Why are the rivers so high? The main reason has been a massive snowmelt produced by a well above-normal snowpack and warmer than normal temperatures during the past week.
First, the snowpack. Take a look at the latest SNOTEL totals (percentage of normal). Much of the Northwest is well above normal---and that is after quite a bit of melt the last week.
And then there was temperature. Here are the temperatures for Spokane and Seattle (yellow lines) for the past two weeks (red and blue lines are normal highs and lows, respectively). There was a surge of well above normal temperatures about a week ago (into the 80s in Spokane!) that initiated a surge of melting. And temperatures have been above normal the last few days.
Or take a look at the deviation of the temperature from normal for the past week. Really warm for much of the Northwest and northern Rockies, which is consistent with large snowmelt.
The surge of melt water was quite obvious in the total storage of the Yakima Basin (see below)--really zoomed up starting with the warm spell.
Large snowmelt and surging rivers in Spring are not unusual in our region, and was a real problem for Portland and other riverfront cities before the hydro dams were put in.
Perhaps the region's greatest spring flood occurred June 5-6, 1894, when an above-normal snowpack was melted quickly by a heat waves, pushing the flood level to 34.4 feet in Portland, and destroying the town of Cascades, Oregon. Another great flood in 1948 destroyed the town of Vanport, Oregon. With a line of dams storing much of the spring snowmelt, flooding is thankfully only a minor nuisance along the Columbia these days.
from Cliff Mass Weather Blog http://ift.tt/2q7FthU
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