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05/20/2013 10:14 am ET Updated Jul 20, 2013

Severe Weather For Oklahoma, Kansas Expected In The Wake Of Destructive Twisters

May 20 (Reuters) - The central United States braced for violent thunderstorms on Monday that could bring more hail, heavy rain and tornadoes to the region stricken by a deadly twister over the weekend.

The National Weather Service said severe storms would likely pummel the Ozarks and the middle Mississippi Valley, with northwest Arkansas, far southeast Kansas, southern Missouri, most of Oklahoma and northern Texas facing the greatest risk.

"A very moist atmosphere will become quite unstable again today," the forecasters said. "This combined with strong favorable winds aloft will result in a risk of a few strong tornadoes, very large hail and damaging winds in the most intense storms."

A massive storm front hammered the region on Sunday with fist-sized hail, blinding rain and tornadoes, including a half-mile-wide twister that struck near Oklahoma City.

One man was killed at a mobile home park in the town of Bethel Acres near Oklahoma City and 21 people were injured in storms throughout the state, said Keli Cain, a spokeswoman for the Oklahoma Office of Emergency Management.

News reports said hundreds of homes and buildings were damaged or destroyed in the Shawnee area east of Oklahoma City.

More than two dozen tornadoes were spotted in Iowa, Oklahoma, Kansas and Illinois, according to the National Oceanic and Atmospheric Administration and local news reports. Hail stones, some as large as baseballs, were reported from Georgia to Minnesota, NOAA said.

Oklahoma Governor Mary Fallin declared 16 counties disaster areas, according to Jerry Lojka, a spokesman for the state emergency management department.

Just after 6 p.m. on Sunday, the National Weather Service's storm prediction center in Norman, Oklahoma, posted a Twitter alert on a tornado about to strike Pink, a town on the edge of Oklahoma City.

"Large tornado west of Pink!" the post read. "Take cover RIGHT NOW in Pink! DO NOT WAIT!"

The storm also prompted an unusually blunt warning from the central region of the National Weather Service, which covers 14 states.

"You could be killed if not underground or in a tornado shelter," it said. "Complete destruction of neighborhoods, businesses and vehicles will occur. Flying debris will be deadly to people and animals."

Pat Slattery, National Weather Service spokesman for the U.S. Central region, said the advisory was part of a new warning system being tested after a tornado killed 158 people in Joplin, Missouri, in May 2011.

A NOAA assessment of the Joplin storm found that "when people heard the first tornado warning, they did not immediately seek shelter. They looked for a secondary source to confirm the tornado," Slattery said. "That got some people killed."

Slattery said the new advisory was reserved for severe tornadoes with the potential to form into "supercell" storms, which produce powerful winds and flash flooding. (Reporting by Steve Olafson, Jane Sutton, Chris Francescani and Ian Simpson; Editing by Doina Chiacu and Xavier Briand)

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The mom-to-be knew about the severe weather watch as she checked into the hospital to be induced at 9 a.m. that morning, but says she wasn’t really nervous.

“We’re used to tornadoes and sirens,” the 25-year-old mom told HuffPost in a phone interview. “If you freaked out every time you heard a siren, you’d have an anxiety attack every May in Oklahoma.”

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Sprinkles Cupcakes in Los Angeles plans to donate all of the proceeds from their (ever-popular) Red Velvet cupcake sold on May 22 to support Oklahomans affected by Monday's tornado.

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Mail Service Alternatives Offered In Storm-Damaged Areas

The post office branch in Moore, Okla., was one of the thousands of buildings damaged or destroyed by Monday's twister. To help residents impacted by the disaster, the US Postal Service is setting up mail service alternatives in the area, News9.com reported.

Some of the options being offered include held mail, portable post offices and delivery service through an alternative office nearby. Letter carriers will also attempt to deliver mail wherever possible.

Click here for more.