A tornado siren blared in Iowa this afternoon as a funnel cloud could be seen in the backdrop of the famed speedway outside Des Moines. NBC News’ Maggie Vespa reports on the severe weather threats facing millions in the Midwest tonight.

Sand and dust storms affect about 330 million people in over 150 countries and are taking an increasing toll on health, economies and the environment.

A line of heavy thunderstorms swept across the D.C. region Wednesday evening, unleashing torrential rains that flooded streets and violent wind gusts that toppled trees. The storms produced spectacular skies — including menacing clouds on approach and vivid rainbows upon exiting.

Severe thunderstorms will head east by Saturday, impacting eastern Michigan and parts of northern Indiana. One to two inches of rain is possible as the system tracks east, but some areas could see isolated amounts of three to five inches through the weekend.

Forecasters warn that slow-moving storms could bring heavy downpours to already saturated areas, increasing the risk of flash flooding.

Michigan residents should prepare for a potentially stormy Saturday with a high risk of severe thunderstorms and heavy rainfall across multiple regions of the state.

A night of concerts in Milwaukee's Veterans Park was called off around 6 p.m. July 11 before Hank Williams Jr., Treaty Oak Revival and more performed.

The chance of rain will decrease through the evening on Friday in the Philadelphia region as temperatures once again gradually drop back into the low 70s to start off the weekend.