What is Snow-Water Equivalent (SWE)?
When it comes to how snow will benefit the river, looks can be deceiving. That's why we report snow-water equivalent (SWE), a measure of water contained within the snowpack, rather than the depth of the snowpack alone.
Does 120% of average precipitation and SWE now mean we're set for the winter?
Unfortunately, great numbers in early winter is no guarantee.
First, in the Henry's Fork Watershed, most snow falls between December and February. Precipitation early in the winter (Oct/Nov) doesn't always translate to a good water year, as temperatures can be too warm to accumulate snowpack.
Second, SWE numbers can change day-to-day. For example, in December 2022, SWE went from 146% of average one week to 126% the next, and was 119% of average two weeks later.
How does that work?
SWE values will continue to change daily as each new day brings, or doesn't bring additional snow. In early winter, daily precipitation averages around 0.2 inches and daily SWE accumulations average around 0.3-0.4 across the watershed. So, each day that doesn't receive that amount will see a decrease in those numbers we track relative to average.
It is very easy for early winter above-average precipitation and SWE accumulations to quickly fall back to or even below average without steady precipitation nearly every day, or heavy precipitation at least several days each week. For example, SWE could drop from 126% of average one day down to average (aka 100%), in about 8 days if we don’t get any new snow.
So, what is a good predictor of how much water we'll have for the season?
It is the amount of snow on the ground in April that determines how much water there is for the season.
The mean April-June temperature in the snow-accumulating areas of the watershed—roughly 6,000 feet in elevation and above—is a strong predictor of snowmelt timing as well.