Active AlertsYellowstone National Park

What: Flash flooding caused by moderate to heavy rainfall is possible, especially in terrain and poor drainage areas.
Where: Northwest Wyoming, including Yellowstone National Park.
When: Through Sunday afternoon.
Impacts: Heavy rainfall may result in flooding of rivers, creeks, streams, and other low-lying and flood-prone locations. Flooding may occur in poor drainage areas and around steep terrain.
Additional Details: - A slow moving weather system will move across Wyoming today through Sunday and bring rounds of showers and thunderstorms to the region, especially northern Wyoming. Any shower or thunderstorm could produce moderate to heavy rainfall across the Watch Area. - http://www.weather.gov/safety/flood

You should monitor later forecasts and be prepared to take action should Flash Flood Warnings be issued.