Snow is the West's reservoir. El Niño moves it south, and not reliably.
Most of the West's summer water falls as winter snow. The April 1 snow water equivalent (SWE: the depth of water the snowpack would make if it melted) is the number water managers plan around. The textbook strong El Niño leaves the northern tier short and melting early, and piles snow on the southern Rockies. This route counts how often that has actually happened at the West's long-record snow sites, and how California's big water years line up with the strong events.
April 1 snowpack in strong El Niño winters
SNOTEL and snow courses · 30+ years of recordEach site's April 1 snowpack in strong El Niño winters
Median percent of each site's own 1991–2020 April 1 median. Not detrended: the regional counts below it are.
For a water manager
Snowpack is memory: a low northern snowpack cuts summer runoff 6 to 9 months later, and reservoirs carry deficits for years. One wet winter refills only part of a multi-year La Niña drought. How much each county still owes since 2020 is on Drought and pluvial debt; what tends to follow a strong El Niño (often La Niña, and the dry southern tier that comes with it) is on The year after; each big event's own card is on Event cards.
How it is made: NRCS AWDB first-of-month snow water equivalent for every SNOTEL site and snow course in the eleven western states, keeping sites with at least 30 April 1 values since 1950 and 20 in 1991–2020 and a normal of at least one inch. Each winter's regional value is an NRCS-style basin index, total April 1 snow water over the total of the same sites' 1991–2020 medians (a site that did not report is left out, never counted as zero), with a straight-line trend removed (fitted on ENSO-neutral winters). Early melt is the share of the April 1 snow water still on the ground on May 1, summed over SNOTEL sites that reported both days (1980 on). California's indices are DWR's station averages, fetched from CDEC.
By region and state
Select a row to chart it below. "Strong-winter index" is the region's basin index (total April 1 snow water as a percent of the same sites' 1991–2020 medians), the median over the strong El Niño winters. The last column is April 1 this year, with the number of sites reporting and how many of them were bare. Thirds are of the detrended 1991–2020 winters; expect 3 of 9 in each by chance. *Share of April 1 snow still there on May 1 at SNOTEL sites, strong vs neutral winters.