A Chilly End To February Ahead; Snow For Some

February 19, 2017

9pm Sunday…

Have you noticed the snowflakes in the 7 Day Forecast here at FOX12 or on your weather app?  I’ve got the scoop…

First, We will not be having a snowstorm this week in the lowest elevations of western Oregon or most of SW Washington.  But we should ALL see snowflakes in the air or even and SOME of us will get some snow on the ground.  I don’t think you’ll see me snowblowing a foot of snow in Salmon Creek again:

snow

That said…later this week we’ll be in a weather situation where many of us will see snowflakes.  But this time we won’t have a cold east wind blowing out of the Gorge or a cold air mass to give us a widespread snowstorm.  
Instead we get light showers and clearing periods Wednesday through Friday; it happens to be a colder version of the “showers and sunbreaks” pattern we often see in the winter and spring.  We’ve seen this pattern only two other times this winter…back in early December (it kicked off the fun stuff on the 5th) and on February 6th.  In both of those situations little or nothing fell in the very lowest elevations along I-5.  That includes much of the metro area.  That’s because often in this setup we get a breezy southerly wind coming in off the 50 degree Pacific Ocean, keeping the lowest elevations just a few notches above freezing.  Not always, but maybe 90% of the time.

We can’t predict the sticking snow elevation (the “SNOW LEVEL”) well in these “snow shower” patterns like we have later this week either.  No matter how often someone tells you it’s going to snow down to 750′, or 500′, or 250’…forget that nonsense…I generally stick to 1,000′ increments in these situations nowadays.  That’s because (for example) heavy showers in one location could drag the snow level down to 300′, yet just light showers 10 miles away don’t drop snow at 1,000′!  I’ve seen that happen many times.

In general, the higher up you live the better chance you have for sticking snow later this week.  Cooler nighttime air, by just a couple of degrees, for Wednesday night and Thursday night means a better chance for sticking snow down below 1,000′.  That’s assuming showers keep going all night as well.  That’s why I think the best window for a dusting down in the lowest elevations is the late night and early morning hours Wednesday night through Friday AM.  Of course as we get closer we’ll refine the forecast and give you more detail…as always.

You can see the general idea just by checking out the ECMWF snow forecast through Saturday:

ecmwf_snowthrusat

See the big empty hole here in the valleys?  And as we saw back on February 6th, models often overdo the lowest elevation snowfall in these marginal setups.  Check out the Cascade snowfall!  It looks like some of the best this season with very low snow levels from Wednesday until further notice

marksnow_mthoodfcst

Here’s the ECMWF ensemble chart from this morning’s run.

tseries_850t_000-360_portland

That’s the next two weeks, time goes from left (right now) to right.  Green line is average 850mb temperature for the date (temperature at 4,000′ in celsius), the blue is the operational run and red is average of all the ensemble runs.  This screams a very chilly last week of February and first week of March.  To get sticking snow with onshore flow this time of year we need -7 to -9 at 850mb.  Notice we’re very close to that several times in the next two weeks.  Of course if we lose that onshore flow and get decently heavy showers at some point, it could snow/stick all the way to sea level even as warm as -3 or -4.  So the main message here is that we’ll be flirting with snow in the lowlands several times in the next 2 weeks.  Anything is possible so let’s get the wishcast express loaded up folks!

WishcastingExpress

Chief Meteorologist Mark Nelsen