Oklahoma City endured an approximately eight-hour nonstop onslaught of severe thunderstorms on Wednesday into early Thursday morning, leaving the metropolitan area strewn with debris and impassable roadways.

As tornadoes descended upon the city's southern suburbs on Wednesday night, a woman in the 16500 block of South Midwest Boulevard entered her storm shelter, as instructed by meteorologists.

However, she did not emerge alive. What's remarkable is that she was killed by flooding, not a tornado, according to KFOR-TV and the Oklahoma City Police Department.

The Police Department wrote in a Facebook post on Thursday:

We regret to report that we have our first storm related fatality from last night's severe weather event. A 42 year-old... Posted by Oklahoma City Police Department on Thursday, May 7, 2015

In multiple instances, underground storm shelters — which provide vital protection against powerful tornadoes that frequently target this city in the heart of Tornado Alley — were forced from the ground by the storms late Wednesday and early Thursday.

2nd photo I've seen of tornado shelter erupting out of ground due to floodwaters. Via Cassie Todd on @koconews FB. pic.twitter.com/1fvfeZfauB — Tim Ballisty (@IrishEagle) May 7, 2015

The amount of rainfall was unprecedented for any day in May and exceeded the average monthly total. The flash flooding prompted the National Weather Service forecast office in Norman, Oklahoma to issue its first-ever "flash flood emergency" for the city, calling attention to the threat to lives and property on Wednesday night.

While Oklahoma City is no stranger to deadly tornadoes, drowning in a storm shelter is a rarity even for this city. According to Rick Smith, the warning coordination meteorologist for the Weather Service in Oklahoma City, there is only one other such death in his office's database. That one occurred in August 2007, when the remnants of Tropical Storm Erin were passing through.

The entry in the database reads simply: "While seeking shelter from the severe weather, an elderly lady drowned while in her storm shelter."

It doesn't typically rain this much in Oklahoma City, but clearly the two deaths since 2007 point to a flood-related vulnerability in storm shelter designs that could make people more hesitant to go into underground shelters the next time a warning is issued. This could increase the number of tornado deaths and injuries.

Graph showing rainfall in Oklahoma City through the end of May 6, 2012, compared with other years. Image: Climate Central

With 7.10 inches of rain, May 6 was the third-wettest day of any calendar day on record (for any month) stretching back to 1891.

According to the research and journalism organization Climate Central, the two-day rainfall total was the second-highest such total on record as well. The 24-hour rainfall total was a 1-in-25 to 1-in-50 year event, Climate Central found (although the six hour rainfall total was more like a 1-in-100 year event, using Weather Service data.)

To put the staggering rainfall total in further perspective, Climate Central found that the 7.10-inch rainfall total was 71.4% of the typical rainfall that Oklahoma City would receive for the year to date.

Over the long-term, the frequency and severity of heavy precipitation events like the one on Wednesday are increasing in the U.S. due in part to manmade global warming, with the fastest increases occurring in the Northeast and Midwest.

Underground shelters can be redesigned to be more flood-resistant, but the bottom line is you're always safer in a storm shelter, except in a flash flood, in which case you should head for higher ground.