By Daniel Trotta (Reuters) – Much of the United States will experience another heat wave this week, with above-normal temperatures forecast for the Pacific Northwest, Southern Plains and the Lower Mississippi Valley. The extreme heat is fueling a fast-moving California wildfire, just went of Yosemite National Park. The Oak Fire has destroyed 10 structures and is 10% contained. The U.S. heat wave followed record heat that killed hundreds if not thousands of people and sparked wildfires in Europe. Following is an explanation of what is causing the heat waves, according to scientists. WHAT IS A H…