Taste of Summer in Early Spring

The last few days have been the warmest since October, with Seattle Tacoma Airport reaching 70F twice (see image below).  We are talking about being 15F above normal!

Let's compare this to the same 4 week period last year.   Generally similar, although last year had more days above normal.

And during the last few days we have been seeing the classic summertime low clouds on the coast, with a minor marine push this morning.  Enough to bring some light drizzle to the early visitors to the new 520 bridge.  Friday morning was the first try for the marine air, as shown by the image at 9:30 AM, with tendrils through the Strait and the Chehalis Gap south of the Olympics.

 By 2 PM, the shallow low clouds had retreated to the the coast.  Still plenty of snow in the mountains.

Last night and this morning, an upper level trough moved towards the coast (see 500 hPa, roughly 18,000 ft at 11 PM Friday).

This weak trough, and its associate upward motion, deepened the low level marine layer and enhanced the onshore pressure gradient.  The result:  more persistent clouds and some very light drizzle.  Here is the visible satellite picture at 9:15 AM Saturday--Puget Sound was covered.

 At 2:30 PM most of the low clouds had burned off, with a remnant left to the NE of the Olympics.  No wonder folks on the 520 bridge got a sunburn.

The dry conditions will end tomorrow night as a modest front moves to the coast during the late afternoon and into the interior around midnight (see forecast precipitation for the three hours ending 8 PM).

But don't be too depressed.  ANOTHER strong ridge will develop midweek with more temperatures near 70F.