Summer Weather (July-September):

Daytime temperatures in Yellowstone National Park are often in the 70s to low 80s at lower elevations. Nighttime temps are usually chilly and may drop below freezing at higher elevations. Most summer days are sunny to partly cloudy, although afternoon thunderstorms are common.

Spring & Fall Weather:

Daytime temperatures can range from the 30s to the 60s with overnight lows in the teens to single digits. Heavy snow accumulation is common in the spring and fall and the weather can change abruptly.

Weather Forecasts:

The Weather Channel
National Weather Service
The Weather Underground
NOAA Radar Image