The formation of a distinct pattern of sea surface temperatures in the middle of the North Pacific Ocean can predict an increased chance of summer heat waves in the eastern half of the U.S. up to 50 days in advance.The pattern is a contrast of warmer-than-average water coming up against cooler-than-average seas. When it appears, the odds that extreme heat will strike during a particular week — or even on a particular day — can more than triple, depending on how well-formed the pattern is.