(Reuters) - Flooded Arkansas and Oklahoma were bracing for more rain that will feed the already swollen Arkansas River, forecasters said on Monday, as Missouri deployed the National Guard in anticipation of further storms.

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With millions of Americans under flood warnings on the Memorial Day holiday, Kansas, Nebraska, Iowa and Illinois got deluged while Oklahoma and Arkansas got a reprieve for the most part.

“But that’s going to change as we have a system moving across there coming into midweek. The rain falling over the portions of the Central Plains into the upper Midwest will add to the river flows upstream,” said Jim Hayes, meteorologist with the National Weather Service’s Weather Prediction Center.

Floodwaters may take weeks to recede, Hayes said, as the heavy rains upriver will take time to drain.

The pattern of drenching thunderstorms and tornadoes, which has been recurring in the Central Plains states since last week, has caused flooding in several river cities.

All 77 Oklahoma counties were under a state of emergency, a spokeswoman for the Oklahoma Department of Emergency Management said. The city of El Reno was still cleaning up from the tornado that killed two people and injured 29 on Saturday.

ACTIVATING THE GUARD

In Tulsa, officials were monitoring the Arkansas River after the U.S. Army Corps of Engineers raised the flow at the upriver Keystone Dam by 65% since last week to 275,000 cubic feet per second. The heavier flow is testing two aging levees in Tulsa, the City of Tulsa said.

Flooding and severe weather have killed six and injured 107 in Oklahoma, state officials said.

In neighboring Missouri, Governor Mike Parson on Monday activated the National Guard to respond to flooding and prepare for severe storms this week, his office said in a statement. Tornadoes and severe storms that slammed the state last week killed three people and destroyed or damaged hundreds of homes.

Arkansas and Oklahoma have already activated their National Guard forces.

As the Central Plains dealt with rain, a heat wave set record daily temperatures in parts of the U.S. Southeast, where the mercury rose 10 to 15 degrees Fahrenheit above normal for this time of year, Hayes said.

Savannah, Georgia, hit 99 degrees Fahrenheit (37 Celsius), breaking a record for the day of 98 degrees set on May 27, 1989. Macon, Georgia, also hit 99, tying a record set in 1953.