We’ve got massive snow accumulation in the western USA this year. It’s not just a few roads being late to open due to the excessive snow clearance effort (WA, CO), the snowpack is way above average this year. Good news for water supply; bad news for riverside communities. Just take a look at the extent of this – the map from 5th June below shows percent of normal snow-water equivalent. The measurements are mostly off the scale. From the lack of blue and green dots, I’m taking it that the red ones are errors.
From mid-May there has been concern over the amount of snow and the potential for flooding, but instead of melting rapidly the snowpack has persisted, and even continued to accumulate.
As of today (09 June) almost all states listed here (with the exception of Alaska, Arizona and New Mexico, which have much less than normal) are showing vast excesses of snow for the time of year. For example Utah:
And a different format depiction for Wyoming:
Below is yet another way of looking at it – here for the Upper Colorado (link: http://snowpack.water-data.com/uppercolorado/index.php), specifically the feed into Lake Powell. Now this really made me sit up. The levels were ticking along a bit above average until just after mid-April, then they began to rise, and rise, and rise. This says two possible things are happening – either more snow has been falling, and/or temperatures are just not rising enough to melt what is there. Either scenario says ‘cold’.
On 5th June:
“April 15th is the date of maximum snowpack and basinwide snowpack is currently 66.6% of the April 15th average
Snowpack is 277.9% of the June 5th average.”
This got me thinking – at what rate does snow melt? I mean we’ve got some truly gargantuan snow drifts in places – how likely is it that significant proportions of them will remain in places that have not retained snow in summer for years?
The last slide in this EPA presentation gives ranges for melt rate with a degree-day factor. The range seems to be 0.07-0.150 inches per day per degree F. So if we take the example that Anthony Watts posted recently of a 22ft drift in Colorado and look up appropriate degree day figures for the region (calculated between now and the end of September for a 32F base)*:
Then for the higher altitudes we have 1500-2600 degree days before we might expect reasonable additions if not accumulations of snow again, but we’re guessing at what the actual melt rate might be for any given location. So…here is a range:
[Update: The table above was produced on the assumption that the melt rates referred to depth of snow. Having covered a lot more background reading on this today I think I should have read the melt rate as "inches SWE/day*F". Current rates of melting from the NOAA summary table are 0.1-1.6 inches SWE/day. For a 22ft starting snowpack (estimated as 150 inches SWE) my back-of-envelope calculation suggests melt rates would need to be sustained at >1.25 inches SWE/day to remove this depth of snow by the end of September.]
That suggests to me that at higher altitudes there’ll be significant snow ‘left behind’ this year. Those white patches on the horizon will be a welcome return for many – cameras at the ready. It is just one year, but how many ‘just one years’ does it take?
*Coop, L. B. 2010. U. S. degree-day mapping calculator. Version 4.0. Oregon State University Integrated Plant Protection Center Web Site Publication E.10-04-1: http://uspest.org/cgi-bin/usmapmaker.pl