Yellowstone National Park

• Wyoming State Parks
• Yellowstone National Park
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Weather

Summer: Daytime temperatures are often in the 70s (25C) and occasionally in the 80s (30C) in lower elevations. Nights are usually cool and temperatures may drop below freezing at higher elevations. Thunderstorms are common in the afternoons.
Winter: Temperatures often range from zero to 20F(-20 to -5C) throughout the day. Sub-zero temperatures over-night are common. The record low temperature is -66F (-54C). Snowfall is highly variable. While the average is 150 inches per year, it is not uncommon for higher elevations to get twice that amount.
Spring & Fall: Daytime temperatures range from the 30s to the 60s (0 to 20C) with overnight lows in the teens to single digits (-5 to -20C). Snow is common in the Spring and Fall with regular accumulations of 12" in a 24 hour period.
At any time of year, be prepared for sudden changes. Unpredictability, more than anything else, characterizes Yellowstone’s weather. Always be equipped with a wide range of clothing options. Be sure to bring a warm jacket and rain gear even in the summer.

http://www.nps.gov/yell/planvisit/orientation/weather/index.htm

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