Nearly 70 million Americans are under some type of heat alert on Monday.
Heat indices in the 100s will be widespread again this afternoon from the Plains to the East Coast, with much of the Mid-Atlantic and Northeast seeing their hottest temperatures so far this year.
Slight relief reaches the Northeast by midweek as temperatures fall several degrees, but temperatures will remain above average all week.
The heat will also move into the Northwest with 90-100 degrees for parts of interior Washington and Oregon state.
Isolated strong to severe storms for northern Plains
Strong to severe thunderstorms will be possible Monday over parts of the north central Plains as a cold front responsible for bringing relief to the heat across the Northeast is expected on Tuesday.
Large hail, damaging winds, flash flooding, and even tornadoes are possible.
Heavy rainfall associated with any storms that develop could cause isolated flash flooding.
The threat of severe weather and heavy rainfall shifts to the Midwest, Middle Mississippi and Ohio River Valleys on Tuesday.
Monsoonal showers and thunderstorms will move into the Southern Rockies this week. Some of those thunderstorms could spark dry lightning, causing wildfires to start.
Fox News' Brandon Noriega contributed to this report.