Spring Snowpack and Flood Outlook – April Update

The potential for spring flooding due to snowmelt in 2018 is well above average across most of northern and eastern Idaho and Montana. Meanwhile, the spring flood potential is slightly below average for southwest Idaho and southeast Oregon.

The storm track over much of the winter brought above normal precipitation values to much of the region. However, it was a tale of two cities on the northern end (cooler) and the southern end of the storm track. Cooler conditions on the northern portion of the storm track led to above normal snow values across Washington, northern Idaho, and western Montana, while the southern end of the storm track brought warmer conditions to Oregon, southwest Idaho, Nevada and Utah, leading to a below normal snow pack. The following graphic illustrates the amount of snow pack compared to normal for mid April.

2018-4-14

 

Winter 2016 and Spring 2017 Flood Summary

This past winter and spring had its share of flooding across southeast Oregon and southwest Idaho.  Not only did spring runoff bring flooding to rivers and streams, but ice jams and snow melt caused flooding during the winter as well.  The stage was being set for an active spring flood season as far back as October 2016, when 150 to 400 percent of normal precipitation occurred across much of the region which moistened the soil profile.  The winter storm track brought well above average snowfall to most of southeast Oregon and southwest Idaho, with extreme snowfall across lower valleys.  A relatively cool and wet early spring was the final piece of the puzzle to ensure abundant spring runoff.  An indicator of how wet this past winter and spring have been, water supply forecasts for the April through September period rank in the top 10 for most of southeast Oregon and southwest Idaho, dating back to 1970.  Additionally, all major reservoir systems either have filled or are expected to fill.  Record high precipitation was seen across many areas from December 2016 through June 2017, shown in the figures below.

pnw_cl (3) pnw_cl (4)

The map below shows March 1 snow pack along with areas where flooding had a significant impact.

flood

Although the threat of snow melt flooding has diminished, summertime thunderstorms can pose a serious flood risk.  Areas of steep terrain and areas burned by wildfire are at particular risk for flash flooding due to thunderstorms.  For flood safety information, visit http://www.floodsafety.noaa.gov/.  For the latest river conditions, see http://water.weather.gov/ahps2/index.php?wfo=boi.

Wildfire Burn Scars are a Flood Risk Infographic

Wildfire Burn Scars are a Flood Risk

Heavy rain can produce flash floods, mudslides & debris flows over burned areas from wildfires.  Water repellent soils are formed when organic material such as trees, scrubs, plants and litter burn at high intensity (high temperatures), causing water repellent compounds to become vaporized which then condense on cooler soil layers below the surface, which prevents the soil from absorbing water after a fire. During heavy rains, water cannot penetrate water repellent soil layers, so it runs off like pavement which causes dangerous flash flooding, debris flows and mudslides.