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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

February 6, 2020

Dry conditions relative to climatology have persisted over the past month in the Sierra Nevada, California and Nevada, leading to the lowest snow water equivalent (SWE) percent of median at the HUC-6 river basin scale in the lower 48. All of the Sierra Nevada HUC-6 basins are currently below 80% of median SWE, with the Walker River basin in the eastern Sierra Nevada the lowest at 61% of median. Small storms throughout January allowed for minor snow accumulations in the region, but this was not enough to keep up with climatology as January is one of the wettest months of the year. While snowpack is below normal for the Sierra Nevada, conditions are much improved compared to early February numbers in other recent snow drought years such as 2015 and 2018, when record, or near-record, low SWE was measured for the same time period.
 
The biggest change in snow drought conditions since early January has been in the Washington Cascades. A number of SNOTEL stations in the region recorded the wettest January on record that led to substantial SWE gains and rapid changes from near record low to slightly above normal snowpack. However, the percent of median SWE is not distributed evenly across elevation with lower elevation stations still only at 50-80% of median. Storms during early January and again during the first week of February have been accompanied by warm temperatures and high freezing levels. This has led to decreasing SWE in some cases at lower elevations.
 
Other areas of concern in the lower 48 include the Sawtooth and Pioneer Mountains in south-central Idaho, where a cluster of stations are reporting 65-75% of median SWE, and southwest New Mexico with three stations below 20% of median SWE. Basins in southeastern Arizona are at 80% of median SWE.
 
In Alaska, well below normal snowpack continues for the Kenai Peninsula. Several SNOTEL stations are reporting less than 50% of median SWE; Grouse Creek Divide SNOTEL at 700 ft. elevation is reporting the lowest at 30% of median SWE. The persistence of low snowpack for the region into mid-winter is most recently being driven by a lack of precipitation (meteorological drought) and large storms in January, but large SWE deficits initially developed in December with heavy precipitation, warm temperatures, and rain at lower elevations (warm snow drought).​

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 February 3, 2020. Scales ranges 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 to blue) in UT, CO, WY, MT, below normal SWE (orange to red) in the Sierra Nevada of CA and NV, a mix of above and below normal SWE in the Pacific Northwest. In the bottom panel the map of Alaska shows below normal (red to orange) in south central Alaska and above normal (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 February 3, 2020. Scales ranges 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 to blue) in UT, CO, WY, MT, below normal SWE (orange to red) in the Sierra Nevada of CA and NV, a mix of above and below normal SWE in the Pacific Northwest. In the bottom panel the map of Alaska shows below normal (red to orange) in south central Alaska and above normal (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 February 3, 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 September 30, 2020 at the Paradise, WA SNOTEL. The black line shows current water year accumulated precipitation through February 3, 2020, the blue line shows current water year SWE through February 3, 2020, and the grey and red lines show long-term average precipitation and SWE, respectively. Currently, precipitation and SWE are both slightly above normal.

USDA Natural Resources Conservation Service (NRCS) water year accumulated precipitation and snow water equivalent (SWE) relative to climatology at Paradise, Washington. In the past 30 days SWE has increased 29.4 inches and precipitation has totaled 29.6 inches. The SWE percentile has made a remarkable improvement from 8th on January 1, 2020 to 62nd on February 3, 2020. Graphics can be found at NRCS.

A scatterplot of X-axis snow water equivalent (SWE) percent of 1981-2010 median and Y-axis elevation (meters) for SNOTEL stations in the Washington Cascades from February 3, 2020. Data is from USDA NRCS. Colors of dots represent water year to date precipitation percent of 1981-2010 average and range from 0 (brown) to 80-120% (white) to greater than 180% (dark teal).

USDA Natural Resources Conservation Service (NRCS) SNOTEL stations in the Washington Cascades on February 3, 2020. Nearly all stations have improved to near-to-above normal precipitation shown by white or green filled circles. However, several stations that are mostly below 1200 m are still reporting SWE less than 80% of median.

 

 

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.