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Snow Drought

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

April 30, 2020

Snowmelt season is underway and the combination of above normal temperatures with below normal precipitation over the past month has led to accelerated melt rates and degrading snow drought conditions over large areas in the west with the maritime ranges being in the worst condition. Temperatures over the past seven days have been above normal over nearly the entire western US while early April warm anomalies were confined to mostly California, Oregon, and Washington with below normal temperatures elsewhere.

Northern California and Southern Oregon have seen some of the biggest decreases in percent of normal snow water equivalent (SWE) over the past month. From March 31 to April 26, the Klamath basin dropped from 75% to 50%, and Upper Sacramento basin from 96% to 66%. Low snowpack and precipitation, and above normal temperatures have been the primary drivers of drought for the region with widespread below normal streamflows and impacts to ranching and farming. Accumulated water year-to-date runoff is very low throughout much of western Oregon, and near historic lows in southwest Oregon. A two category downgrade of the US Drought Monitor has occurred in this region over the past month with D2 (severe drought) and D3 (extreme drought) currently depicted, resulting in 3 SW Oregon counties declaring state drought emergencies. The Humboldt Basin in Nevada is also in poor shape at 49% of normal SWE which will likely lead to drought impacts into the spring and summer as this basin provides surface and groundwater to major agricultural regions in northern Nevada.

For Colorado, winter-like storms have allowed the snowpack to continue to build in the northern half of the state. The Front Range had a particularly snowy April (37.3” of snowfall) that led to record seasonal snowfall at Boulder, Colorado with 152” since October 1 and broke the previous record of 134.7” set in 1909. Snow melt and below normal snowpack exists in the southern part of the state with the Upper San Juan basin, critical to Colorado River water supply, at 85% of normal.

​For Alaska, snowpack continues to degrade in the Kenai Peninsula basin which is currently at 34% of normal SWE. Several waves of above normal temperatures combined with below normal precipitation have accelerated melt in this region. Turnagain Pass SNOTEL station, at 1880’ elevation, currently has the lowest SWE in the 37 years of record at 17.5” with the normal being 39.5”. All other stations in the Kenai Peninsula (with the exception Kenai Moose Pens) are at the 7th percentile SWE or lower based on SNOTEL stations.​

Two panels show USDA Natural Resources Conservation Service (NRCS) percent of 1981-2010 median snow water equivalent (SWE) over the western U.S. (top) and Alaska (bottom) for April 26, 2020. Scales range from <0% (red) to 100% (white) to >200% (blue). Filled dots show individual SNOTEL stations and filled polygons shows HUC-6 river basin averages from all stations within a given basin. In the top panel a map of the Western US shows above normal SWE (green) in northern ID, western MT, WY, northern CO, and northern AZ, below normal SWE (orange to red) in AZ, southern CO, CA, NV, southern ID, OR, and WA. In the bottom panel the map of Alaska shows below normal (orange) in south central Alaska and above normal (green-blue) in central Alaska.

Two panels show USDA Natural Resources Conservation Service (NRCS) percent of 1981-2010 median snow water equivalent (SWE) over the western U.S. (top) and Alaska (bottom) for April 26, 2020. Scales range from <0% (red) to 100% (white) to >200% (blue). Filled dots show individual SNOTEL stations and filled polygons shows HUC-6 river basin averages from all stations within a given basin. In the top panel a map of the Western US shows above normal SWE (green) in northern ID, western MT, WY, northern CO, and northern AZ, below normal SWE (orange to red) in AZ, southern CO, CA, NV, southern ID, OR, and WA. In the bottom panel the map of Alaska shows below normal (orange) in south central Alaska and above normal (green-blue) in central Alaska.

USDA Natural Resources Conservation Service (NRCS) percent of 1981-2010 median snow water equivalent (SWE) over the western U.S. (top) and Alaska (bottom) for April 26, 2020. Only stations with at least 20-years of data are included in the station averages. Shaded polygons show percent of median SWE for HUC-6 (hydrologic units) river basins. For an interactive version of this map please visit NRCS.

A time series graph showing for October 1, 2019 through July 1, 2020 at the Klamath Basin. The black line shows the current water year snow water equivalent (SWE) through April 27, 2020, while the blue line shows 1981-2010 maximum SWE, the green line shows 1981-2010 median SWE, and the red line shows 1981-2010 minimum SWE. Current SWE values are below median  for the entire season with a sharp drop in SWE in the last few weeks.

USDA Natural Resources Conservation Service (NRCS) water year accumulated snow water equivalent (SWE) (in.) (black) at Klamath Basin compared to 1981 - 2010 maximum (blue), median (green), and minimum (red). Graphics can be found at NRCS.

A time series graph showing for October 1, 2019 through July 1, 2020 at the Kenai Peninsula. The black line shows the current water year snow water equivalent (SWE) through April 27, 2020, while the blue line shows 1981-2010 maximum SWE, the green line shows 1981-2010 median SWE, and the red line shows 1981-2010 minimum SWE. Current SWE values are below median for the entire season with a sharp drop in SWE in the last few weeks to near minimum of the 30 year record for April 27th.

