layer of air thousands of feet above the surface that prevents warm air at the surface to freely rise, inhibiting convection. During the winter, this is almost always not an issue. Once the cold front clears, any dry air gets pretty well mixed out in the mid-levels, thus, surface parcels (of air) can freely rise. If the day becomes highly unstable by California standards (>1000 j/kg CAPE values, for instance), one issue can arise: and that's widespread weak to moderate-strength convection, limiting further destabilization, and overcrowding the valley with cloud cover and precipitation. This is generally more of a problem with low-shear setups, but occasionally occurs with decently sheared days. Otherwise, if widespread convection doesn't exist due to already notable instability combined with lift, typically lift will not be overpowering, allowing somewhat more sporadic/scattered convection to develop and remain discrete. The more discrete a thunderstorm remains, the better odds it'll have in the long run to mature, rotate, and produce whatever it becomes capable of, whether that be tornadoes or simply hail.
Thunderstorms typically first develop on the west side of the valley, unless strong convergence exists elsewhere. One place deeper in the valley where convergence has many times allowed development is from about Sacramento northward to the Sutter Buttes, sometimes offset to the east. This is where a band of supercells developed in October 2012, dropping multiple tornadoes across the southern and central valley into the lower foothills. The west valley (and sometimes northern delta) convergence zone is typically where the most robust, long-lived supercells develop, for instance the May 2011 event that spawned the aforementioned EF2 in Butte county, and a classic, somewhat larger than "normal" supercell in 2014 that dropped tornadoes in Glenn county. Supercells that develop in the middle of the valley are typically the smallest, but of course size doesn't matter when it comes to tornado producing storms, as this overview should prove.
Once you've got all of the above, to allow surface based supercells to develop and thrive, they can do just that… that is, until they reach the foothills. The thunderstorms themselves can remain fairly strong up into the sierra, but the rotational features usually don't survive past 3000ft. In October 2012, a tornado touched down in Lake of the Pines, which lies in the mid-foothills, as one of multiple tornadoes spawned from mini-supercells that developed during an event late in the month. Another touched down between Oroville and Paradise up in Butte county, in 2011, but this was borderline valley-level. Both of these locations are under 3000ft, thus it seems there's an overall threshold these storms can withstand before loosing their well-defined rotation due to getting torn up in the low-levels by the higher elevation surface, though, if a storm is stronger than most recently, it wouldn't surprise me to see a storm try to drop something slightly higher in elevation.
I've written this overview many times over the years, but I feel this is the most accurate, understandable version yet. My goal is to eliminate the generalization that California is a boring weather state, it's exceptionally variable by season. California gets rocked by major storms, floods, wind events, snowstorms and blizzards in the mountains, and even the occasional tornado or a few. It's not an easy-going state weather-wise, and presents major complications for forecasters and computer models at times given the expansively diverse terrain and location.
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