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First heat wave of the year in the books weeks before official start of summer

Twenty-one million people were under heat advisories again on Tuesday, with temperatures expected to soar into the 90s and feel close to 100 degrees.

The East Coast is officially experiencing the first heat wave of the year with Washington and Philadelphia having endured three consecutive days of 90-degree temperatures, weeks before summer actually starts.

In fact, the last several days have been so hot that more than a dozen locations stretching from the Northern Plains to New England just experienced their hottest first week of June on record.

Burlington, Vermont's high of 96 degrees on Monday was not only a daily record but the hottest they've been so early in the season.

This heat wave, which began over this past weekend, is expected to continue Tuesday and into Wednesday before a major cooldown knocks temperatures down on Thursday and Friday.

About 21 million people were under heat advisories again on Tuesday, where temperatures 10-20 degrees above average will make it feel more like late July than early June. Boston and Minneapolis were two of the larger cities experiencing unseasonably warm weather.

As if air temperatures in the upper 80s and 90s weren't enough, for locations with additional high humidity, the temperatures will feel more like the mid-to-upper 90s to close to 100 degrees.

A few spotty record highs were also possible Tuesday afternoon across New England and the Northern Plains, including the cities of Portland and Caribou in Maine and Fargo, North Dakota.

Temperatures will stay hot on Wednesday, especially across the Upper Midwest, where cities like Minneapolis and International Falls could set record highs.

This prolonged period of record-setting temperatures comes nearly two weeks before the official start of summer, which occurs on June 20.

The combination of heat and humidity will also continue to spark showers and thunderstorms through Wednesday for much of the eastern half of the country. The area with the best chance for heavy downpours includes portions of Oklahoma, Texas, Arkansas and western Mississippi.

In these areas 2-4 inches of rain could fall, with locally higher amounts up to 8 inches. For that reason, a flash flood watch is in effect, including 3 million people, until Wednesday morning.