A Busy Weather Week Ahead: Low Elevation Snow Update

December 3, 2016

10am Saturday…

I’ve been checking all the latest maps/models and meteorological goodies over my morning drink.  Doesn’t everyone else wake up on a Saturday morning and do that as well???

December-February is winter as far as meteorologists are concerned, and west of the Cascades just about all our winter weather (snow/ice/big storms) occurs in December/January most years.  Right on schedule, next week will be our first wintry week of the season with a:  1) strong cold front Sunday morning,  2) marginal snow event for the lowlands Monday, and 3) possibly a widespread snow/ice event in the metro area and (for sure) Gorge Thursday.  Beyond that we’ll most likely warm up again.

MONDAY SNOW

A strong cold front will pass through the region very early Sunday morning.  In fact by the time you wake up it’ll already be down around 40 degrees and there may even be snow mixed in up in the hills, but no sticking below 1,500′.  Then at midday the rain stops and skies break open to sunshine.  Sunday afternoon looks great!  Partly cloudy and mainly/all dry with light wind.  Last chance to hang your Christmas lights in dry and comfortable weather for a while.  Late Sunday night and the first part of Monday is when it gets real interesting.  A “cluster” of showers moves inland, right after skies have cleared overnight.  At this point it’s marginally cold enough to get sticking snow down to the lowest elevations west of the Cascades.  But if we get a burst of heavy enough showers, we could easily get some of that snow to stick down here at the valley floor.  Very tough call in these situations.  If you want it to snow, we have the overnight cooling to help us, yet we also have a mixing southerly wind that tends to keep the snow level off the valley floor.  Models are in very good agreement showing .15″ to .30″ precipitation during this time.  That would be enough for 1-3″ snow accumulation if it all made it to the ground.  That’s very unlikely in the lowest elevations.  But I’m not paid to just see-saw back and forth over what may or may not happen…this is what I think is most likely…

SUNDAY-MONDAY HIGHLIGHTS

  • Rainy start Sunday, then dry and partly cloudy, no sticking snow below 1,500
  • During Monday AM commute, EVERYONE WILL SEE SNOW IN THE AIR as snow showers arrive before sunrise
  • If you live ABOVE 1,000′, expect 1-2″ snow on the ground by mid-morning
  • If you live in the higher hills around town (West Hills/Mt. Scott/Sylvan) near 1,000′, expect a trace to 1″
  • At the surface (where most of us live) a dusting is possible, maybe even up to 1″ on the lawns/barkdust
  • I DON’T EXPECT MAJOR IMPACTS TO THE A.M. COMMUTE ON HIGHWAYS/FREEWAYS, but probably very slow as we all gawk at the snow
  • SOME SNOW WILL GET ON ROADS HIGHER UP IN HILLS (for sure above 1,000′)
  • Monday afternoon commute will be uneventful with temperatures in the upper 30s and scattered light showers

So obviously we’re all going to really excited about seeing the first snow of the season, but a bunch of snow accumulating on metro highways is pretty unlikely in this pattern.

Note the different model snow forecasts.  It’s important not to read these too “literally” because the terrain of the model may not match the actual terrain.  For example, notice on all three there is significant snow on I-5 between Longview and Woodland.  That’s because the narrow gap the Columbia River cuts through the hills isn’t resolved on this resolution model, so it just thinks the hills continue right across the low areas.  The West Hills, Chehalem Mtn, and south Salem hills show up nicely on the WRF-GFS (middle image) though.  The same thing happens in the Gorge.  Most models (unless they have less than 5 km resolution) just see the Gorge as a low mountain pass.  Thus Cascade Locks may be seen as a 2,000′ pass instead of at sea level, similar setup for Hood River too.

ecm_snow_10pm-10ammon

mondaypm_snow

rpm_snow_12z

Whatever happens the first half of Monday, by afternoon it’ll just be snow/rain showers in the lowest elevations as temperatures “warm” up to 40 or so.

We might have a leftover flurry Monday night or Tuesday, but for now it looks like far less moisture available.  Beyond that time we go into dry and cold for a couple of days.

LATE WEEK SNOW/ICE

On Wednesday a cold east wind starts blowing out of the Gorge and that sets us up for what we call a “transition event”.  Here in the lowlands of Western Oregon and SW Washington there are two ways we get snow (generally).  The first is the showers flowing onshore like Monday which involves no cold/dry continental air coming in from the east.  On Monday the lower elevations of the Gorge have the same weather we do in the western valleys.   This first situation rarely provides much snow and it’s always a real marginal situation.  I don’t think I’ve ever seen it below 30-32 degrees in this pattern.

The 2nd way we get snow is when cold air is in place (often via cold air moving in from the Gorge) and then moisture returns from the west overhead.  This typically is at the end of a cold period.  That pattern is how we get our big/significant snow/ice storms.  That setup is coming either Wednesday night or Thursday.  And this far out details are very much subject to change.

By Wednesday night a very strong east wind will likely be blowing through the Gorge and out into the metro area.  10-12 millibars easterly gradient = brrrr!  That’s the usual gusts to 100 at Vista House and 70 mph in most of the usual windy settlements at the west end.  At the same time a warmer front is approaching.  Check out the WRF-GFS map for late Wednesday night and you can see thick cold air dammed up in the Eastern Gorge, pouring out the west end:

slp-120-0000

I’ve been around this block a few times…that setup, along with a bunch of precipitation arriving, screams SNOW STORM IN THE GORGE and/or ICE STORM FOR PART OF THE GORGE.  Or most likely a combo.  If the air is cold enough, this could be a snow/ice event in the metro area too.  We won’t really know the details of that until Wednesday when the colder air is in place.

LATE WEEK HIGHLIGHTS

  • Some sort of significant snow/ice storm is likely in the Gorge either late Wednesday night or Thursday
  • It MAY be cold enough for snow and/or freezing rain in at least parts of the metro area too
  • Whatever happens will most likely occur overnight Wednesday night into Thursday

Stay tuned!

Tonight I’ll be out in Seaside for the Providence Festival of Trees.  It’s a great event to raise money for the Providence Seaside Hospital Foundation.  Stop by and say hello if you see me.  Just don’t shake my hand…I think I have some sort of cold coming on…poor timing…

Chief Meteorologist Mark Nelsen