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Remember these guys?

The drought that parched much of the nation during the past year didn’t just stunt crops — it also stunted the annual yield of tornadoes. And an unseasonably chilly spring is so far helping to keep the hellish twisters at bay — although weather forecasters warn that trend may be short-lived.

During the past 12 months, the U.S. was hit by an estimated 197 tornadoes rated EF1 or stronger on the Enhanced Fujita scale, which ranks tornadoes according to their destructive potential from a low “0″ up to a devastating “5.” That was the lowest number of such tornadoes during any 12-month period since record-keeping began in 1954 — well below the previous low of 247 recorded between July 1990 and June 1991.

That’s in huge contrast to the onslaught of tornadoes that tore deadly paths of destruction through the nation in 2011, which was a record-busting year of tornadoes galore.

These tornado statistics come from NOAA National Severe Storms Laboratory researcher Harold Brooks. He also noted in a blog post that the number of people killed by tornadoes during the past year — 7 — was the lowest since 1899. Here’s a graph lifted from his post:

(click to embiggen)
NOAA
(Click to embiggen.)

The record tally of tornadoes in 2011 had people wondering then whether climate change was to blame, and the sudden dearth of the storms has people again wondering the same thing. It certainly feels like one of those boom-bust weather cycles that we expect from climate change. But there doesn’t appear to be any evidence directly linking the recent tornado cycle to global warming.

The dearth of tornadoes over the past year was linked to the lack of moisture in the air amid the shortage of rainstorms nationwide. There are divergent views on whether the recent droughts affecting the tornado states were caused by climate change — although climate models do predict more droughts in central North America, which is often a vast playpen of deadly twisters.

Likewise, we can’t singularly blame climate change for the cold snap that recently hit the Great Plains and the Midwest. But climatologists have drawn links between global warming and the weather patterns that delivered the cold spurt.

Climate Central delves into the tornado/climate question:

The drought that enveloped the majority of the lower 48 states during the past year has contributed greatly to the paucity of tornadoes, since the dry conditions have robbed the atmosphere of the water vapor that fuels severe thunderstorms. Other tornado ingredients, such as strong upper-level winds and atmospheric wind shear, have also been missing. …

Since tornado seasons vary considerably from one year to the next due to natural variability, it is unclear that the absence of tornadoes during the past 12 months has anything to do with global warming, just as it’s unclear if the 2011 tornado outbreaks were connected to it, either.

Tornadoes are complicated beasts, affected not only by moisture and temperature but also by wind shear and other factors.

Meanwhile, the Weather Channel warns that the cold conditions that have been recently keeping tornadoes at bay might soon break:

[T]he stubborn cold air of this past week will … gradually give way to more typical warm and humid air returning from the Gulf of Mexico into the central and southern Plains, to the east of a sharpening dryline.

With that said, the polar jet stream will remain well to the north in Canada through at least mid-week, rather far north for early May. Instead, weaker wind flow aloft, despite the upper-level system limping east from California, will be in play.

What that means is while severe thunderstorms and some tornadoes are possible in the Plains this week, the weaker wind flow aloft may keep this episode from reaching a full-fledged, widespread outbreak [of tornadoes] that May is so notorious for.