"We urge everyone to use caution as storms continue across our state," Lt. Gov. Jari Askins said.
Askins approved paperwork declaring the state of emergency at the request of Gov. Brad Henry, who is out of state on business, The Oklahoman reported.
The declaration makes it possible for local governments to pursue reimbursement of costs associated with recovery from the storm and is an early step toward pursuing federal assistance if needed, the newspaper said.
Oklahoma City officials warned motorists to stay home Monday and not underestimate the danger of driving on flooded streets.
Oklahoma City was slammed by persistent heavy rain and flash flooding, affecting Monday's morning travel, AccuWeather.com reported. Other cities such as Tulsa, Okla., St. Louis, Indianapolis and Cincinnati could see similar heavy storms as the system pushes east.
Forecasters said spotty, isolated storms in the Deep South also could trigger flash flooding. Tropical air and associated high pressure were expected to remain entrenched over the southeastern United States this week.
Oklahoma City spokeswoman Kristy Yager said widespread flooding was reported throughout the city and passable thoroughfares were clogged with traffic, The Oklahoman reported.
People already on roadways or thinking about traveling "need to find a parking area on high ground and just wait for this to pass," Yager said. "This is treacherous. They can be late to work. Their life depends on it."
Yager said the city's storm-water drainage system couldn't keep up with the rain.
Rainfall totals of up to 7 inches have been recorded in some parts of the city in just a few hours, The Oklahoman said.
High water also forced closures of several state and interstate highways in Oklahoma County, state police said.