Snow Today updates daily images on snow conditions and relevant data, and also provides monthly scientific analyses from January to May, or more frequently as conditions warrant.

SNOW COVER PERCENT

SNOW COVER DAYS

CHANGE IN SNOW WATER EQUIVALENT

TOTAL SNOW COVER AREA

MEDIAN SNOW COVER DAYS > 800m

SNOW WATER EQUIVALENT

Snow cover on the downhill

March 11, 2020
  • The typical date for annual maximum snow cover has passed.
  • Barring a major change in snowstorm activity, the snow cover across the West will continue to decline with the seasonal warming.
  • February continued to see a trend of below average snow cover across the Western United States.
  • Late January and early February storms replenished snow cover in some areas, but drought continues to impact California.
  • February 2020 had the sixth lowest snow cover for the month in the 20-year satellite record.
  • Despite below average snow cover, the Snow Water Equivalent (SWE) remains near average or above average for many locations across the West, outside of California, Arizona, southern Oregon, and south/central Idaho.
Overview of conditions
Table 1. February Snow Cover in Western United States, 2020 vs. the 20-year satellite record
Snow-Covered Area Square Kilometers Square Miles Rank
2020 715,000 276,00 15
2001 to 2019, Average 795,000 307,000 --
2001, Highest 1,159,000 447,000 1
2015, Lowest 457,000 176,000 20
2019 1,143,000 441,000 2

As averaged for February 2020, snow-covered area for the Western United States was 715,000 square kilometers (276,000 square miles) (Table 1). This was the sixth lowest (or fifteenth highest) snow cover in February in the 20-year satellite record. For comparison, there was 80,000 square kilometers (31,000 square miles) less snow-covered area in February 2020 than the average computed over the 2001 to 2019 reference period. The February with the greatest snow cover since 2001 was in 2001, when an average of 1.16 million square kilometers (448,000 square miles) blanketed western North America, 444,000 square kilometers (171,000 square miles) more than in 2020. The February with the least snow cover since 2001 occurred in 2015, when snow covered an average of only 457,000 square kilometers (176,000 square miles), 259,000 square kilometers (100,000 square miles) less than 2020. Comparing February to the same month a year ago, there was 428,000 square kilometers (165,000 square miles) less snow cover in February 2020 than in 2019. Through February 2020, the lowest daily snow coverage was 636,000 square kilometers (246,000 square miles) occurring on February 29, 2020, while the highest daily snow coverage was 765,000 square kilometers (295,000 square miles) occurring on February 4, 2020. As of this post, the annual maximum snow cover in 2020 also occurred on February 4.

Between October 1, 2019, the start of the water year, and February 29, 2020, there were 15 snow-covered days when averaged over elevations greater than 1200 meters (3,900 feet) in the Western United States (Figure 1b). Over that domain, there were 10.5 snow-covered days at the start of February 2020, 0.6 fewer days than the 2001 to 2019 average at the start of February. By the end of February 2020, there were 1.9 fewer snow-covered days than the 2001 to 2019 average. From the start to the end of February, the current year had a smaller increase in snow-covered days, relative to more average years (Figure 1b).

Figure 1a. The solid black line represents the total snow-covered area across the Western United States during February 2020, with February 28 circled at the end. The grey area shows the interquartile range where 50 percent of the historical snow-covered area from 2001 to present falls. The dashed line in the middle of the gray area shows the historical median. 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.

Credits
K. Rittger, Institute of Arctic and Alpine Research (INSTAAR)

Figure 1b. The solid black line represents the total snow-covered days across the Western United States for elevations higher than 1,200 meters (3,900 feet) during February 2020, with February 28 circled on the end. 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 grey area shows the historical median. Note that snow-covered days are calculated from snow-covered area data. Therefore, the dotted line at the bottom depicts 2015, the lowest year on record for snow-covered area; while the dotted line at the top depicts 2008, the highest year on record.

