Snow to Sea Level?

September 11, 2006

Hah, got your attention!  It’s a slow sunny afternoon (but very nice) in Oregon, so I figured we’d look forward to winter; at least the stormy/snowy/cold part, not the constant rain.
As discussed in GREAT detail in the previous post’s comments, weather maps suddenly changed over the weekend.  We have a strong upper level ridge right over Oregon this afternoon.  500mb heights are near 590 dm, which is classic summertime sunshine and little/no marine layer along the coastline.
But over the next 60 hours, the ridge suddenly retrogresses, leaving it out near 145-150W longitude by Thursday morning.  That opens the back door wide for several strong disturbances to drop south out of the Yukon and Northwest Territories of Canada, carving out quite a strong trough of low pressure directly overhead.  Sound familiar?  We would be approaching snow/cold freak-out stage if it was anywhere between November 15th-March 5th or so.  The constant live coverage would be ready to start at a moments notice. 
But it’s not winter, so no snow to sea level and no WINTER BLAST coverage.  Instead a sudden downturn to Octobery weather for at least a few days.  No change tomorrow, except for a strong marine push by evening.  Wednesday is the transition day with strong westerly flow developing above and marine air at least 5000′ thick overhead.  Then Thursday we get a cool & unstable airmass overhead for scattered showers.  This pattern is not really conducive to a prolonged period of steady rain, unless the low sits right overhead and we get a cluster of showers together (that could happen).  Still cool/showery Friday, then maybe a break before a 2nd system comes through later Sunday or Monday.  That one is similar to what we’d see in winter when a warming/wetter system approaches from the west and wipes out the cold air.  Long range maps all over the place but the ECMWF and 12/18z GFS seem to point us back to warmer weather again, but below 80 degrees.
How low will the snow go?  Looks like 850mb temps drop to around 2 deg. Thursday-Friday AM.  500-1000mb thicknesses don’t drop below 545 dm. (5,000′ snow in winter).  So I doubt it’ll stick below 4,500′.  But Mt. Hood will sure be white once skies clear, maybe on Saturday?
The big picture says…enjoy the warm sun on Tuesday!  Mark