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]
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
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).
(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.