Cold spring air quells tornadoes, so far

With cold and snow hanging on in much of the nation many weeks later than normal, spring 2013 is about as different from spring 2012 as it possibly could be.

But 2013, so far, is following 2012’s lead in one area: lack of tornadoes.

Through Monday, there had been 217 tornadoes nationally, according to the Storm Prediction Center. That’s about half of what can be expected through April 22, based on the six-year average from 2005 to 2011.

After an epic 2011 with more than 1,800 confirmed tornadoes and 551 fatalities — the most deaths since 1936 — the following year produced half as many tornadoes in the United States.

Tornado deaths were still high in 2012 at 70, more than half of which occurred in a single early March outbreak in the Ohio and Tennessee river valleys.

Besides that outbreak and another in mid-April in the central and southern Plains states, tornadoes in 2012 were sporadic nationally, owing largely to summer like warmth pushing the jet stream north into Canada early in the spring.

Only three people have been killed by tornadoes so far in 2013, and none died in the last six months of 2012.

Some experts have suggested that 2012 may have been the weakest tornado year, or nearly so, in 62 years of official tornado record-keeping.

Tornado numbers are not easily tracked over decades because the lack of Doppler radar, fewer chasers and spotters in wide-open areas, less extensive instant communication, less widespread visual recording technology and less intensive post-storm documentation led to many tornadoes likely being missed from 1950 to 1990 or so.

Several hundred to sometimes more than a thousand tornadoes may not have been counted in each year of the 1950s and 1960s. Prior to 1950, there are no reliable estimates on national tornado numbers.

So far in 2013, repeated atmospheric patterns of blocking high pressure in the northern latitudes have forced the jet stream far to the south.

The result has been that cold air from Canada and the Arctic has penetrated unusually far south in March and April.

These repeated intrusions of cold air have kept warm, moist weather patterns that would more likely spawn tornadoes from overspreading the central and eastern United States on a consistent basis.

Whether this will flip into an active tornado season in May and June is the subject of much conjecture.

The argument for it would be that having cold and snow so far south this late in the season would mean that boundaries between cold and warmth will be retreating northward later than normal.

These boundaries are often the breeding ground for the kinds of storm systems that can produce multiple tornadoes. Late snow pack also increases surface moisture available for severe storms in the Northern Plains and Upper Midwest.

The argument against it would be some tendency in this early spring to flip dramatically between the colder, wetter pattern and a warm, dry pattern. In other words, it is possible this spring does a 180-degree flip from winter to summer in much of the nation. You already remember Southwest Virginia going from snow to 90 degrees in six days earlier this month.

There are strong signals a cold-to-heat flip may in fact occur at least over the next couple of weeks, as many of the areas still receiving heavy snow experience abnormal warmth, leading to flooding concerns.

Whether tornado numbers rebound nationally this year, it only takes one ripping through a populated area to ramp up the fatality figures.

Weather Journal runs on Wednesdays.

Weather Journal

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