The Big Meltdown Followed By A Cool Down

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.

The Warmest April Day in Seattle History

We broke major records today.

The warmest April day on record at Seattle (89F).  The previous record was 85F.

The warmest April day in record at Olympia (88F).  Previous record was 87F.

The warmest April day on record at Bellingham (83F).  Previous record was 78F.

We tied the record number of days hitting 80F or more at Seattle (two).  We will beat that record tomorrow.  It was also the warmest day since last August.

A number of local observing sites got to 90F or above.  I mean a LOT of locations--here is the map:

Not impressed yet?   The temperature at 850 hPa (around 5000 ft) at Quillayute, on the WA coast, was an amazing 16.2C.  (see plot, with light red line showing the daily record, and blue dot showing today's temperature).

It was the second warmest temperature at that level ever observed during the first four months of the year.  The warmest?  Earlier this month, April 7th, with 16.6C.

It was so warm that we started getting weird mirage effects because of superheated air over cold water.  Here is a picture sent to me by Tony Nahra, showing a marvelous superior mirage (lofting of objects on the opposite shore). The only other alternative explanation is a new Amazon complex.  I like the mirage better.

Why so warm?  Big ridge over the western U.S. aloft.

At 850 hPa (about 5000 ft), there was southerly flow with enough of an easterly component to give some offshore flow that reduced the marine influence.

And weak pressure variations at the surface (so no cool northerlies like on April 7th).  The meteorological planets were aligned for heat.