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After the Fire – Weekly Snow Reports

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National Weather Service Burn Map

Weekly Snow Reports for Burned Areas:

Please Note: These reports are prepared weekly as an informational tool for emergency managers, county officials, conservation groups, landowners and the public. Snow and weather conditions change hourly, so please consult the NRCS Snow Survey website or the National Weather Service website for the most up-to-date information in your area.

If you can look uphill from where you are and see an area burned by wildfire, you are at risk.

See an Interactive Burned Area Map from the National Weather Service

Lands damaged by wildfire experience a higher risk for flash flooding. Even areas that aren’t traditionally prone to flooding are at risk, because the burned soils can’t absorb as much water. Flooding after a fire can cause significant soil erosion and trigger debris slides (such as ash, rocks, and burned trees) that can damage roads and infrastructure.

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Pictured: A view of the damage from the Canyon Creek Wildfire that blazed through John Day, Oregon in the summer of 2015.

Flash Flooding, Snowpack & Rainfall

The potential for flooding depends greatly on rainfall intensity. Heavy rainfall on melting snow is a trigger for flash flooding. Generally, you can look at the density of the snowpack to gage snowmelt. Higher-density readings are an indicator that the snow is getting ready to melt. A general rule of thumb is that half an inch of rain falling in less than one hour is enough to cause flash flooding in a burned area, but this can be more or less depending on the fire severity and the steepness of the terrain.

Understanding the current snowpack conditions in your area and the potential for rain-on-snow events can help you stay better informed and prepared to anticipate flash flooding after a wildfire.

The USDA Natural Resources Conservation Service (NRCS) Snow Survey Program in Oregon measures snowpack and other pertinent data at about 300 mountain sites across Oregon and Washington. This data is available in near real-time online, so be sure to check the website for the latest information.

What factors should I look for in the snowpack as it relates to flood risk after a fire?

Heavy rainfall–and heavy rainfall in conjunction with melting snow–are the primary factors to consider for flood risk. There are several indicators you can look for to gage when the snowpack is starting to melt.

First, check the snowpack density at the sites in your area. If densities reach 40% or more, there is a greater risk for flooding if a moderate or heavy rainstorm falls on top of that snow. A high snow density means that the snowpack is reaching a point where melting is imminent. Density is calculated by dividing the snow water equivalent (SWE)–which refers to the amount of water stored in the snow–by the snow depth, multiplied by 100%. A 40% dense snowpack means that the snow is 40% water.

It’s also important to note the trend in SWE. If there’s a pattern of decreasing SWE levels, this also means the snowpack is beginning to melt, and thus the chances for flooding during a rainstorm are higher.

Keep in mind that it’s normal to see a gradual decrease in snow depth as the snow settles–so a downward trend in depth doesn’t always indicate snowmelt. As the snowpack begins to melt, which typically begins after the peak of the season in March and April, there’s a greater risk for higher run-off as the snowmelt flows into streams and rivers.

Click here to view the Northwest River Forecast.

How long will there be an elevated risk of flash flooding?

This depends on the severity of the wildfire and how much erosion occurs, but most burned areas are prone to flash flooding and debris flows for at least two years after the wildfire. It could take many years for trees and plants to re-establish. Vegetation is the primary factor that slows down the precipitation run-off that creates flash flooding and debris flows. Each burned area poses its own unique risk due to many factors including proximity to population centers, burn severity, steepness of terrain, and size of the burned area.

Click here to view a PDF version of this fact sheet. (PDF, 2 MB)

Contacts:

NRCS Oregon Snow Survey
Scott Oviatt, Supervisory Hydrologist
Phone: 503-414-3271
Email: Scott.Oviatt@or.usda.gov

National Weather Service
Pendleton, Oregon
Phone: 541-276-7832
www.wrh.noaa.gov/pdt