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
May 28, 2020
In the western United States, many observation stations are reporting snow water equivalent (SWE) of zero. Late May into June is climatologically when many mid-to-low elevation locations have completely melted out. At this time, long-term medians, or “normals”, of SWE is near zero. Low seasonal snowpacks and unseasonably warm temperatures in states like California, Nevada, Oregon, Washington, Arizona, and New Mexico have led to melt out occurring about one to three weeks earlier than normal. Other locations, like the Wasatch Range in Utah, saw above normal SWE on April 1 but have melted out already or are on the trajectory to melt out early due to extended warm and dry conditions. The poor late season snow conditions will lead to below normal mountain runoff this summer in these states as well as southern Colorado and parts of southwestern Idaho. The northern Rocky Mountains typically melt out a bit later due to higher elevation and colder climate. A near-to-above-average snowpack still remains in the northern Idaho panhandle and northwest Montana.
In Alaska, with the exception of parts of the southern Kenai range and one station in central Alaska, most locations have reached zero SWE. In the Fairbanks region of Alaska, above normal seasonal snowfall, particularly in March and April, led to melt out about one week later than climatology. In southcentral Alaska, the Kenai mountains melted out approximately a full month earlier than normal due to well below normal snowfall. For example, the Grandview SNOTEL (elevation 1100’) reached zero SWE on May 20. Its normal melt out date is June 18.
Typical drought impacts are likely in locations where the snowpack melted out early. These impacts include: reduced mid- and late-summer streamflows, increased stream temperatures, lower than normal soil moisture, early drying of vegetation and increased vegetation stress, and increased high elevation fire potential.
USDA Natural Resources Conservation Service (NRCS) snow water equivalent (SWE) station values over the western U.S. (top) and Alaska (bottom) for May 25, 2020. For an interactive version of this map please visit NRCS.
USDA Natural Resources Conservation Service (NRCS) May-July forecast total volume for the 50% exceedance probability (median May-July total runoff), percent of 1981-2010 median, over the western U.S. issued May 1, 2020. Shaded polygons show percent of median for HUC-6 (hydrologic units) river basins. For an interactive version of this map please visit NRCS.