INDIANAPOLIS — After several quiet years, tornadoes have erupted in the United States over the last two weeks as a volatile mix of warm, moist air from the Southeast and persistent cold from the Rockies clashed and stalled over the Midwest.

On Monday, the U.S. tied its current record of 11 consecutive days with at least eight tornadoes confirmed on each of those days, said Patrick Marsh, warning coordination meteorologist for the federal Storm Prediction Center. The previous 11-day stretch of at least eight tornadoes per day ended on June 7, 1980.

“We’re getting big counts on a lot of these days and that is certainly unusual,” Marsh said.

The National Weather Service had already received at least 27 more reports of tornadoes Tuesday, suggesting that the record for consecutive days would be broken once the official totals are in.

The weather service has received 934 tornado reports so far this year, up from the yearly average of 743 observed tornadoes. More than 500 of those reports came in the last 30 days. The actual number is likely lower, however, because some of the reports probably come from different witnesses who spot the same twister.

The U.S. has experienced a lull in the number of tornadoes since 2012, with tornado counts tracking at or below average each year and meteorologists still working to figure out why.

“A lot of people are trying to answer that, but there’s no definitive answer,” Marsh said.

The recent surge in tornado activity over the past two weeks was driven by high pressure over the Southeast and an unusually cold trough over the Rockies that forced warm, moist air into the central U.S., sparking repeated severe thunderstorms and periodic tornadoes.

Those conditions are ripe for the kind of tornadoes that have swept across the Midwest in the last two weeks, said Cathy Zapotocny, a meteorologist for the weather service in Valley, Nebraska. The unstable atmosphere helped fuel many of the severe winter storms and subsequent flooding that ravaged Nebraska, Iowa and Missouri earlier this year.

Zapotocny said the number of tornadoes this year was “basically normal” until the surge this week. May is typically the month with the highest incidence of tornadoes, usually in the Plains and Midwestern states collectively known as Tornado Alley, where most of this year’s twisters have hit.