Tornado passes across south Oklahoma City, Monday, May 20, 2013. (AP Photo/The Oklahoman, Paul Hellstern)

It’s the great depression for tornado activity in the U.S.

At least 400 fewer tornadoes than average have touched down in the U.S. this year, making it one of the quietest years on record for twisters, according to the National Weather Service (NWS).

Whereas an average of 1,260 tornadoes form each year in records dating to the early 1950s, only 823 have occurred in 2014 through November, says Greg Carbin, warning coordination meteorologist at the NWS Storm Prediction Center, in Norman, Okla.

(Greg Carbin, NWS-SPC)

Examining the chart of tornado count history, one could easily jump to the conclusion that 2014 is not only among the quietest tornado years, but THE quietest year. But Carbin says the data have too much wiggle room to definitively make such a claim.

“It will be difficult to make a final declaration that 2014 is the ‘all-time’ lowest year on record for a couple of reasons,” Carbin says. “One, the year is not over and the final tally is not complete. Two, report inflation in later years, and likely under-reporting in early years, introduces errors in the annual tornado counts.”

Today’s observing technology (i.e. radar) and the proliferation of storm spotters mean few tornadoes are unreported – some may even be double counted, whereas it’s likely some tornadoes were missed in earlier decades. The National Weather Service attempts to correct for these biases using statistical methods, but cautions the actual count estimates are inexact.

“The tornado record is fraught with problems, even in the modern era,” says Carbin. “Many tornado records are suspect.”

In addition to the number of actual tornadoes, the number of tornado and severe thunderstorm watches issued by the National Weather Service is also way down in 2014, per the map below:

Difference from 10-year average in the number of severe thunderstorm and tornado watches in 2014, January-November (Carbin-NWS)

2014 joins 2012 and 2013 as very inactive years for tornadoes. “When adjusted for report inflation through November, the last three years in a row have fallen well below the mean,” Carbin says.

Interestingly, the causes for the tornado depression each of the past three years vary considerably. They don’t seem connected, Carbin says.

In 2012, a summer heat dome (or upper level ridge of high pressure) shut down activity over the central U.S. after an active early spring. In 2013, the pattern was generally too cold for tornadoes in the early spring, became very favorable in May (recall the Moore and El Reno, Okla. tornadoes), but then suppressed activity over the summer. The colder than normal weather pattern in 2014 has something to do with this year’s low numbers, “but there is much more to it” Carbin says.

The current three-year down period for tornadoes fits into a recent trend towards extreme tornado season “volatility”, or large year to year differences in the number of twisters – which researchers are trying to better understand.

Related: Studies: Tornado seasons peaking earlier, becoming more volatile

The lull in tornadoes coincides with a down period for hurricanes as well. It has been 9 years since a major hurricane (category 3 or stronger) struck the U.S., the longest period on record. However, it would be a stretch to connect the downturn in these two types of tempests since they form under very different conditions (although a small percentage of U.S. tornadoes come from landfalling hurricanes).

Related: Unprecedented lull in major hurricane landfalls continues as Atlantic season comes to a close