Dangerous severe storms will sweep across the eastern half of the U.S. this Memorial Day weekend, affecting a number of major cities from Saturday to Monday.

The storms will start Saturday, with 18 million people from Texas to Iowa under a high threat of severe storms, the strongest of which will hit Oklahoma, Kansas and Missouri, including Oklahoma City and Tulsa, Oklahoma; Joplin, Missouri; and Wichita, Kansas.

Discrete thunderstorms are expected to form over Oklahoma and Texas by 6-8 p.m. ET and will push east into Missouri and Iowa overnight.

Also overnight, a few long-lived supercells are expected. These storms are capable of producing intense tornadoes, giant hail and destructive wind gusts.

The storms will continue to push east on Sunday, moving into the Midwest and Ohio Valley. They are expected to affect 42 million people in cities such as Chicago, Indianapolis, Nashville, St. Louis and Cincinnati.

Damaging wind gusts are most expected across the Midwest, but tornadoes and large hail are also possible.

The storms will finish off on the East Coast on Monday, with a slight risk of severe weather issued to the mid-Atlantic. In this region, including Baltimore; Washington, D.C.; and Charlotte and Raleigh, North Carolina, 27 million are at risk of experiencing strong to severe thunderstorms.

The primary hazard to watch out for is severe wind, but a storm or two could be capable of producing large hail or a tornado.

With this active storm pattern comes the risk of flash flooding, especially across the mid-Mississippi Valley. In total, 3 million are under flood alerts including in cities such as Memphis, Tennessee; and Tupelo, Mississippi.

Rainfall totals will generally range from 1-2.5 inches through the weekend, with localized higher amounts of 3-plus inches possible where training storms develop.

Southern heat

While the South won’t be facing thunderstorms Memorial Day weekend, it will be facing extreme heat.

Summerlike temperatures will affect the southern Plains and the Gulf Coast as highs soar to 10-20 degrees above average.

Heat alerts are in effect for 7 million across southern Texas on Saturday, including in Austin, San Antonio, Corpus Christi and Brownsville, as temperatures will climb as high as 100-115 degrees.

Nearly two dozen record highs will be threatened Saturday afternoon as temperatures reach into the 90s-100s in Brownsville and Houston, Texas; Key West, Florida; and New Orleans.

On Sunday, more heat will cover the South, with more than 20 record highs threatened in Corpus Christi; Miami and Orlando, Florida; Baton Rouge, Louisiana; Dallas, Houston and San Antonio.

Extreme fire conditions

Four million are under alerts for critical fire weather conditions across the high and southern Plains from Colorado to Texas, including Albuquerque and Santa Fe, New Mexico; and El Paso, Texas.

Newly formed fires are at risk of rapidly spreading due to the dangerous combination of dry vegetation, 30-45 mph winds and low relative humidity.

This article was originally published on NBCNews.com