Coldest Weather in 4 Years Ahead!

December 2, 2013

I just finished this afternoon’s forecast, plus caught up on a bazillion emails from the Thanksgiving Weekend.  Big picture weatherwise is that we have a cold period coming up with several chances for snow or some sort of frozen precipitation.

 

The Highlights

  • Showers should end the next few hours (by 8pm).  As expected, since the snow level didn’t drop very quickly, the best you’ll see below 1,000′ is a flake mixed in with a dying shower.  I think anyone above 1,000′ could get a dusting before we dry out.
  • Cold, sunny, and dry the next 3 days.  Temps will gradually trend downhill, first from increasing cold wind out of the Gorge, then increasing cloud cover Thursday should keep us real chilly.  I have 40, 36, 34 for the next 3 days of high temps here in Portland.  Low temps will be nothing unusual tonight, but then plummet Tuesday night and Wednesday night to the lowest so far this year.  Widespread 15-20 degree temps away from the Gorge (wind keeps temps up), and maybe some 10-15 degree readings in the coldest outlying spots like Vernonia, Battle Ground, and Hillsboro.
  • Models now show a rare (for us) cold and slightly wet system swinging south out of western Canada, then sliding down the Pacific Northwest coast.  If so, it’ll be cold enough for all snow, probably even at the Coast.  Assuming this moisture does arrive, we could actually see a widespread light snowfall Thursday night and Friday morning west of the Cascades.
  • Models also show a brief colder blast of air behind that for even colder temps Saturday/Sunday.
  • For Sunday/Monday, our models are all over the place but seem to be trying to merge moisture from the southwest and another blast of cold air from the north.  This pattern in the past has historically produced our best snow/ice storms.  So some sort of “high-impact” event, as we say, could be coming at that time.  Still 6-7 days away so that could easily change.
  • We haven’t seen this long of a cold stretch since December 2009.

 

For the hardcore weather geeks

Showers moving over the metro area right now, but it looks dry upstream in Western Washington, so the idea of drying things out quickly after 7pm is probably still on track.  It also appears the snow level in the Coast Range is down to at least 1,500′, so the atmosphere is cooling quickly.  Just a little too slow for us to get sticking snow in the lowest elevations.  It’s interesting to note that the timing of this is pretty much the same as shown many days ago, but the airmass was also expected to be cooler at this time on those model runs back on Tuesday/Wednesday last week.  Right now is when we’d be getting our snow with the last of the showers…too bad.

East wind (dry and colder air) begins late tonight as the modified arctic air moves in.   Such dry air means bright sunshine for a couple of days though too.

I raised tomorrow’s high to 40 degrees based on MOS forecast.  Plus it seems like often we over-forecast the first day when the cold air moves in when we get great mixing, but the brunt of the cold stuff hasn’t totally trickled in.

Things now are looking real interesting Thursday night and Friday morning.  The GFS, ECMWF, and GEM all show a stronger shortwave trough swinging south through Western Canada, bringing a 2nd surge (a reload!) of modified arctic air later Friday and Saturday.  Now that the trough digs offshore a bit on models, that spins up a surface area of low pressure that swings moisture down the coast.  The track is similar on all models showing the main snow heading through the southern half of Western Oregon.  If this solution continues to hold, we could see a widespread (light) snowfall across the entire west side of the state, maybe part way up into SW Washington too.  Something similar to this happened during the December 1998 cold spell.  I remember the westside, downtown, and coastline got snow and the eastside got nothing as a cold low scooted down the coastline.

Whether we get moisture or not that day, cloud cover should keep our high temps around freezing, colder if we actually get snow.  Then a real chilly day Saturday with sunshine.

Sunday and Monday are REAL interesting as some of the models are showing energy cutting under the strong upper-level ridge to our west and combining, in some way, with more energy and cold air coming down from the north.  This general pattern has given us widespread snow/ice storms in the past.  And, just as in the past, we’ll see wide variations in solutions for that period over the next 4-5 days.  I remember the January 2011 (or was it 2012) event where at one point we expected a massive snowstorm here as low pressure would move in to our south.  However, over the following days models moved the surface low much farther north and we ended up with a trace to just several inches across most of the metro area.

So, to summarize, some real weather fun is possible over the next week and we MIGHT finally see some sleddable snow in our area for the first time in several years…maybe.  Lots of “possible”, “might”, and “maybe” in this post eh?  My 7 day forecast has more decent snow chances (and cold) than I’ve had in several years too.

Here are the 12z GFS ensemble and 12z ECMWF ensemble charts:

tseries_850t_000-360_Portland

 

tseries_850t_000-384_Portland

Chief Meteorologist Mark Nelsen