late snow

Today was a spectacular early spring day with temperatures reaching the lower 60s just about everywhere east of the Coast Range

Todays Observed Highs OrWa 2017

This is due a weak and “splitting” jet stream.  Most of the action is sliding by to our north

Mark Jet Stream

Now that inversion season is over, we don’t get significant fog stuck in the valleys either.  This same setup with weak onshore flow might have left us with all-day long fog or low clouds in December/January.

Models are in very good agreement that the mild ridging over us will move farther offshore as a new ridge develops over the Gulf of Alaska this weekend.  By Saturday it looks like this…

Mark Jet Stream2

This is a classic setup for a snow “close-call” in the lowlands.  A cold upper-level trough digs south out of western Canada, and then cold north or northeast wind at the surface follows as the upper-low moves south of us.   If it was winter (Thanksgiving to late February) I’d be thinking we have a decent chance of getting sticking snow out of this, but in mid-March everything has to be EXACTLY RIGHT (or wrong) to get sticking snow near/at sea-level.

Right now it appears a surface low will “spin up” along the Washington coastline Friday, sending rain inland along with an increasing southerly breeze.  That means cold rain Friday with high temps 45-50 degrees.

Then on Saturday models generally agree that surface low is either right over us or shifting slow south offshore.  That potentially allows colder arctic air to slip south into the Pacific Northwest.  That would be on a north or easterly wind (through the Gorge).  The regularly awful (with cold weather) GFS model thinks the airmass will be so cold it stays in the 20s in the central/eastern Gorge all day Saturday.  That’s not going to happen.  More likely 10-15 degrees warmer than that as ECMWF and GEM models show. Our 7 day forecast numbers here:

Data Driven Forecast Highs Next 7 Day Meteogram

Precipitation intensity Saturday plus how much cold air (if any) arrives will determine our snow chances.  If we get cold easterly flow before precipitation runs out, then we could easily see snow in the air just about anywhere later Saturday or Saturday night.  During the daytime Saturday we’ll be up around 42-48 degrees.  That’s a good 10 degrees below average for mid-March.  It seems prudent to put those “mixed showers” in the forecast for Saturday so we did.  I should have a much better idea by Thursday, two days ahead of time.

By the way, we’ve only seen measurable snow in March (in Portland) 5 times in the past 50 years!  We saw 0.5″ last year and in 2012.  That March 22, 2012 event is the latest on record at the airport/NWS office.  Those records go back 80 years.

Portland Snow Latest 22

There was one snowfall in the old downtown record books on April 1, 1935.

Portland Snow Latest 2

All models forecast that cold upper-level trough to move south quickly Sunday.  That puts us back into lots of sunshine, a cold east wind, and likely dry later Sunday and Monday.  Then as high pressure gets closer temperatures jump back into the 50s, if not Monday then Tuesday.

That’s the real story; our unusually dry February/March will continue.  The ECMWF and GEM models both show only about 1/2″ rain in the next 10 days!  Just the ECMWF is shown here

ecmwf-deterministic-oregon-total_precip_inch-4705600

Chief Meteorologist Mark Nelsen

Recommended for you

Load comments