Climate Impacts Research Consortium

  Findings:

• Over 90% of snow monitoring sites with long records (more than 40 years worth) in the American West show declines, according to CIRC research (Mote et al. 2018).

• Using a hydrologic model to corroborate these results, CIRC researchers concluded that this translates to the snowpack equivalent of 25 to 50 cubic kilometers of water (roughly 6–12 cubic miles). To make that really big number make sense for their readers, the researchers compare it to Lake Mead, which can hold up to 32 km3 (not quite 8 cubic miles) of water (Mote el al. 2018).

• Watersheds in the Pacific Northwest that receive a mix of rain and snow and derive a substantial portion of streamflow from spring snowmelt are most sensitive to future warming expected during the winter months (Vano et al. 2015).

• The Cascade Mountains in Oregon and Washington are expected
  to be particularly hard hit by declines in snowpack with a projected decrease of 65%—or 37.5 km3—in April 1 snow water equivalent (SWE) storage—by the 2080s under the high emissions scenario (RCP 8.5) (Gergel et al. 2017).

• By the mid-21st century (2040–2069) under the high emissions scenario (RCP 8.5), every SNOTEL site in the West is likely to see less snowfall and that snowfall is more likely to come in extreme snowfall events (Lute et al. 2015). (SNOTEL sites are automated snow- observing stations.)

• Sites that currently experience average winter temperatures that hover just above freezing are projected to see the largest decreases in the amount of snow that falls during extreme snowfall events, declining 20–50% from historical records. These include most of the SNOTEL sites in Oregon and Washington (Lute et al. 2015).

• In the Cascade Mountains are some of the hardest hit SNOTEL sites, which are projected to experience a 35–70% reduction in snowfall from historical levels (Lute et al 2015).

• Low snowfall years will become common in the Cascades by the mid- 21st century, whereas high snowfall years will become exceedingly rare (Lute et al. 2015).

• The water year 2014–2015 was dubbed a “snow drought” because precipitation was near-normal while abnormally warm temperatures led to record low snowpack. Record low spring snowpack measurements were set at 80% of mountain recording sites (or SNOTEL sites) in the Western United States (Mote et al. 2016).

• Spring snowpack in 2015 was the lowest on record for Oregon—89% below normal—and tied for lowest on record for Washington (Mote et al. 2016).

• Anthropogenic forcing added about 1° C (1.8° F) of extra warming to the water year 2014–2015 exacerbating the “snow drought” (Mote et al. 2016).

Why is April 1st used in Snow Monitoring?

April 1st marks the point of maximum snowpack accumulation for much of the American West.

April 1st also marks the turning point after which snowpack begins to melt. Although the exact date of peak snowpack may occur earlier or later than this—depending on factors, such as location, elevation, and year-to-year variability—streamflow forecasts using SWE numbers on April 1st have long been used for predicting summer streamflow. So much so that April 1st snowpack numbers act as a kind of bellwether for summer water supplies.

Years with low April 1st snowpack tend to be followed by low flows in rivers and streams during the summers that follow. This can sometimes lead to or exacerbate water scarcities, especially in snowmelt-dominated basins.