Friday night's storms battered areas in and around Oklahoma City with high winds, heavy rain and hail.
The Oklahoma Medical Examiner's Office announced nine fatalities in the state and said five of the victims have not been identified.
In Missouri, authorities said three people died from severe flooding in the wake of the storms.
Streets turned into rivers, with stranded cars submerged in water as high as their door handles in some places. CNN said a massive sink hole off a major road developed due to the deluge, halting traffic.
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The National Weather Service's Storm Prediction Center in Norman, Oklahoma, said the first tornado developed around the city of El Reno before moving into parts of Oklahoma City, spawning others.
Local broadcaster KOCO reported that 77 people had been admitted to hospitals with storm-related injuries.
Missouri Governor Jay Nixon, who toured the widespread devastation to assess the damage, urged residents of his state to avoid walking or driving through flooded areas.
As the extent of the devastation in Oklahoma became clear, the work of cleanup crews was complicated by downpours that drenched the region overnight.
As the storms approached, Oklahoma City's Will Rogers World Airport evacuated passengers to an underground tunnel and suspended incoming and departing flights. It re-opened around 3:30 am, but all early departures had been canceled, officials said.
Power company OG and E meanwhile reported 74,093 outages by mid-afternoon yesterday and the American Red Cross has opened shelters for those in affected areas.