Heat is the number one killer -- and it’s heading to the most densely populated part of America for a prolonged period.

Dangerous storms and extreme heat will affect millions this week from the Midwest to the Northeast, with flooding and high temperatures forecast.

A heat wave forecast to envelop the Midwest and the Northeast with triple-digit weather is expected to arrive on the heels of the first official day of summer. But before the sizzle, the country's midsection and much of the East Coast will have to endure at least a couple of days of severe weather.