Snow Water Equivalent: To SWE, or not to SWE

Snow observations from the dense CoCoRaHS network provide forecasters and other users data and information that would otherwise not be available. The measurements of snow and related snow water equivalent measurements are optional but extremely beneficial. These extra observations take more time and often present challenges that you won't encounter just measuring rain.

There seems to be a good deal of misunderstanding about what exactly constitutes 24-hour Snow Water Equivalent (SWE) and how it is measured. The definition is included in our Glossary of Terms on the CoCoRaHS web site.

The 24-hr Snow Water Equivalent (SWE) is NOT your Gauge Catch, even if the precipitation is all snow. Many observers melt what is in the rain gauge, report that correctly as their Gauge Catch, and then incorrectly copy that value into the 24-hour Snowfall SWE field.

"If all of the precipitation was snow, aren't the gauge catch and 24-hour SWE the same?"

Maybe.

The amount melted in the gauge and the 24-hour Snowfall SWE are two distinct and separate measurements and often not the same value. They might be the same, especially in a light snowfall with little or no wind. An entry should be made in the 24-hour Snowfall SWE field only if the observer takes a snow core from the board or other flat surface, melts the core and measures it. This is the value that is entered for 24-hour SWE.

Why do we do a separate measurement of the snow water equivalent? The 4-inch rain gauge is not a great snow collector. Depending on conditions during a snow event, especially wind, it may catch too much or too little of the snow. If done correctly, the SWE measurement is usually more representative of the amount of water in the snow that fell.

If you do not take and measure a core of snow (and it is optional), then the 24-hour SWE value should be left as NA.

Here is a graphic that demonstrates how to take a core of snow for a 24-hour SWE measurement.

The 24-hour SWE measurement should be done at the same time as you measure your 24-hour New Snowfall, which should be when the snow ends and before any settling or melting occurs. It often will not be at your regular observation time. For example, if the snow ends late in the afternoon you should measure the snow (and SWE) then and report it at your next regular observation time the following morning.

We have just updated our Winter Weather Measurements training slide program which explains the procedures for measuring new snowfall, new snowfall SWE, snowpack, and snowpack SWE in more detail. If you are interested in making these important winter measurements please take time to review this training. You can view the training program by clicking on the image below.