Weekend Snow Possibilities & Cold Weather Next Week

February 15, 2018

5pm Thursday

The coldest airmass we’ve seen in late February since at least 2011 will be arriving Sunday night.  Now that we’re within 3 days of possible snow at the lowest elevations, it’s time to get into a bit more detail.

HIGHLIGHTS FOR WESTERN VALLEYS

  • There is no chance for lowland snow through Saturday night...we’ll have typical rain showers through this time.  In fact quite windy & mild Saturday (50-52 high)
  • During the daytime Sunday, the sticking snow level will hover in the metro area hills (close to 1,000′).  But we’re talking off/on snow showers.  Sunbreaks in-between combined with typical daytime heating keeps the “stickage” in the hills.  Showers will be light as well which helps.
  • Roads below 1,000′ should remain snow-free during the daytime Sunday
  • Sunday evening colder air rushes in, but it’s very dry air too.  Most likely snow showers change to light flurries and then end.  Temperatures drop below freezing all areas
  • Any water left on roads Sunday evening will freeze, plus some spots in the western valleys may see light snow accumulation on those roads.  There will be icy/snowy areas on some roads for the Monday AM commute but it may not be widespread
  • Total snow at the lowest elevations?  Could be NOTHING for many of us through this period…not a real wet pattern.  I’d be surprised to see much above 1/2″ in the city.  1-3″ more likely up above 1,000′ of course.

 

TO SUMMARIZE:  AT THIS TIME, I THINK “SLEDDABLE” SNOW IS UNLIKELY FOR MOST OF US IN THE LOWEST ELEVATIONS.  LESS THAN 1″ IS LIKELY FOR MOST

This graphic summarizes my snow thoughts:

Beyond Monday morning, the vast majority of our models keep us dry through Wednesday.  The airmass will be the coldest so far this season.  Very dry air combined with clear skies could drop most of us into the teens Monday night and/or Tuesday night.  But under bright and strengthening late February sunshine, afternoon highs should still make it close to 40 each day.  Think typical Central Oregon winter weather.

WHAT ABOUT COAST & GORGE?

COAST: Too warm during the daytime Sunday for sticking snow out there, but ZERO to 1/2″ would be my best guess for coastal towns Sunday evening and night.

GORGE:  We don’t have a cold easterly wind blowing in the Gorge, so not much different from westside.  A TRACE to 2″ is most likely Sunday evening through Monday AM.

 

TECHNICAL DETAILS

For a snow/cold event, models are in surprisingly good agreement through at least Monday.  All show the upper-level trough digging down right over us Sunday and Monday from the north.  The sequence of events sure looks like the November 2010 freeze.  The cold air just drops in from the north instead of coming at us through the Gorge.  850mb temps are in -6 to -8 range during the daytime Sunday, which this time of year should be (just barely!) enough to keep sticking snow off the valley floor.  If we had more vigorous showers forecast by models I would go for sticking in spots during the day.   Then they are all in good agreement drying things out quickly after 00z Sunday PM.  Folks this is a classic case of moisture disappearing right when the cold air arrives (like Nov 2010).  Note the 4pm Sunday to 4pm Monday precipitation forecasts from both WRF-GFS and ECMWF…almost nothing

And you see the snowfall forecast through Sunday 4pm…which seems a bit high even though it’s showing all of 1/2″ at most in the metro area.  Check out the Cascades!  Yep, this should be the best weekend of the entire winter up there!

How about the cold?  I’ve checked out the February 2006 and February 2011 late season cold snaps.  A few interesting points…one is that the cold airmass modifies quickly compared to midwinter events.  The other is that with full sunshine and weak Gorge wind, it’ll be easy to get highs up into the upper 30s.  Thus my forecast highs of 38 both Monday and Tuesday.  Both ECMWF and GFS bottom out 850mb temps around -11 to -12 over Portland on Monday.  That’s real cold for this time of year, although not record low.   It’s interesting that we’ve never made it down to -14 over Salem after the first few days of the month.  This is also not going to be a big east wind event for us, even Monday and Tuesday.

Later next week?  Since we have more cool troughs dropping in on us later in the week, the most likely scenario is a switch to onshore flow and rapid modification of the airmass.  That would lift snow levels above the valley floor in the 2nd half of the week.  That happened in 2006 and 2011…not much snow going out of the cold airmass since there is no Gorge wind to hold in the cold air.

I will be off for my weekend the next two days, but back in Sunday.  If there’s anything dramatically different from what I’ve posted above, I’ll update this blog on Saturday.  Otherwise you can assume nothing has changed.

Chief Meteorologist Mark Nelsen