Credits
K. Rittger, Institute of Arctic and Alpine Research (INSTAAR)
Conditions in context

Snow cover in the Western US peaked in early February 2020 and has been generally declining since then (Figure 1a). In the West, snow-covered area typically peaks in early February, but the timing of the peak varies annually between early January and early March depending on snow accumulation amounts and temperatures. Analysis of the last 20 years shows the timing of maximum snow cover in 2020, as of this post, fell almost exactly on the average date (Figure 2).

Figure 2. This graph shows the date of maximum snow cover over the Western United States for the 20-year satellite record. The dashed line indicates the average date for the maximum. The red dot shows the maximum snow cover date for the 2019 to 2020 snow cover year.

Credits
K. Rittger, Institute of Arctic and Alpine Research (INSTAAR) & M. Raleigh, National Snow and Ice Data Center (NSIDC)

Comparing snow cover at the end of February 2020 to the average for 2001 to 2019, less snow cover topped the lower elevation areas surrounding major mountain ranges like the Sierra Nevada and Cascades. Less continuous snow cover was also present in areas like Arizona, western Montana, and northern Idaho (Figure 3). The lack of snow cover in many of these regions resulted in 2020 being the sixth lowest February snow cover in the 20-year satellite record for the Western United States (Table 1). Although the storm frequency was comparable to previous Februaries in areas like California, northern Montana, and Idaho, the storms were not as intense, notably in California. Throughout February, the drought in California intensified and expanded north in the state. However, frequent storms in Utah, Colorado, Wyoming, and southern Montana led to above average snow cover and Snow Water Equivalent (SWE) conditions in those states. Among these states, Wyoming ended February with notably higher snow cover than average (Figure 3).

Figure 3. These maps show snow conditions for February 28 across the Western United States. The upper left map shows the percent of snow cover for February 28, 2020. The upper right map shows the average percent of snow cover for February 28 over the 2001 to 2019 record. The bottom map shows the difference between 2020 and the 2001 to 2019 average. The darker the blue the more snow cover in 2020 than the 2001 to 2019 record; the darker the red the less snow cover in 2020 than the 2001 to 2019 record.

Credits
K. Rittger, Institute of Arctic and Alpine Research (INSTAAR)

In contrast to the timing of maximum snow cover in early February, SWE tends to reach its maximum value at mountain sites between March and May in the Western United States. However, SWE patterns in February hint at a preview of what the snowpack may look like during the time of typical maximum accumulation, unless there are major storms in the coming weeks. February 2020 started with slightly above average SWE conditions in nearly all states except California (Figure 4, left). That trend prevailed through the end of February with most regions remaining at or above average (Figure 4, right). California’s drought spread northward through the Sierra Nevada and into parts of the southern Oregon Cascades, with portions of the most southern mountains in Idaho showing slightly drier conditions as well. Sites in central Arizona started at near average conditions at the beginning of February but ended the month at 50 percent or below average. Colorado fared slightly better than Utah across the month as did eastern Oregon and northern Wyoming.

Figure 4. These maps show Snow Water Equivalent (SWE) at monitoring sites at the beginning and end of February 2020. SWE is expressed as percent of average conditions for the beginning and end of February at each site, with warmer colors indicating below average SWE, or less water, and cooler colors indicating above average SWE, or more water. The green shading delineates mountainous areas as represented in Environmental Protection Agency data.

Credits
M. Raleigh, National Snow and Ice Data Center (NSIDC)

The winter snowpack continued to build in many areas with SWE in February increasing over 5 inches at many stations (Figure 5). The Pacific Northwest, northern Colorado and Utah, western Wyoming, and the Idaho-Montana border had several stations with large increases in SWE. Lower elevation stations in California had a loss of SWE as much as 2 inches, because of the combination of melt and little new accumulation. Higher elevation stations in California gained only an inch of water.

Figure 5. This map shows the change in Snow Water Equivalent (SWE) in inches during February. There were larger increases in the Pacific Northwest compared to the southern mountain ranges in California, Utah, southern Colorado, New Mexico, and Arizona. The green shading delineates mountainous areas as represented in Environmental Protection Agency (EPA) data.

