7 of 7 Evie True, 9, right, plays in her pool Monday in Lewiston with her cousin Lila Washington, 9. The two have been lifelong companions and swim as often as they can. Andree Kehn/Sun Journal With temperatures expected to reach 90 degrees or higher Tuesday, this latest blast of hot air could become an official heat wave, according to the National Weather Service. The agency defines a heat wave as three consecutive days of at least 90 degrees. With Sunday and Monday reaching that threshold in many parts of Maine, and Tuesday and Wednesday forecast to break that threshold, Mainers should prepare for a couple more days of air conditioners, fans and other cooling measures.