Monday Morning Snow: Who Gets It and Who Doesn’t

December 4, 2016

3pm Sunday…

It’s a  refreshing day out there with partly cloudy skies and just a few light showers.  I came over the Coast Range midday and not a snowflake, or even snow-dusted tree in sight.  Very nice.  But tomorrow morning/midday will be far different.  So glad we never forecast snow (or anything close to it) for Sunday here at FOX12.

Let’s jump right into the highlights for all of you that just want to know what to expect tomorrow without the hype or technical details…

THE BIG PICTURE

We will not be having a snowstorm tomorrow in the lowest elevations of western Oregon or most of SW Washington.  But we should ALL see snowflakes in the air or even briefly heavy snow falling and SOME of us will get some snow on the ground.

We can’t predict the sticking snow elevation (the “SNOW LEVEL”) well in these “snow shower” patterns like we have tomorrow.  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.

HIGHLIGHTS

Partly cloudy skies tonight with a few light rain/snow showers as temps cool into the mid 30s

  • Between 4-9am a batch of snow showers move through NW Oregon and SW Washington (during the commute)
  • Lowest elevations of all the I-5 corridor cities (including Portland): expect ZERO-1″ accumulation during that time.  If the showers are heavy enough, up to 1″, if it’s just light stuff that doesn’t drag enough cool air down for “stickage”, then forget it…just flakes in the air or a dusting on grass/cars.
  • Up around 1,000′ and above expect 1-3″ snow during that time.  < Pretty confident on this.

I expect no big traffic issues on major highways/freeways in the metro area, I think the AM Commute should be okay in the cities and lowest elevations.  MAYBE some slush up at Sylvan on Hwy 26 briefly if the showers are heavy enough.

There will be snowy/slushy roads up around 1,000′.  Or at lower elevations in central/northern Clark county where the showers will be heavier.  Possibly Scappoose/St. Helens as well.

Regardless of what happens during the AM Commute, afternoon/evening temps remain well above freezing so lowland roads will be bare and that dusting to 1″ will have melted.  It’s unlikely we get a sudden freezing in the evening hours there.  Of course there could still be leftover snowy/slushy spots up around 1,000′ and above.

COAST:  Too warm, just rain/snow showers mixed

GORGE:  I-84 (near sea level) remains clear, hills up around 1,000′ get snow just like westside though

COAST RANGE SUMMITS:  Snowy at times, especially early.  First icy driving of the year.  A little better early/mid afternoon with warming temps.

We dry out tomorrow night and Tuesday

A GREAT TOOL FOR CHECKING YOUR ELEVATION I love this one: http://www.whatismyelevation.com/##  Put in your address and it’ll tell you how high up (or how low) you live.

 

TECHNICAL STUFF FOR THE WEATHER GEEKS

If we don’t get heavy showers tomorrow morning, there could easily be nothing accumulating anywhere below 1,000′ due to a marginally cold atmosphere (only -5 to -6 at 850mb), breezy/mixing southerly wind, and no cold air mass already in place.  Let’s pretend it’s the old days (like 1995-2000), and we don’t have these high-resolution models:  I’d just be forecasting “sticking snow above 1,000′, with no accumulation likely in the lowest elevations”.   When I look at the WRF-GFS cross-section, that’s very marginal for snow.  I prefer to see the “zero” line (32 degrees F) a bit closer to sea level with that well mixed southerly flow setup.  Again, if we don’t get heavy showers this is going to be a non-event in the metro area.

wrf_cross_section

I think our RPM model’s forecast of snow accumulation pretty much represents what I’m forecasting on-air tonight and in the highlights above:

rpm_snowaccum_metro

The heavier stuff is over the hills and higher terrain of the metro area.  Do not read maps like this too “literally”.  By that I mean (for example), you shouldn’t think the northern part of Lake Oswego could see 1/2″ but the south side gets an inch.  Models aren’t that great yet.  That model data is then contoured by a separate program too, introducing other issues.  The general idea is that we’re not getting a big snow event tomorrow, but it’ll be a close call for many of us and there will probably be plenty of two-hour school delays in the hills.

For some reason the WRF-GFS model from the UW is very bullish on Clark County snow:

wrf_snow_bymondaypm

It also thinks 2″ is going to fall over Portland’s central/eastside areas.  I doubt that will happen.  Forecasting exact snow amounts any day is tricky, but forecasting them off of randomly placed showers is a bit crazy.

You can also see on the map above what I talked about with snow level forecasting.  Lighter showers in the higher elevations southeast of Oregon City produce almost no snow in those hills up around Redland near 1,000′, yet 2″ at PDX near sea-level.  You get the idea hopefully…don’t read too much into any one model in this sort of pattern.  It IS notable that both the GFS and ECMWF generate almost nothing over us:

ecm_snowendingmondaypm

gfs_snowendingmondaypm

So there you have it.  We do dry out Tuesday and Wednesday, and on Wednesday we’ll get a strong and very cold east wind.  That sets us up for some sort of freezing rain and/or snow event by Thursday morning in the Gorge, and likely in a good part of the metro area too.  The good news is that upper-level westerly flow is resuming with lots of wet weather just beyond with onshore flow.  That’s going to dispatch the cold air coming through the Gorge relatively quickly.  I think it’s a 1 day event for the metro area Thursday.  It COULD last into Friday in the Gorge…we’ll see.

Chief Meteorologist Mark Nelsen