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Snow Drought

What is 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)

Current situation and impacts in the West

April 12, 2018: Similar to April 1, snowpack is generally below average in the southern Cascades, Sierra Nevada, and the Colorado River Basin while above average in the northern Cascades with the northern Rockies experiencing well above average snowpack. Snowpack is also below 1981-2010 medians in south and southeastern Alaska. [Next update scheduled for April 26, 2018]

A map of the United States, focusing in on the Western US, shows the percent of period of record (POR) median snow water equivalent (SWE) based on snow course and SNOTEL site data from the United States Department of Agriculture Natural Resources Conservation Service. The SWE percent of POR median values range from <0 to >200%. Most of the California Sierra Nevada mountains are below to normal, generally ranging from <0% to 100% of median. The same is true for the southern Cascade Range and Blue Mountains in Oregon with just above median in the Washington Cascades. The northern Rockies through Wyoming and Montana have above median SWE, ranging from 100 to >200%. Meanwhile the Colorado River Basin and the southern Rockies are below median, ranging from <0% to 100%. Finally, much of the Northern Plains through Montana and North Dakota are well above average with the exception of northeast Montana and western North Dakota. A map of the United States, focusing in on Alaska, shows the percent of period of record (POR) median snow water equivalent (SWE) based on snow course and SNOTEL site data from the United States Department of Agriculture Natural Resources Conservation Service. The SWE percent of POR median values range from <0 to >200%. The SWE percentage values range from <25% to >200%. Most of central Alaska is above median snowpack, with values ranging from 150 to >200%. Parts of eastern alaska are more near normal. Snowpack is below median values in southern and southeastern Nevada, reaching as low as 25-75%.

USDA Natural Resources Conservation Service (NRCS) percent of period of record median snow water equivalent (SWE) over the western U.S. (left/top) and Alaska (right/bottom) for April 11, 2018. Source: NRCS

A map of the United States, focusing in on the Western US, shows the SNOw Data Assimilation System (SNODAS) percent of average (2004-2018) snow water equivalent (SWE) for April 9, 2018. The SWE percent of average is displayed using a ranging from 0 to 800%. Most of the California Sierra Nevada mountains are below normal generally ranging from 25-90% of normal. The same is true for the southern Cascade Range and Blue Mountains in Oregon with just above average in the Washington Cascades. The northern Rockies through Wyoming and Montana have above average SWE, in some places as high as 400%-800% of normal. Meanwhile the Colorado River Basin and the southern Rockies are below average. Finally, much of the Northern Plains through Montana and North Dakota are well above average with the exception of northeast Montana and western North Dakota.

SNOw Data Assimilation System (SNODAS) percent of average (2004-2018) snow water equivalent (SWE) for April 9, 2018. Image made using Climate Engine

Snowpack is still below average in the Sierra Nevada. A strong atmospheric river occurred the first couple days of April in the Sierra with very high snow levels that led to decreases in the snowpack despite heavy precipitation. Snowpack remains well above average in the northern Rockies. On April 9, SWE in the Belly Basin (northern Montana) was the second highest in the SNODAS record (2003-2018).

Two graphs which display daily SNOw Data Assimilation System (SNODAS) percent of average (2004-2018) snow water equivalent (SWE) appear side by side; the Lake Tahoe Basin in northern California and northwestern Nevada appears on the left and the Belly Basin in northern Montana on the right. On the x-axis is the water year from Oct 1, 2017 to Oct 2018 and on the y-axis is daily SWE in millimeters. A black line on each graphs shows the 2003-2018 average and a blue line shows the accumulation of SWE for this water year up to April 9, 2018. For the Lake Tahoe Basin in the northern Sierras, SWE has been well below normal with rapid decrease following April storms. In the Belly Basin and the northern Rockies snowpack is near record levels.

(Left) SNODAS basin-wide average SWE time series for the Lake Tahoe Basin (northern California/northwestern Nevada) and (right) for the Belly Basin (northern Montana) through April 9, 2018. Blue line shows this year, black line is the 2003-2018 average, and grey shading denotes different percentile ranges. 


Snow Drought Tools