It May (or may not) Snow Thursday Night

December 3, 2013

2pm Tuesday…

Cold air is gradually working its way in this afternoon, it’s definitely not COLD.  High temps are occurring now in the lower 40s, not exactly an “arctic blast”.  It didn’t help that clouds hung in all night across most of the metro area, keeping temps way above freezing for most of us.  But hang on, we’re only feeling the edge of the cold air mass now.  Colder air will continue pouring in the next few days.  Here is the latest


  • Gradually getting colder the next 4 days.  Highs tomorrow mid-upper 30s, then 32-35 Thursday, then likely near/below freezing Friday-Sunday.
  • Low temps under clear skies and dry air drop into the 15-25 degree range tonight, then most of us in the teens tomorrow night.  Hope you winterized your pipes and irrigation systems.  I just blew out my drip irrigation for my garden and turned on the heat under the chicken water…can’t be having those ladies get thirsty.
  • Sunny skies again tomorrow, but clouding up the 2nd half of Thursday.
  • Latest models say the snow MAY stay just south of the Portland/Vancouver metro area Thursday night and Friday morning.  Snow looks very likely from Salem down to Eugene, but we will be at the far northern edge of the snow band.  Plenty cold, that’s not a problem, but models have shifted the moisture slightly farther south.  And we’re not being wishy-washy, but with any weather system any time of the year SOMEONE is always on the edge of the rain/no rain line right?  It looks like the metro area may be in that spot this time around.  So right now it’s possible we see nothing in the metro area Thursday night and Friday morning.  BUT if models are just 50 miles off (quite possible), we’d have an inch or two of snow in Portland during that time…stay tuned!
  • Cold, cold, cold.  There is a 2nd surge of cold air coming in through the Gorge that arrives Friday morning.  It’s pulled down from Eastern Washington as the weather system passes by to our south.  It’s going to be gusty and quite cold that day.  We haven’t had a day that chilly for a few years.
  • Likely just cold and clear Saturday and maybe Sunday too.
  • Big question mark on the return of moisture later Sunday through Tuesday.  Models are all over the place.  Our 7 Day forecast will probably change and look drier when we update later this afternoon.

For the hardcore weather fans

I know there was some whining in the comments about the cold air not moving in fast enough.  Very typical (the gradual airmass cooling).  I remember in 2009 or maybe it was 2008 when I expected a  Dec. 1990 or Feb. 1989 type arctic blast.  I think this is more the exception than the rule from what I’ve seen the past 10-15 years.  We’ll just gradually cool the next few days, although the surface low sliding down the coast Thursday night and Friday morning will dramatically strengthen the east wind through the Gorge.   That’ll keep the temps heading downhill.

I checked all the 12z and 18z models (no 18z GFS yet), and they are all in agreement with the surface low coming down the coastline exactly where the ECMWF has been showing it since yesterday morning.  Take a look at the 18z NAM, 12z GFS, and 12z ECMWF:


Pretty good agreement, and only 60 hours away.  Note that I picked the closest approach of the precipitation on each of these models.  It pulls away to the south afterwards.  I did notice the 18z NAM was much closer/wetter with precipitation than the 12z NAM.  Here’s what it looked like at the same time as the last map above:


Hmmm, then take a look at our 18z RPM model, it has a low in almost the same position, but wants to throw up moisture farther north and faster.  Take a look at 11pm Thursday evening, which shows snow falling across most of Western Oregon:


Then the total snowfall it generates…slightly drier than it’s 12z run.


I think the takeaway from this is that numerical weather models are in quite good agreement on a 2nd surge of cold and dry air moving south later Thursday, spinning up a surface low pressure, and bringing light snow to some areas between Kelso and Medford in the interior of western Oregon and maybe Long Beach to Newport on the coast. Right now the odds are that it may just barely miss the Portland metro area. But a slight shift of 50-100 miles north or south will make all the difference.  That surge of colder air means Friday (with some cloud cover and gusty east wind) and Saturday will be the coldest days this week, with only a couple of degrees of warming Sunday.  I saw an online forecast claiming 40s for Sunday…that sure isn’t going to happen.

A change is up in the Monday-Tuesday forecast.  The interaction of the cold northerly flow and lots of moisture from the south appears to be off the table for now.  But whether we get some sort of transition event (snow/freezing rain) at that time is still a big question.  Whatever happens wouldn’t occur now until later Monday or Tuesday as it appears the strong upper level ridge to the West may not allow much “undercutting” of Pacific moisture.  Flip a coin on the last two days of the 7 Day forecast this afternoon.

I do notice ensembles are in good agreement with a warming trend after Sunday.  The 12z GFS and 12z ECMWF both agree we return to a near-normal (temp-wise) airmass as we head towards the middle of next week:



Chief Meteorologist Mark Nelsen