A strong high pressure system is expected to build across the West, setting off a "historic" heat wave from northern Utah to southern California. Temperatures in Phoenix could reach 100 degrees by St. Patrick's Day, breaking the record for the earliest 100-degree day set on March 26, 1989. The 10-day forecast from the Climate Prediction Center puts the heat wave probability at nearly 100% in some places, a rare level of confidence. High temperatures are expected to be 15-25 degrees above normal for this time of year. This comes after a record-hot winter in Phoenix, which last year saw the second-warmest year on record for America's hottest big city. The heat is likely to persist, leading to rapid melt of the remaining snowpack.