(CNN) Over 7 million people are under flash flood watches across the Desert Southwest and the Rocky Mountains as a plume of moisture moves in from the Pacific.
"Tropical moisture will surge northward over the Southwest into parts of the Great Basin/Central Rockies through Thursday," the Weather Prediction Center said Wednesday.
This influx of moist air could lead to flash flooding across the region from Wednesday into Thursday.
"Flash flooding and debris flows will be possible, especially over burn scars, mountainous terrain, and narrow canyons," according to the National Weather Service in Grand Junction, Colorado.
The prediction center issued a moderate risk -- level 3 of 4 -- of excessive rainfall over the Great Basin, and the central, southwest and southern Rockies through Thursday morning. Rainfall totals of 1 to 4 inches are possible.
The storms "may end up being the convective event of the 'summer,' " the weather service in Salt Lake City wrote.
The deep moisture plume will gradually shift east Wednesday night into Thursday morning.
"Periods of moderate to heavy rainfall are expected through Thursday night," said the weather service office in Grand Junction. "Additionally, strong to severe thunderstorms are possible, most likely across the Four Corners area, which could lead to even heavier rain rates, hail, and damaging winds."
The weather service also warns of danger from flooding in national parks in the region. Slot canyons and small streams are expected to experience flash flooding, so visitors are urged to stay weather aware.
The flooding is expected to peak Wednesday. By Friday, drier weather returns to the region.