By Rich McKay (Reuters) – An arctic blast that gripped much of the United States on Saturday left more than 700,000 without power, at least 16 dead from weather-related car crashes and thousands stranded due to flight cancellations. Plummeting temperatures were expected to bring the coldest Christmas Eve on record, and energy systems across the country were strained by rising demand for heat and storm-related damage to transmission lines. The latest outage numbers are a sharp drop from the 1.8 million U.S. homes and businesses left without power as of early Saturday morning, according to track…