What Is Snow Drought?
Snowpack typically acts as a natural reservoir, providing water throughout the drier summer months. Lack of snowpack storage, or a shift in timing of snowmelt from that reservoir, can be a challenge for drought planning. Few drought metrics include storage and release of snow water. Several years of low snowpack, especially across the western U.S., have led to many studies looking into the causes and impacts of reduced snow storage (see Resources) and the creation of a new definition of drought called Snow Drought.
Snow drought is defined as period of abnormally low snowpack for the time of year, reflecting either below-normal cold-season precipitation (dry snow drought) or a lack of snow accumulation despite near-normal precipitation (warm snow drought), caused by warm temperatures and precipitation falling as rain rather than snow or unusually early snowmelt. (AMS Glossary of Meteorology)
Snow-dominated regions face several challenges due to snow drought and its impacts:
- Summer Water Availability: Snow droughts reduce the amount of available water for spring and summer snowmelt. This, in turn, reduces streamflow and soil moisture, which can have impacts on water storage, irrigation, fisheries, vegetation, municipal water supplies, and wildfire.
- Winter Water Management: Warmer winter storms lead to rain instead of snow at higher elevations in mountain regions that can create challenges for water management and flood mitigation strategies, particularly when dealing with extreme events.
- Outdoor Tourism and Recreation: Many local economies and industries rely on snowpack and river flows from snowmelt to support their outdoor industries such as skiing, rafting, and fishing.
- Ecosystems: Lack of snow can disrupt ecosystems over shorter and longer timescales.
Current Situation and Impacts in the West
June 25, 2020
This June 25 current situation update will be the last for the 2020 Water Year as snowpack and snowmelt are past peak values. Updates will resume in the 2021 Water Year (i.e., beginning October 2020). Access to the tools is available year round.
The 2019-2020 snow season ended with a few regions in snow drought, including part of the Lower Colorado River Basin in Arizona and New Mexico, the Sierra Nevada, the Great Basin, parts of the Cascade Range, and south Central Alaska. In terms of total snow covered area for the western United States as a whole from MODIS satellite data, from mid-winter through the melt season this year wound up being not too far from average when considering the 2001-2019 distribution.
Above normal temperatures from mid-April through mid-June in the Southwest, and particularly in the Colorado River Basin, have led to accelerated snowmelt in the region. Parts of Colorado, New Mexico, and Arizona saw record high 60-day mean temperatures for the period of April 22-June 20, 2020 (relative to 1979-2015 rankings).
The combination of low snowpack and above normal spring temperatures has led to below normal observed streamflows. Large areas across the West exist with 28-day streamflows ending June 22 below the 25th percentile, with pockets of stations below the 5th percentile. Forecast June-July total runoff (which includes most of the 28-day period referenced above) is expected to be below normal for the Colorado River Basin despite a healthy mid-winter and early spring snowpack for the upper Basin. At Lake Powell, which is at 53% of capacity as of June 21, 2020, the May inflow was 66% of normal and the total April-July inflow forecast is 57% of normal. Another area of concern for water supply this summer is the California-Oregon border region. The Klamath Basin forecast for total June-July runoff (peak runoff months) is only 8% of normal.
Snow drought conditions have played a big role in the ongoing or developing drought status in the US. Drought Monitor. The latest map from June 23, 2020 indicates that 46% of the Western United States is in at least moderate drought (D1), including some drought in every state. In comparison only around 5% of the West was in drought at the end of the 2018-19 snow season. Additionally, extreme drought (D3) has taken hold in pockets in Utah, California, and Oregon, and large parts of Southern Colorado and northern New Mexico.