“July has re-written climate history," said the head of the World Meteorological Organization.
Louisiana Gov. John Bel Edwards warned that there could be “a considerable amount of overtopping” of levees.
Scientists warn that heat waves are becoming increasingly common due to human-caused climate change.
Planting season is the slowest in decades as farmers grapple with floods, rains and sodden fields.
The latest round of damaging weather in the central U.S. came a day after violent storms killed one person and injured at least 130 in Indiana and Ohio.
At least three people died in a tornado in Golden City, Missouri, on Wednesday. Extreme weather has also threatened parts of Oklahoma, Kansas, Iowa and Illinois.
An intense storm system swept through Oklahoma, western Texas and southern Kansas on Monday, spawning several tornadoes that caused some damage.
The National Weather Service placed several counties under a tornado watch, calling it a "particularly dangerous situation."
Hundreds of schools canceled classes in Minnesota, Nebraska and South Dakota, where the governor closed state offices in much of the state for a second day.
More than 10 million people were reportedly under flood warnings Friday.
The storm brought blizzard conditions to parts of Colorado, Wyoming, Montana, Nebraska and South Dakota.
Meteorologists on Monday gave the deadly Lee County tornado an EF4 classification, saying the Sunday twister had wind speeds of about 170 mph.
Over 20 people were killed, and the death toll is expected to rise.
A PIX11 News interview with Diane from Brooklyn has some Twitter users calling on her to be a meteorologist.
Polipoli Spring State Park on Maui turned white as Haleakala saw 4-foot drifts.
Good for sledding, bad for driving.