California Spinnin’: Tornadoes Strike The Golden State

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How unusual is it for tornadoes to touch down in California? Image by blondieb38

How unusual is it for tornadoes to touch down in California? Image by blondieb38

Several tornadoes touched down around Sacramento on Wednesday, as the east coast was reeling from Winter Storm Xenia. There was no connection between the tornadoes and Xenia, other than the fact that a tornado and an extra-tropical low pressure system are both spinning areas of low pressure.

A History Of Tornadoes In California

Though tornadoes are not unheard of in California, they are rare, averaging four per year from 1961 to 1990. That compares to the leading state, Texas, which averages 137. A more revealing statistic is the number of tornadoes per year per 10,000 square miles. That levels the playing field when behemoths Texas and California are compared to smaller states.

California records 0.26 tornadoes per year by this metric; only five states in the continental U.S. have lower numbers (there has never been a tornado in Alaska), all west of the Rocky Mountains.

What Are Favorable Conditions For Tornado Formation?

Without getting into all the physics involved, it is possible to characterize the atmospheric conditions that produce the most, and most severe, tornadoes as follows: a moist flow of air near the surface with a dry flow from a different direction above.

Simply put, the dry air on top of wet creates an unstable air column when it is lifted, and the change of direction of wind with height starts the column of air spinning.

Where Do The Favorable Conditions For Tornado Formation Exist?

Much of the eastern United States is prone to conditions conducive to tornado formation. Warm and moist air streams northward from the Gulf of Mexico. The flow above has often crossed the Rockies and the moisture has been wrung out. Southerly flow of warm, humid air at the surface; westerly flow of dry air above: instability and spinning; tornado heaven.

Which State Gets The Most Tornadoes Per 10,000 Square Kilometers?

Waterspouts sometimes move over the Florida peninsula and become tornadoes. Courtesy Noaa

Waterspouts sometimes move over the Florida peninsula and become tornadoes. Image courtesy NOAA.

The answer to this question is a little surprising: Florida. The Sunshine State has a double sea breeze in the summer (a sea breeze on both coasts), and where the two sea breezes meet there is a zone of sufficient instability to cause tornadoes. Most Florida tornadoes are of minimal strength — little more than glorified waterspouts or dust devils — because the air is normally quite humid at all levels.

The entire area east of the Rockies is subject to favorable conditions for tornadoes at some time, but ‘tornado alley’ cuts through Texas, Oklahoma, Kansas, and Nebraska. However, any place where the Gulf moisture gets a foothold and dry air flows above can experience powerful tornadoes.

What Happened In California?

The California tornadoes really have the same cause as much of this winter’s bad weather: a jet stream depressed to the south of its normal position. While most of this winter’s storms have been over the eastern part of the country, the wind pattern has recently flattened, so that strong jet stream winds now blow from coast to coast. Significant ripples in the jet stream (troughs) cause lifting. A pronounced trough produced the tornadoes in California on Wednesday.

As the trough that caused the California tornadoes moves across the country, there is a good chance of tornadic activity in the northern Gulf Coast states, where the moisture-laden air has taken up residence. Severe weather could occur today from Texas to Alabama.

On Saturday, the highest potential for damaging tornadoes and thunderstorms moves to Georgia and the Carolinas, perhaps as far north as Virginia. By Sunday, the trough moves off the coast, but a cold front trails back into Florida where some severe weather could occur.

Is This A Harbinger Of Things To Come?

Though the climate is certainly changing in response to an increase in greenhouse gases, the California tornadoes cannot be linked to climate change, and tornadoes in California will most likely continue to be rare.

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