(NewsNation) — As the first day of summer approaches, dangerous heat is forecast for a wide swath of the nation, according to the National Weather Service.

Above-average temperatures are expected across the central U.S. and Deep South during the last full week of June, the NWS forecast.

These temperatures will shift eastward Monday to encompass more of the Midwest and Great Lakes, with a few daily high-temperature records possible between Nebraska and northern Wisconsin.

Excessive heat over the Heartland is expected to advance east across the Mississippi Valley and Mid-South during the first half of the week.

Heat advisories and excessive heat warnings are in effect as the heat index approaches 100F across much of the northern Plains.

In addition to the heat, the NWS forecast showers and thunderstorms across parts of the Southwest and southern Rockies, bringing bring isolated chances for flash flooding.

A few developing thunderstorms will have the potential to produce damaging wind gusts, large hail and isolated tornadoes across the northern High Plains, according to the NWS. Heavy rain may accompany embedded thunderstorms throughout the northern Rockies and much of Montana, as well.