Its that time of the year again: when mountain snowpacks turn the corner from accumulation to melt!
The early April snow survey and water supply forecast from the British Columbia River Forecast Centre showed snowpacks well above normal in the upper Fraser Basin. And as historical data from real-time monitoring networks show, once snow melt starts it tends to go rapidly.
This post examines the most recent snowpack data for BC and points to regions where the accumulation/melt corner has (or has not been turned). But first: a bit of background.
Background
From a river forecasting perspective, its not the depth of snow that’s important, its the amount of water contained in the snowpack. This depth of water is called the snow water equivalence, or SWE. SWE is a function of depth and snow density - while depth is relatively easy to measure automatically, density is not. So automatic measurements of SWE are made by snow pillows which are essentially giant antifreeze-filled bladders that measure the total weight of the snowpack.
I am using data compiled by the fine folks at the BC government, who make all this data accessible. Current year SWE (hourly): http://env.gov.bc.ca/wsd/data_searches/snow/asws/data/SW.csv… Historical SWE (daily): https://pub.data.gov.bc.ca/datasets/5e7acd31-b242-4f09-8a64-000af872d68f/daily_asp_archive.csv
Upper Fraser Basin
So to the current SWE situtation - snowpacks in the Upper Fraser River basin are definitely above normal. And most aren’t showing signs of turning the corner yet. Here’s the first plot, showing the data for the Yellowhead Station (elevation: 1860 m). The plot on the left shows the current year’s daily SWE observations (orange), along with the historical average for each day of the year (solid blue) and the 25th and 75th percentiles (dashed blue). And for the record I used a smoother on the mean values and percentiles to reduce some noise…
The plot on the right shows the daily change in SWE. Again, orange dots are the current year, and blue is the average change (based on the blue line shown on the left). Positive values show accumulation, and negative values show melt/runoff. Note the steepness of the curve: when mountain snowpacks start to go, they really go. But the Yellowhead station isn’t quite there yet - based on the historical observations though I’d expect it to happen in the next few weeks.
Some more stations below: McBride (1611 m), Barkerville (1520 m) and Revolution Creek (1690 m).
It looks like only the Barkerville station, the lowest elevation station of the four, has turned the corner so far this year. The snowpacks at all sites are way above normal for this time of year: and nearly double at Revolution Creek! With the clear skies and warm weather we’ve had this weekend I would guess we will start to see these stations turn the corner shortly. And while these sites represent a very limited sampling of a large area, its not a surprise that the professional river forecasters are concerned for the upcoming snowmelt flood season.
Future work will use time-series analysis to examine current and historical rates of snowpack depletion and the date of the turning point to see if there are any signs of climate change on these mountain snowpacks.
JMS
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Snow pillow locations in the Upper Fraser River Basin.