USDA Natural Resources Conservation Service (NRCS) water year accumulated snow water equivalent (SWE) (in.) (black) at Kenai Peninsula compared to 1981 - 2010 maximum (blue), median (green), and minimum (red). Graphics can be found at NRCS.
 

Snow Drought Tools

Example image of a NRCS SNOTEL and Snow Course Data
Point maps and interactive maps of snow water equivalent, snow depth, and snow density from the Natural Resources Conservation Service (NRCS) Snow Telemetry (SNOTEL).
Example image of a Climate Engine SNODAS image showing snow water equivalent
Climate Engine uses Google’s Earth Engine for on-demand processing of satellite and climate data via a web browser. Click for SNOw Data Assimilation System (SNODAS) snow water equivalent (SWE) maps and time series over the western U.S from your day of choice compared to average.
Example image of a NOHRSC National Snow Analyses map
Gridded snow data from the National Operational Hydrologic Remote Sensing Center (NOHRSC), also available in an interactive map.
Example image of a CA-NV River Forecast Center map
The CNRFC interactive website provides a full set of observations and forecasts, including snow data, observed and forecasted freezing levels, and streamflow forecasts.
Example image of a Colorado Basin River Forecast Center map
The CBRFC interactive website provides a full set of observations and forecasts, including snow and river conditions and water supply forecasts.
Western Water Supply Forecast Map
Website gives user access to all the western RFC water supply webpages.
Example image of a Northwest River Forecast Center map
The NWRFC interactive website provides a full set of observations and forecasts, including snow and river conditions and water supply forecasts.
Example of a Snow Cover Map
Daily maps, including animation tool, of northern hemisphere snow cover (white) and ice extent (yellow) from the U.S. National Ice Center. Click for current data.
Example image of a National Snow Probability Forecasts map
National Operational Hydrologic Remote Sensing Center (NOHRSC) snow probability forecasts depicting the probability of snowfall reaching or exceeding 4, 8, or 12 inches in the next 24 hours to 72 hours.
Example image of a CW3E West Coast Freezing Level Forecast map
The Center for Western Weather and Water Extremes (CW3E) uses GEFS forecasts to show probabilities for the western states’ watersheds of the freezing level being above or below the terrain height, i.e. forecast near-surface temperatures being above or below freezing, and precipitation falling as rain or snow.
Example image of a CW3E West-WRF Model
West Weather Research and Forecasting (West-WRF) is an ongoing effort at the Center for Western Weather and Water Extremes (CW3E) to develop a regional weather prediction system, including 3-hour and 24-hour snow, tailored to western U.S. weather and hydrological extremes.
Example image of a NRCS Streamflow Forecast Map
Available in spring and summer for the Western U.S., forecasts of percent of monthly average flow compared to data from 1981-2010.
Example image of a Sierra Nevada Water Storage Tracking map
Daily reservoir storage and snowpack update for the Sierra Nevada.
Example image of a California Data Exchange Center Snow map
Snow course and snow sensor information from California Department of Water Resources, including snow water content maps and time series by Northern, Central, and Southern Sierra. Click for actual conditions.
Real Time Spatial Estimates of SWE
Experimental research product provided by the CU-Boulder and NASA JPL that provides near-real-time estimates of snow-water equivalent (SWE) for the Sierra Nevada in California from mid-winter through the melt season.
Sample SWE Map for the Intermountain West
Experimental research product provided by the CU-Boulder and NASA JPL that provides near-real-time estimates of snow-water equivalent (SWE) for the Intermountain West from mid-winter through the melt season.
Example image of a Airborne Snow Observatory map
NASA/JPL, in partnership with the California Department of Water Resources, has developed the ASO, an imaging spectrometer and scanning lidar system, to quantify SWE and snow albedo. Click for actual conditions.
Colorado’s Decision Support Systems SNODAS Tools
Colorado’s Decision Support Systems SNODAS Tools process the national SNODAS gridded dataset daily to provide data products, including Snow Water Equivalent and Snow Coverage statistics for Colorado water supply basins.
Example image of a CA-NV Snow Water Equivalent map
Monitoring from the UCLA Drought Monitor of current observed snow water equivalent (SWE) percent of average for nearly all California Department of Water Resources snow pillow stations.
Example of a PNW SWE map
Monitoring from the University of Washington Drought Monitoring System of current observed snow water equivalent (SWE) percent of average for nearly all NRCS SNOTEL stations, California DWR snow pillow stations, and a selection of British Columbia government snow pillow stations.
NRCS Water Supply Outlook Reports
Water supply outlooks produced monthly from January to May.
Example image of a CVTEMP map
NOAA Southwest Fisheries Science Center tool for modeled and observed temperature and flow data for the Sacramento River associated with Shasta Reservoir, Shasta Dam Operations, and meteorological conditions.
US Water Watcher Example Map
The US Water Watcher tracks water conditions from exceptionally wet to exceptionally dry using a number of different metrics including snow.
Example of NW Climate Toolbox Map
Snow Water Equivalent Percentile (1981-2010) based on VIC-gridMET data available through the Northwest Climate Toolbox HydroClimate Mapper at monthly intervals on the first of the month.
Intermountain West Climate Dashboard
Providing situational awareness of climate, drought, and water resources for the Intermountain West Drought Early Warning System including briefings.