Beware of Crazy Snowpack Percentages!
Many folks are interested in our mountain snowpack, from hikers and agricultural interests to those concerned about wildfires.
Well, the latest snow water map from the USDA SNOTEL network is an eye opener and a study of contrasts (see below). For the Olympics, we are now at 283% of normal, for the fire-prone Okanogan highlands of northeast Washington at 265%, and the Blue Mountains southeast of Walla Walla is around 190%. The western side of the Cascades is very close to normal. In contrast, east of the central to southern Cascades, snowpack is below normal, ranging from 58 to 35%.
How can we have such huge variation over one state? Is this believable?
One has to be careful about interpreting such snow percentiles of normal. On April 1st, the day water resource agencies like to use as a benchmark, the snowpack around the state was very close to normal. But then we had a warm spell in April and stronger than normal westerly winds that resulted in a faster-than-normal melt in some locations (such as the eastern slopes of the Cascades).
During the season of snowpack melt off, small differences can produce wacky percentages of normal, either above or below normal.
Take Waterhole, a site in the Olympic Mountains where snowpack is nearly 300% of normal today (blue is this year and red is normal for snow water equivalent or SWE). That location was above normal in March and early April and then hit the normal rapid decline in May. Right now, the amount is above normal by a huge percentage....but there is hardly any snow left (and will be gone in a few days). The huge percentage above normal is really meaningless.
Now consider a site on the eastern slopes of the central Cascade (Morse Lake), where the percentile is very low (around 20%). Again, we started out with a normal snowpack, but the warm weather caused a quicker than normal melt-off (by a week or so.) The melt-out is a few days early, but you would think the end of the world is here because of the advance of a few days.
And even with an earlier melt-off, in the Yakima drainage much of the water was saved in the reservoir system, ready to aide agriculture and fish passage (see plot below). In the plot you can see the rapid rise of the stored water (blue line) in April and the fact the water level is now above normal (red line). Last year is the green line.
At Stevens Pass, the melt-off was June 5th, 3 days later than normal (which means more snow than normal) and the view today at Paradise on Mount Rainier still shows plenty of snow (see below)
The bottom line is that we need to be careful when interpreting snowpack numbers in late spring and early summer, since differences that are meaningless can produce HUGE percentile anomalies from normal. And as we go into this summer, the snowpack and water situation is quite reasonable and not far off from normal. Agriculture will have its water and one should not expect unusual wildfire activity. Something not to worry about.
Well, the latest snow water map from the USDA SNOTEL network is an eye opener and a study of contrasts (see below). For the Olympics, we are now at 283% of normal, for the fire-prone Okanogan highlands of northeast Washington at 265%, and the Blue Mountains southeast of Walla Walla is around 190%. The western side of the Cascades is very close to normal. In contrast, east of the central to southern Cascades, snowpack is below normal, ranging from 58 to 35%.
One has to be careful about interpreting such snow percentiles of normal. On April 1st, the day water resource agencies like to use as a benchmark, the snowpack around the state was very close to normal. But then we had a warm spell in April and stronger than normal westerly winds that resulted in a faster-than-normal melt in some locations (such as the eastern slopes of the Cascades).
During the season of snowpack melt off, small differences can produce wacky percentages of normal, either above or below normal.
Take Waterhole, a site in the Olympic Mountains where snowpack is nearly 300% of normal today (blue is this year and red is normal for snow water equivalent or SWE). That location was above normal in March and early April and then hit the normal rapid decline in May. Right now, the amount is above normal by a huge percentage....but there is hardly any snow left (and will be gone in a few days). The huge percentage above normal is really meaningless.
Now consider a site on the eastern slopes of the central Cascade (Morse Lake), where the percentile is very low (around 20%). Again, we started out with a normal snowpack, but the warm weather caused a quicker than normal melt-off (by a week or so.) The melt-out is a few days early, but you would think the end of the world is here because of the advance of a few days.
And even with an earlier melt-off, in the Yakima drainage much of the water was saved in the reservoir system, ready to aide agriculture and fish passage (see plot below). In the plot you can see the rapid rise of the stored water (blue line) in April and the fact the water level is now above normal (red line). Last year is the green line.
At Stevens Pass, the melt-off was June 5th, 3 days later than normal (which means more snow than normal) and the view today at Paradise on Mount Rainier still shows plenty of snow (see below)
The bottom line is that we need to be careful when interpreting snowpack numbers in late spring and early summer, since differences that are meaningless can produce HUGE percentile anomalies from normal. And as we go into this summer, the snowpack and water situation is quite reasonable and not far off from normal. Agriculture will have its water and one should not expect unusual wildfire activity. Something not to worry about.
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