Why does a heat wave sometimes follow a cool spell in the spring and early summer?

While strange but true, many a heat wave has come on the heels of a cool spell in the Mid-Atlantic Region. The reason is that the same cool, Canadian high pressure area that brings in the cool air moves off the east coast and then pumps up warm air on its back side. Remember, winds blow clockwise around high pressure areas in the northern hemisphere, so as the high settles off the east coast, winds turn southwesterly, and this brings in the warm and sometimes humid air. A very notable example of this occurred in April 1976, when the temperature reached 96 degrees on April 18th in NYC after a cool spell a week earlier.