Stretch of Tornadoes, Severe Weather Since Late April One of Most Prolific in 8 Years

More than 500 reports of tornadoes have been received by the National Weather Service in the past 30 days, notes Sam Lillo, a doctoral candidate at the University of Oklahoma. The actual number of tornadoes has yet to be confirmed, pending NWS damage surveys from recent activity.

It's the first 30-day period with at least 500 tornado reports since a stretch that included the April 2011 Super Outbreak, according to Patrick Marsh, warning coordination meteorologist at NOAA's Storm Prediction Center (SPC).

Almost 2,200 reports of severe weather, including 328 reports of tornadoes, have been logged from May 17 through the morning of May 28 in the U.S.

Almost 150 tornadoes have been confirmed since May 17, according to Katie Wheatley of UStornadoes.com.

Lillo's research found the only 8- to 14-day periods that were more active for tornadoes in the U.S. occurred in April 2011, late May 2004 and in early May 2003.

"Last week was just incredibly long, and even now with severe weather threats outside of Oklahoma, there's the feeling of 'here we go again'," said Lillo. "I can only imagine this feeling is strongest for the operational forecasters covering these threats every day."

From May 17-28, the NWS issued 642 tornado warnings, according to the Iowa Environmental Mesonet (IEM).

In just one 24-hour period ending Tuesday morning, the NWS issued 124 tornado warnings, the most in such a period since April 28-29, 2014, according to Daryl Herzmann of IEM.

May is typically the month with the most tornadoes in the U.S. with an average 279 tornadoes. That's an average of 9 tornadoes each May day.

Interestingly, there hasn't been a single day in this stretch since mid-May with an egregious number of tornado or severe reports that would distinguish it as a singular historic outbreak like the Super Outbreaks of 2011 or 1974 or the May 3, 1999, Plains outbreak.

According to SPC statistics since 2000, there are one to five days each year with at least 400 reports of severe weather in the U.S.

So far not one day in this current stretch has topped 400 severe reports.

Instead, it's been just a steady stream of severe weather, often over the same, relatively confined area, with an elevated number of tornado reports each day.

That's including four separate deadly nighttime tornadoes in Iowa, Missouri, Ohio and Oklahoma.

(STUDY: Nighttime Tornadoes More Than Twice as Likely to Be Deadly)

A stagnant jet stream pattern is the culprit of this severe weather siege and rounds of torrential rain and subsequent major river flooding.

The jet has taken a persistent southward dive over the western U.S., sending impulses into the central U.S., where warm, humid air was in place to generate the instability to ignite severe thunderstorms and tornadoes.

Lillo compared the longevity and anxiety of the current severe weather siege to the late-May 2013 tornado swarm in central Oklahoma, including Moore, El Reno and Shawnee.

"After that stretch all I wanted was the summer heat ridge," said Lillo, referring to a dome of high pressure aloft that typically suppresses severe weather and tornadoes in mid- to late summer in Oklahoma and the southern Plains. "I just didn't even want to think about tornadoes for the rest of the year."