Cold Night Ahead

December 7, 2009

We haven't been below 18 degrees in 10 years at PDX. It MIGHT happen tonight, IF the wind dies and IF clouds stay away.

A very chilly day.  A went from Corbett to Cornelius visiting a school, so I feel like I saw the whole metro area.  It was not quite as cold as I had hoped, but an official high of 31 degrees.  With such low dewpoints and decreasing wind (should go calm at PDX later), we have the potential to drop into the teens tonight even here in the city.  Of course something always has to screw it up right?  Check out the clouds approaching from the north on the satellite looper.  This is from a shortwave moving south through British Columbia.  Most of the energy should stay offshore, so we’re keeping the forecast dry.  Our RPM model shows cloud cover increasing towards morning, but other models do not.  Hopefully they are right.  We’ll also hope our model is wrong for precipitation too.  It’s 12z run showed snow in a line from Astoria to Eugene (heading SE).  But the 18z kept more energy offshore like other models.

I hate to jump on this too quickly, it’s only Monday, but it appears that models MAY be coming into some sort of agreement on moisture making it into the Pacific Northwest by Saturday.  For each of the last 3-4 days we have needed to move the onset of freezing rain or snow back one day as the northern stream keeps wanting to keep the southern jet to our south.  Of course we are in the “no-mans” land most of this week, but supposedly the southern stream should push farther north this weekend.

11pm Update:  I’ve looked at every single model to try and figure out what’s going to happen this weekend.  What a mess!  All models agree on the northerly flow interacting with the southerly subtropical jet, with the southerly one eventually taking over.  We are going to be right in the middle.  I don’t want to bring anyone down, but  I COULD see a setup where we get out of our arctic air with very little frozen precipitation.  A gradual warmup with cloud cover and weak onshore flow Friday through Sunday.  Depends on the positioning of a low offshore. 

Also, something I’m seeing on both the GFS and ECMWF is a strengthening northern stream oriented west-east to our north by Saturday/Sunday.  That gives us much lower pressure to the north and weak onshore flow…it’s all up in the air for now…

Chief Meteorologist Mark Nelsen