To understand the changes in SWE through February the following were analyzed: the number of snowstorms, the average amount of water delivered across the storms, and the amount of snowmelt. A higher than average number of February storms (Figure 6, left) brought snow cover back to slightly within 50 percent of the historical average (Figure 1). However, the storms delivered less water on average (Figure 6, right) in most of California, northern Idaho, western Montana, and parts of the Pacific Northwest.

Figure 6. The left map shows the number of snowstorms delivering greater than 0.5 inches of water in February 2020. Many locations had more snowstorms in February 2020 than an average February. The right map shows the water delivery of February snowstorms, comparing 2020 snow totals to average snow totals in February. The green shading delineates mountainous areas as represented in Environmental Protection Agency (EPA) data.

Credits
M. Raleigh, National Snow and Ice Data Center (NSIDC)

In February, some melt occurred in all regions of the Western US (Figure 7) with the highest melt relative to average in California, the Pacific Northwest, New Mexico, and Arizona. Locations in the Rocky Mountain states (Colorado, Utah, Wyoming, Idaho, and Montana) had the least snowmelt relative to the average. In some areas like California, the melt occurred in tandem with weak snowstorms (Figure 6, right) to deliver low SWE conditions. In other areas like central Arizona, the melt (Figure 7) appears to have been greater than the gains from new snowstorms, leading to the drastic change from near-average SWE to very low SWE. Changes in SWE (Figure 5) in the Pacific Northwest were buffered from elevated melt rates (Figure 7) by larger snowfall amounts from new storms (Figure 4).

Figure 7. This map compares total snowmelt (inches of water) in February 2020 versus the average. Warmer color dots indicate more snowmelt in February 2020 than in an average February, and cooler colors indicate less snowmelt in February 2020 than in an average February. Here snowmelt is defined as reductions in measured Snow Water Equivalent (SWE), so other losses—such as evaporation—are included in the total snowmelt. The green shading delineates mountainous areas as represented in Environmental Protection Agency (EPA) data.

Credits
M. Raleigh, National Snow and Ice Data Center
NASA SnowEx 2020 update: Snow water equivalent and snow-covered area

The relationship between snow cover and SWE has long been a topic of interest and research to help estimate the water stored in snow. Relationships between snow cover and SWE can be used to understand regional or basin wide patterns and averages but are not always transferable and do not provide information on the spatial distribution of SWE, a goal of the NASA SnowEx campaign. This month’s SnowEx update examines the variability of snow cover in regions around each SnowEx study site (Figure 8) and compares historical snow cover and SWE (Figure 9).

Many of the SnowEx sites are in regions that are consistently snow covered for the entire snow season. Satellite data for the immediate region (Figure 8, left column) surrounding each study site at 2 square kilometers (.77 square mile) show little variability because of this site characteristic. For example, the Lakes and San Joaquin Basins, American River Basin, and Sagehen Creek—all located in California—show little sign of the drought in the Sierra Nevada that is clear in the regional analysis (Figure 3 and Figure 4). By analyzing snow cover from the surrounding region (Figure 8, right column) including the nearest 360 square kilometers (139 square miles), snow cover across different elevations and topographic characteristics are included, and the variability between years becomes clearer as does the regional drought in California. These data can provide insights into areas with below average snow, like in California, but is less informative in area with above average snow, like in Niwot Ridge and Fraser Forest in Colorado. This is because snow cover surrounding those areas is already at or near the maximum coverage (Figure 8) for both the immediate and surrounding regions.

Figure 8: These graphs show snow cover as a percentage around the immediate area (left column) and surrounding area (right column) for five NASA SnowEx study sites. In the left column, the solid black line represents the total snow-covered area in 2 square kilometers (.77 square mile) regions corresponding to the center of each site. In the right column, the solid black line represents the total snow-covered area in a 360 square kilometers (139 square miles) region around each site. The grey area shows the interquartile range where 50 percent of the historical snow-covered area from 2001 to present falls. The dashed line in the middle of the gray area shows the historical median. The dotted line at the bottom depicts 2015, the year of the lowest snow-covered area; the dotted line at the top depicts 2008, the year with the highest snow-covered area. The variability between years is much greater for the surrounding region than the immediate area around each SnowEx study site.

