Thursday, February 23, 2012

Snow data II

Water Year 2012 Graph for PARADISE SNOTEL in Washington

Notice to Reader.-- This blog entry has been superseded and moved to a Web page on Mt. Rainier weather overview, weather data and snow data. Please update your bookmarks and links.

Original Post - Please use links above to new Web page.

Above is the graph for the precipitation and snow water equivalent of snow for the Paradise SNOTEL site operated by the NRCS. I wanted to show folks what these folks do and what data they provide.

There are different types of snow data which can be confusing to readers and where it's easy to misunderstand the numbers cited in the literature, magazines and newspapers. These are snowfall, snowpack and snow water equivalent.

The term snowfall is obvious. It's the snow that falls measured in inches. This where the NPS publishes the annual figures (through 2007 - PDF). It's the most often cited statistic about the snow at Mt. Rainier NP. The data is usually determined at a site or from a collector, usually read daily.

The term snowpack is also obvious. It's the snow on the ground, again measured in inches (depth). This data, shown for the Paradise site, is also from a site either with marked poles or pressure sensors. This number changes during the season from a variety of reason, including new snowfall, snowmelt to runoff, ablation, compaction and melting-refreezing from pressure.

This is often the snow collected at and transmitted from remote sites in the NRCS's SNOTEL network of sites. It's often the easiest to collect and transmit, but it also requires calibration to be useful for other purposes, namely the snow water equivalent.

This term isn't so obvious and is defined as the equivalent of inches of water for a specific snowpack. This is where they visit the remote sites and take snow samples from the depth to the soil and then measure the depth (snowpack) and weight (as water). This is then converted to an equivalent water to snow, in inches.

This number determines the density of the snow, meaning the number of inches of water per foot of snow (or the reverse for other calculations). Usually dry snow is about 2-4 inches of water per foot of snow. Wet snow is 6 or more inches per foot. Mt. Rainier normally get wet to very wet snow where the Rocky Mountains get dry to very dry snow.

This number determines the amount of runoff in water is in the snowpack. It's important for water managers when modelling and projecting spring snowmelt into reservoirs for reservoir management and for river basin water resources management. This work usually starts in January, the beginning of the permanent snowpack (meaning now sudden severe rain-on-snow events) and goes through the final snowmelt in June-July when it's gone from at or below about 6,000 feet elevation.

The graph has been updated and has the following paragraph

You can see the SWE for 2011 is above average for this year along with the annual precipitation. Occasionally you will see the trend of the two lines differ for awhile. This is often due to field surverys to get new snow depth and density data, and a recalibration of the telemetry data posted to the Web. Snow (field) surverys are usually monthly.

Anyway, that's the lesson for the day. And you can find these sites on the map of weather sites.

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