It's been such a quiet year for twisters across Tornado Alley that weather researchers are searching for answers.
The National Oceanic and Atmospheric Administration's Storm Prediction Center described the tornado season on Friday as "incredibly quiet," with the national tornado total as May ended at little more than half the average number to date.
"Why the nation has experienced so few this year is not clear, although researchers are searching for clues," a statement issued by the Storm Prediction Center said Friday.
Storm chasers have adopted an almost wistful tone about one of weather's most violent storms.
"One day we will tell our grandkids great tales about back when there were storms to chase," Ian Livingston tweeted on May 19.
May is the heart of tornado season across the Great Plains, but the 158 tornadoes that occurred this year were little more than half the typical year's total.
One key component is that much of the nation's heartland endured unusually cold temperatures in April and then saw temperatures surge to summer-like readings in May.
Minneapolis, for instance, experienced one of its coldest Aprils on record and then in May set a record for its earliest 100-degree day ever. Wichita endured its fourth-coldest April and then its second-hottest May.
The thunderstorms that most often spawn tornadoes feed off the instability in the atmosphere that occurs when masses of cold air from the north collide with warm, moist air from the Gulf of Mexico.
But those scenarios haven't been common this year.
"There weren't a lot of severe weather opportunities," said Ken Cook, meteorologist-in-charge of the Wichita branch of the National Weather Service. "It was cold, and then it was hot."
Just three of the 431 tornadoes through May 31 across America were rated as strong as EF3 on the Enhanced Fujita Scale. There have been only three deaths from tornadoes, far below the average total of 38 for the end of May.
The low number of strong tornadoes is significant, weather officials say, because more than 85 percent of tornado deaths occur in tornadoes rated EF3 or higher — or twisters with winds topping 135 miles an hour.
The U.S. went a record 305 days, from May 17 of last year until March 18 of this year, with no tornadoes that were EF3 or stronger.
One reason for the low death toll, weather officials say, is simply the lack of tornadoes.
Kansas defied the quiet May, logging 34 tornadoes. That's just a handful below the state's annual average for what is historically its busiest tornado month of the year.
But Kansas was also far and away the busiest state for tornadoes in the country in May, with Colorado a distant second at 19. And Kansas didn't see its first tornado of 2018 until the first day of May.
Weather officials say better public awareness may also help explain why there have been so few tornado deaths so far this year. Efforts to improve the nation's preparedness and response to tornadoes have ramped up substantially since the devastating 2011 tornado season, which saw the most tornado fatalities in decades.
NOAA has invested in new satellites, upgraded radar, faster supercomputers and better computer models to improve forecasts and warnings.
The quiet tornado season has meant the social media accounts and pages of storm chasers and tornado researchers resemble December, filled with reminiscences of past chases rather than fresh photos and videos of this season's severe weather.
Sluggish tornado totals figure to persist as June arrives, forecasters say: Current weather patterns are more common for late July, which is typically a sleepy time for tornado development except in the northern Great Plains.