First Snow In Cascades This Weekend, Plus Cool Showers in Lowlands

September 26, 2019

7pm Thursday…

Get ready for a real taste of October weather this weekend!  Luckily that does NOT mean a ton of rain.  Let’s just say it’ll feel a bit more “refreshing”.

Summary

  • Nothing dramatic is expected (weather-wise) below 4,000′ over the next few days
  • Cooler showers arrive Friday afternoon in NW Oregon and SW Washington.  Then expect light showers off/on Saturday.  Sunday might be dry west of the Cascades.  There will be plenty of sunshine between showers the next few days, not gloomy at all.
  • Coastal weather looks great!  Not many showers Saturday OR Sunday.
  • Temperatures drop well below normal for late September, but frost is unlikely for 90% of us west of the Cascades
  • Light snow is possible down to around 4,000′ or maybe a bit below both Saturday and Sunday mornings.  Trace-2″ at Government Camp, but possibly 6-10″ up at Timberline Lodge by Sunday afternoon when things dry out
  • Passes should remain clear, although a brief morning dusting or icy spot is possible both Saturday and Sunday mornings.

This weather pattern has been forecast quite well by our weather models.  For the past week they’ve been showing a cold pool of air dropping south through western North America.  Sure enough, this evening a cold “upper-level trough” is right over Juneau, AK

500vor.12.0000

By Friday afternoon it is right over Washington, this IS our “arctic outbreak” pattern in winter.

500vor.36.0000

Then by Saturday afternoon it’s over NE Oregon.  A large upper-level ridge has developed in the eastern Pacific and cold northerly flow extends from the Yukon down to California!

500vor.60.0000

We get a surge of showers ahead of it Friday afternoon and evening, then the majority of shower action Saturday evening and beyond moves south and east of us.  At the same time high pressure at the surface turns our wind flow “offshore” late Saturday.  That gives us an easterly wind Sunday and Monday, drying things out with plenty of clearing.

The WRF-GFS (from UW) cross section over Portland shows the big cool down in the airmass overhead.  Time goes from right to left, starting with this morning and ending on the left side with Sunday afternoon.    I’ve drawn the zero degree line (celsius) in blue.  The 850 horizontal line is around 4,000′.  You see it goes from around 50 degrees (F) at that elevation today to about 32 degrees late tomorrow night.  It remains within 5-10 degrees of that freezing mark through at least Monday morning.

cross-section

That means it’s reasonable to say anything above that 4,000′ elevation will fall as snow beginning sometime after sunset Friday.  Models are showing plenty of mountain precipitation tomorrow night through Sunday morning.  I see the 18z ECMWF is giving the higher parts of Mt. Hood 6-10″+.  That seems reasonable.  Same thing in the central Cascades of Oregon and at least 2-6″ in higher parts of Eastern Oregon too.

web_ECMWF_snowaccum_hourly

Our RPM model thinks there will be spots over 10″ up around Timberline and higher.  Seems reasonable and goes with the Euro forecast

RPM Snow Accumulation Mt Hood Zoom In

There are even hints that at least a dusting could fall down to Sisters, Bend, and Redmond by Sunday morning with a cold northerly wind blowing moisture “upslope” into that area.  This is a classic hallmark of an “arctic blast” in that area.  Northerly flow moves uphill from the Columbia River into Central Oregon, clouds form, and snow is squeezed out when the air rises.  We don’t get many situations with flow from that direction but it’ll probably happen over the weekend.

The big snow/cold event will be to our north and east.  Look at all the Winter Storm Watches and Warnings across Washington, Idaho, & Montana.  Some spots in the Rockies could see 3-4 feet of snow!  Very early, even for that area.

Winter Weather Advisory 2017

So how “cold” will we get in the lowlands west of the Cascades?  For most of us Sunday will probably be the coolest day.  Even with abundant sunshine we’ll barely make it to 60…maybe only upper 50s

Forecast Max Temperature NWS NDFD Grids

If you’re a gardener like me you might be worried about frost.  I think that’s unlikely for most of us west of the Cascades.  Maybe either Monday or Tuesday morning around Tillamook, Vernonia, Banks, Battle Ground.  Just the very coldest outlying areas.  The rest of us remain above 35 degrees.  Your warm weather veggies won’t like it, but it’ll sure give them the hint the “end is near” for this growing season.

Forecast Min Temperature NWS NDFD Grids

What you’ll notice most Saturday through early next week will be the refreshing air and bright sunshine and of course much cooler mornings.

Enjoy your weekend!  Chief Meteorologist Mark Nelsen