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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

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.
 

Graph showing the USDA Natural Resources Conservation Service (NRCS) snow water equivalent (SWE) basin values over the western U.S. and Alaska for April 1, 2020.

USDA Natural Resources Conservation Service (NRCS) snow water equivalent (SWE) basin values over the western U.S. and Alaska for April 1, 2020. For an interactive version of this map, please visit NRCS.

 

Graph from the Snow and Ice Data Center showing the total snow-covered area across the western United States from MODIS for 2001-2020.

Total snow-covered area across the western United States from MODIS for 2001-2020. The black line shows 2020. The grey area shows the interquartile range where 50 percent of the historical data from 2001 to present falls. The dashed line in the middle of the gray area shows the median of the historical data from 2001 to present. The dotted line at the bottom depicts 2015, the lowest year for snow-covered area; the dotted line at the top depicts 2008, the highest year for snow-covered area. For more information, please visit the National Snow and Ice Data Center.

 

Map showing the mean temperature percentiles for the period April 22-June 20, 2020 based on gridMET. Percentile rankings are based on the 1979-2015 period of record. 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).

Mean temperature percentiles for the period April 22-June 20, 2020 based on gridMET. Percentile rankings are based on the 1979-2015 period of record. For more information and maps, please visit the Northwest Climate Toolbox.
 

U.S. Geological Survey Water Watch map showing below normal 28-day average streamflow compared to historical streamflow for June 22, 2020. All stations shown are currently below the 25th percentile.
Table explaining the streamflow percentile classes in the U.S. Geological Survey Water Watch map above. 10 to 24 is below normal, 6 to 9 is moderate hydrological drought, less than or equal to 5 is severe hydrological drought, and low is extreme hydrological drought.

USGS 28-day below normal observed streamflow for June 22, 2020. All stations shown are currently below the 25th percentile. For more information, you can visit the USGS Water Watch.
 

USDA Natural Resources Conservation Service (NRCS) June-July forecast total volume for the 50% exceedance probability (median June-July total runoff), percent of 1981-2010 median, over the western U.S. issued June 1, 2020.

USDA Natural Resources Conservation Service (NRCS) June-July forecast total volume for the 50% exceedance probability (median June-July total runoff), percent of 1981-2010 median, over the western U.S. issued June 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.
 

United States Drought Monitor map of the Western region for June 23, 2020, released on June 25, 2020. This map  indicates that 46% of the Western United States is in at least moderate drought (D1), including some drought in every state.

United States Drought Monitor map for June 23, 2020. For more information and weekly updated maps, visit the U.S. Drought Monitor website.

 

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.