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Burn scars pose a flash flood danger
The Alpine Lake Fire in Fremont County burned more than 40,000 acres in 2012, and rainfall hitting the burned region may cause flash floods. NWS graphic

Burn scars pose a flash flood danger

Jul 8, 2013 - From staff reports

The effects of the busy 2012 fire season could still be felt across western and central Wyoming this summer.

In Fremont County, the Alpine Lake Fire burned more than 40,000 acres in 2012 and rainfall hitting the burned region may pose a threat to residents and recreators.

Lands scarred by fire will be prone to excessive runoff from heavy thunderstorm rains. The result could be dangerous flash flooding that happens quickly with little advance warning. The National Weather Service in Riverton is advising residents near fire scars to prepare early and be ready to move to safety should heavy rain occur.

Fires can turn soils and forest litter into an almost pavement-like layer just below the surface. Heavy rains cannot penetrate this layer and surface water runoff is much greater than under normal conditions. A thin soil layer on top of this pavement-like layer and the excessive runoff can quickly slide and move downhill carrying burnt logs, shrubs, rocks and other debris.

"Our biggest concern is that a brief burst of heavy rain could quickly cause flash flooding in these burned areas and providing advance warning in that situation would be difficult," said Chris Jones, warning coordination meteorologist at the NWS office in Riverton. "The response to the heavy rain is so quick that people will need to be prepared ahead of time."

Jones said residents in and near the burn areas need to have a planned, safe evacuation or escape route and an emergency supply kit.

Fremont County creeks that could be affected include Bull Lake Creek, South Fork Bull Lake Creek, North Fork Bull Lake Creek, Hidden Creek, Sheep Creek, Paradise Creek, Little Sand Draw and North Fork Sage Creek.

These flash floods and debris flows are most likely during the first two years following a fire. The steepness of the terrain, the intensity and size of the fire, and the amount and duration of the rainfall are important factors in flooding within burn scars. Jones said half an inch of rain in an hour can be enough to cause flash flooding or debris flows in the burned areas.

Forecasters at the Riverton NWS office are particularly aware of the North American monsoon during the months of July and August. The monsoon has historically posed the greatest threat for heavy thunderstorm rain across Wyoming, Jones said.

The Riverton NWS office has developed a comprehensive website for five of the larger wildfires that occurred in 2012: Alpine Lake, Fontenelle, Gilead, Horsethief Canyon and Sheep Herder Hill. Fact sheets, safety recommendations and drainage maps are available at weather.gov/riverton.

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