Credits
K. Rittger, Institute of Arctic and Alpine Research (INSTAAR)
Snow water equivalent analysis

SWE at SnowEx sites in California went from approximately 15 percent below average in January (see previous post) to over 30 percent below average in February (Figure 9a). The Idaho sites had SWE shifting from slightly above average in January to slightly below average in February. The five Colorado sites had a mix of above and below average SWE in January and February. For example, Senator Beck went from well above average to below while Niwot went from above average to higher. Cameron Pass and Grand Mesa lost SWE as did Jemez River.

The snow cover differences from average in the surrounding (Figure 8, right column) regions around each SnowEx study site appears to be somewhat indicative of SWE (Figure 9a). The regions immediately coincident with the sites (Figure 8, left column) did not show similar relationships. In general, the snow cover seems to be well correlated, but Little Cottonwood and Jemez River are exceptions.

Figure 9a. This chart shows Snow Water Equivalent (SWE) conditions at thirteen NASA SnowEx study areas at the end of February 2020. SWE is expressed as percent of the long-term average value at nearby snow pillow sites from the full National Resources Conservation Service (NRCS) Snow Telemeter (SNOTEL) and California Department of Water Resources (DWR) snow pillow networks, weighted by proximity to each study area. Common colors denote different states, from left to right California, Idaho, Utah, Colorado, and New Mexico.

Credits
M. Raleigh, National Snow and Ice Data Center (NSIDC)

Figure 9b. This chart shows snow cover conditions in 360 square kilometer regions around NASA SnowEx study sites at the end of February 2020. Snow cover is expressed as a percent of the long-term average value from the 20-year satellite record.

Credits
K. Rittger, Institute of Arctic and Alpine Research (INSTAAR) & M. Raleigh, National Snow and Ice Data Center (NSIDC)

Snow Today will continue to provide updates on snow conditions at the SnowEx sites as the 2020 winter evolves. While it is unlikely that snow cover will increase, higher values of SWE may occur especially in high elevations. Measurements are planned until early May. Knowing both the historical average and current conditions provides useful context for scientists collecting and analyzing data in these intensive field campaigns.

References

Rittger, K., M. S. Raleigh, J. Dozier, A. F. Hill, J. A. Lutz, and T. H. Painter. 2019. Canopy Adjustment and Improved Cloud Detection for Remotely Sensed Snow Cover Mapping. Water Resources Research 24 August 2019. doi:10.1029/2019WR024914.

Related references

Painter, T. H., K. Rittger, C. McKenzie, P. Slaughter, R. E. Davis, and J. Dozier. 2009. Retrieval of subpixel snow covered area, grain size, and albedo from MODIS. Remote Sensing of Environment 113(4): 868–879. doi.org:10.1016/j.rse.2009.01.001.

Raleigh, M. S., K. Rittger, C. E. Moore, B. Henn, J. A. Lutz, and J. D. Lundquist. 2013. Ground-based testing of MODIS fractional snow cover in subalpine meadows and forests of the Sierra Nevada. Remote Sensing of Environment 128: 44–57. doi:10.1016/j.rse.2012.09.016.

Dozier, J., T. H. Painter, K. Rittger, and J. Frew. 2008. Time–space continuity of daily maps of fractional snow cover and albedo from MODIS. Advances in Water Resources 31(11): 1515–1526. doi:10.1016/j.advwatres.2008.08.011.

Acknowledgements

Moderate Resolution Imaging Spectroradiometer (MODIS) data: The MODIS Snow Covered Area and Grain-size (MODSCAG) data are provided by Snow Data System (SnowDS) at the Jet Propulsion Laboratory (JPL). The reflectance data are from NASA MODIS.

Snow station data: Snow station data are sourced from the Snow Telemetry (SNOTEL) network by the Natural Resources Conservation Service (NRCS), United States Department of Agriculture (USDA) and the California Department of Water Resources. The data are publicly accessed at www.wcc.nrcs.usda.gov/snow.

Computing: This work utilizes resources from the University of Colorado Boulder Research Computing Group, which is supported by the National Science Foundation (awards ACI-1532235 and ACI-1532236), the University of Colorado Boulder, and Colorado State University.