Wednesday, May 13, 2020

Flash Flood Forecasts

Flash flooding in Little Wildhorse Canyon sadly took the lives of two young girls on Monday.  The situation was one that is common on the Colorado Plateau with precipitation from an afternoon thunderstorm funneled into a narrow slot canyon.  The tweet thread below from University of Utah graduate student Tom Gowan, who was in the area, describe the situation.

Routine weather forecasts produced by the National Weather Service provide general guidance about precipitation, but focused solely on precipitation, not the potential for flash flooding in slot canyons.  For example, below is the zone forecast for the San Rafael Swell region issued Monday morning for Monday calling for a 20% chance of showers and thunderstorms, the latter possibly producing strong gusty winds.

Source: NWS
However, the National Weather Service also produces a southern Utah flash flood potential rating, which is available at  This product is provided to help assess the potential for storm-produced flooding in slot canyons, washes, and other low areas.  The product is based on the flash flood potential rating definitions below, which include Not Expected, Possible, Probable, and Expected.

I wasn't sure how to access an archive of these ratings, but @erincoxnews at Fox13 posted the one below that was issued for Monday showing a rating of possible for the San Rafael Swell, meaning that some slot canyons, dry washes, and small streams may experience flash flooding. 

These flash flood potential ratings are available online at the link above and are also commonly posted at National Parks and other recreation visitors centers.  Get an update if you can before heading out to recreate.

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