The Sacramento valley is sometimes considered a mini-tornado alley, as the valley floor creates a nice flat surface to that of the midwest, favoring surface based convection more than hilly terrain. Tornadoes are much more common in California than many think, and most often occur behind cold frontal passages, instead of ahead of them in the warm sector.

Graphical depiction of a classic setup that could produce mini-supercells in the Sacramento valley.

Graphical depiction of a classic setup that could produce mini-supercells in the Sacramento valley.

The best time for a cold front to pass to maximize the convective threat is in the morning, with clearing by the mid to late morning in time for peak afternoon heating. Usually, stronger systems will have a pretty intense cold pool aloft behind the cold front with surface heating after the front passes. A prime cold pool for good instability would ideally be -25 to -40c at 500mb… and if the valley can get enough clearing to warm surface temperatures into the mid to upper 50s at least, we often see very steep low level lapse rates, and moderately steep mid-level lapse rates… typically around 7.5c – 8c, with low level lapse rates approaching 9 – 10c sometimes in events where the sun is out almost all morning/early afternoon before convective development.

These steep lapse rates, indicative of a sharp temperature gradient with height, usually lead to to surface or near surface CAPE values of around 200 – 800 j/kg when combined with what moisture there is in the low-levels (typical cool-season setups involve dewpoints in the 40s to 50s)… which may not seem like a whole lot to mid-westerners, but it is sufficient for cold core convection to develop and sustain itself. Mixed layer CAPE at the same time may also get up to 200 – 500j/kg well. Some setups can include more moisture/sun and lead to CAPE values in excess of 1,000 j/kg – but don’t happen all too often and can sometimes lead to an overly unstable environment, leading to clouds and weaker convection all day, limiting sunshine for more robust storm development.

The development itself can be caused by lift remaining aloft if we’re in the favorable location of a trough, but in some cases it is caused by convergence zones setting up in parts of the valley. Two convergence zones that I notice a lot are from around Gridley to Nicolaus, down the center of the central Sacramento valley, and another from around Ione to Roseville and further north to the Wheatland/Lincoln area, along the east side of the southern/central Sacramento valley and sometimes extreme northeastern San Joaquin valley.

Shear is a very important part of forecasting potential tornadic convection (doesn’t always have to be thunderstorms), and the first things I look are bulk upper level shear (500mb), then surface to 500mb crossovers. If you see a southerly low level flow combined with a westerly upper level (jet) winds (which creates a 90 degree angle), you can tell there will be favorable vertical shear for convection to strengthen, be sustained, and possibly lead to rotating updraft development depending how strong the shear is.

Another important step to shear forecasting is your hodographs. If we were to be forecasting a chance of tornadic convection/thunderstorms, you want to see a curving type shape from around the beginning to a quarter or half the way through. The mid-west, during more significant tornado outbreaks, may see a loop or two around three quarters of the way through the hodograph, after the curve. In the valley, we often get small curves indicating some weak to moderate (usually 0 – 1km) helicity (75 – 200 M2/S2 range). Curving hodographs also tell the storm motion, which is usually to the right (or east towards the foothills) with right curved hodographs.

We can also get terrain enhanced shear, often either on the west side of the valley near the coastal mountains, on the east side where wind convergence zones can set up, as well as near the Sutter Buttes. Sometimes this shear can work against a thunderstorm, with shear sometimes changing directions too often or actually being too strong, tearing a storm’s updraft apart… though usually it’s more supportive than against it in most cases if instability is strong enough to support a strong and consistent updraft.

You get the favorable position of a trough and/or associated upper low off the west coast, cold pool instability, and shear… you’ll have a chance of strong, possibly supercellular thunderstorms, just like anywhere else in the U.S., or even around the world.