The record breaking temperatures of the past few days and warmer than normal temperatures of this month have resulted in substantial melt of the formidable snowpack we started with on April 1. And the rivers are responding.
To illustrate the snowmelt, here is the plot of snow water content at Stampede Pass at 4000 ft on the eastern side of the WA Cascades (normal snowpack is the light blue). We were near normal around April 1, but have lost about 20% since then.
This massive melt of water is causing a number of rivers on the eastern slope of the Cascades to approach flood stage, with predicted floods on some. To illustrate, here are the hydrographs, showing the observed and predicted river levels and discharge for the Okanogan, Stehekin, and Naches rivers--all are predicted to flood during the next few days.
Spring flooding from snowmelt is not unusual, but it was generally absent last year because the snowpack was so poor.
During the next day, we are about to transition to a far cooler, wetter pattern as the ridge of high pressure weakens and moves westward. The upper flow pattern changes substantially. Here is the 500 hPa (18,000 ft) upper level map for 5 PM today (Wednesday). Big ridge centered over the Rockies, with a weak disturbance (trough) over Oregon--which has brought some thunderstorms to western Oregon and SW Washington.
72 hours from now (5 PM Saturday), the ridge has weakened and westerly/southwesterly flow has invaded the West Coast.
As a result, over the next 72 h precipitation will return: light over WA, but substantial to the south over Oregon and CA (see below). Such precipitation is unusual for CA this late in the season and very welcome.
Want to be impressed?
The forecast total precipitation over the next two weeks by the GFS model is quite wet for the Northwest (see below), with some areas getting 4 inches or more. Temperatures will return to normal. I think folks are ready for some normal weather again.
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