Hints of summer are on the way to the western Plains of the United States. Temperatures in parts of Oklahoma reached 100 degrees on Monday for the first time in 2008. Walters, Grandfield, Altus, Mangum and Hollis, all in southwest Oklahoma, reached 100 degrees. A building ridge of high pressure centered over the central part of the country, combined with southwesterly winds bringing warm, dry air, led to the high temperatures. The arrival of 100-degree heat usually comes in middle to late May in the southern Plains. The hot temperatures arrive later if it has been unusually wet. The earliest Oklahoma City reached triple digits was on April 12, 1972 when the high reached 100 degrees.