Spring heat waves drive record western United States snow melt in 2021


 Throughout the western U.S. snow melted at an alarming rate in April 2021 and by May 1, hydrologic conditions were severely degraded with declining summer water supply forecasts compared to earlier in the winter. The objectives of this study are to 1) quantify the magnitude and climatological context of observed melt rates of snow water equivalent (SWE) and 2) underpin the hydrometeorological drivers during April 2021 based on atmospheric reanalysis and gridded meteorological data. Peak SWE indicated snow drought conditions were widespread (41% of stations between 5th to 20th percentile) but not necessarily extreme (only 9% of stations less than 5th percentile) . Here, using observations from the Snow Telemetry (SNOTEL) network we found record 7-day snow melt rates (median of -99 mm; +/- one standard deviation of 61 mm) occurred at 24% of SNOTEL sites and in all 11 Western states. Strong upper atmospheric ridging that began initially in the north Pacific with eastward propagation by mid-April to the Pacific Northwest Coast led to near-surface conditions across the Western U.S. conducive to rapid snow loss. One heat wave occurred inland across the Rockies the first week of April and then later in April, a second heat wave impacted the Cascades and northern California. We find that ripening of the snowpack by both record high surface solar radiation and air temperatures were factors in driving the rapid snow melt. Equatorial Pacific sea surface temperatures and the La Nina pattern that peaked in winter along with an eastward propagating and intensifying Madden-Julian Oscillation were likely responsible for driving the placement, strength, and progression of the north Pacific Ridge. This study documents the role of an extreme spring “sunny heat wave” event on snowpack, and the cascading drought impacts which are anticipated to become more frequent in a warming world.