From the National Weather Service – Cheyenne, WY

...FLASH FLOOD WATCH THIS AFTERNOON AND EVENING...

.THE ATMOSPHERE OVER SOUTHEAST WYOMING AND WESTERN NEBRASKA IS
VERY MOIST. THUNDERSTORMS THAT DEVELOP THIS AFTERNOON AND EVENING
HAVE THE POTENTIAL TO PRODUCE LOCALLY VERY HEAVY RAINFALL. IN
ADDITION THESE STORMS WILL BE VERY SLOW MOVING WHICH WILL ALLOW
SIGNIFICANT ACCUMULATIONS WHERE IT RAINS. THE STORMS SHOULD END BY
LATE EVENING BRINGING THE THREAT OF FLASH FLOODING TO AN END.