Southeast Utah is part of the Colorado Plateau, a "high desert" region that experiences wide temperature fluctuations, sometimes over 40 degrees in a single day. The temperate (and most popular) seasons are spring (April through May) and fall (mid-September through October), when daytime highs average 60º to 80º F and lows average 30º to 50º F.
Summer temperatures often exceed 100ºF, making strenuous exercise difficult. Late summer monsoon season brings violent storm cells which often cause flash floods.
Winters are cold, with highs averaging 30º to 50º F, and lows averaging 0º to 20º F. Though large snowfalls are uncommon (except in nearby mountains), even small amounts of snow or ice can make local trails and roads impassable.
Get an Arches-area forecast from the National Weather Service