Snow For Some Sunday AM

11pm Saturday…

A cold front moves into Western Oregon and Washington tomorrow morning.  At the same time we have a cool airmass overhead, along with colder and dry air pouring out of the Columbia River Gorge.  The combination of these means we have a marginally good potential for snow to the lowest elevations again.  ANYONE IN THE LOWEST ELEVATIONS MAY SEE SNOW FALLING, BUT ONLY SOME AREAS WILL GET SNOW STICKING ON THE GROUND.

Similar to Christmas Eve morning, pinpointing the locations where we see snow to the valley floor is tough.  Unlike Christmas Eve, everyone will get steady precipitation west of the Cascades by 10am.  So instead it’s all about temperatures then for your Sunday morning snow/rain forecast.  Here’s the forecast graphic I used at 10pm:

I say west metro for the lowest sticking snow because there are strong hints based on the easterly flow and models that cold air will be deepest as it banks up right along the east side of the Coast Range and Tualatin Mtns.  This is similar to the “Forest Grove Effect”, but kind of a weak version of what we’ve seen in the past.  The 00z WRF-GFS and our RPM have a similar look.  Tomorrow morning would be a good time to be in Vernonia, Buxton, Banks, or Forest Grove.  Possibly St. Helens & Scappoose too.  Also I think a dusting to 1″ is possible anywhere higher up in the West Hills.

Your next question is…”what if I live at 600′“?  Then you’ll likely see conditions between those two text panels.  No matter what someone tells you, we can’t forecast snow level with that much detail.  I’ve seen snow level forecasts for 250′, 750′ etc… in the past.  That drives me nuts.  Precipitation intensity and location will affect snow level far more than elevation in most cases.  For example at 1,000′ I had no snow Christmas Eve morning, yet people near sea level around Woodland/La Center had snow because heavier showers were falling in those locations.

For those of you travelling through the Coast Range tomorrow, I’d delay until the afternoon hours when the snow backs off.  Roads should be better at that time.

For the Cascades and Gorge, the issue is that a lot of the moisture won’t even make it that far.  As a result I said just 1-2” in the Gorge and it’ll probably be mainly wet on I-84 through the entire day.  A few more inches in the Cascades won’t make any different on the snow-packed roads either.

Chief Meteorologist Mark Nelsen

Flurries/Showers This Evening

7:45pm Saturday…

Merry Christmas!  Sorry a little late, but hey, it’s only the 2nd day of Christmas; I’ve got 10 more days in which I can say that.

I’m working this evening and I’ll put together a blog post about this upcoming week after 00z models are in.

But, I notice a band of precipitation which no models seem to have accounted for is crossing the Coast Range.  It’s REALLY light stuff, but taking a look at the 4pm Salem balloon sounding:

It’s pretty obvious the atmosphere is cold enough for snowflakes to survive down into the lowlands.  Temperatures are a degree or two above freezing up around 2,000′, but it’s dry air and precip falling into that dry air will evaporate, cooling the atmosphere.  That’s what we call Evaporational Cooling or, as slang, Wetbulbing.  The highlighted column is what the temperature will be if the airmass “wetbulbs”.  Note below freezing all the way down almost to the surface.

So IF the precipitation survives the drier low levels and IF the band doesn’t fall apart entering the valley?  Then you may see snow or snow/rain mixed in the next couple of hours in the metro area.  It’s cold enough so that IF the precip holds together a dusting could accumulate at the top of the West Hills, Mt. Scott, Western Gorge etc…

A lot of IFs, but you get the idea.

Chief Meteorologist Mark Nelsen