Widespread Snow Likely Metro Area Sunday

January 2, 2016

9:30pm…

Things have sure changed quickly in 24 hours..we’ve gone from “just a few flurries” to “wow, it looks like a real snow/ice event is possible tomorrow!” Sunday we may see our first real snow “storm” since February 2014.

HIGHLIGHTS

  • Prepare for widespread snow/ice on metro area roads by Sunday midday/afternoon.  POSSIBLY as early as late morning.
  • GET YOUR DRIVING DONE BY NOON!
  • 1-3″ is likely by late evening.
  • Freezing rain/ice pellets will mix with the snow by evening too.  We may see spots with an icy glaze on top of the snow by late evening
  • Temperatures will only creep up to freezing and then stay below freezing through the Monday AM commute
  • Whatever falls will still be on the roads Monday morning.
  • In the east wind areas (east metro and Gorge) the snow will be blowing and drifting a bit, causing poor visibility.

MarkSnow_PortlandOutlookPercentages

MarkSnow_PortlandOutlookPercentages2

What the heck is going on?

The easy part is the cold…it’s PLENTY cold out there.  Temperatures have been dropping steadily the past 3 days as cold air pours out of the Columbia River Gorge and the airmass cools in eastern OR/WA (see previous post).  Very dry air (at the surface) also means that as precipitation arrives temperatures will drop 3-5 degrees.  It’s called evaporational cooling because the evaporation process removes heat out of the air.  We top out around 33 and then assuming snow shows up midday/pm temps will drop back into the upper 20s.  We also have a clear night and then cloud cover suddenly shows up at daybreak; thus tomorrow should be the coldest day in 2 years in our area.

How Much?

All models have “moistened up” quite a bit the past 24 hours.The 00z NAM-MM5 and WRF-GFS from the UW both bring .20 to .40″ precipitation over the metro area by late tomorrow evening.  If it’s all snow, we could see 2-4″ total.  That would be a snowstorm by our sad mild marine climate definition.  By the way, if models are totally wrong and hardly any moisture makes it here?  We’ll get a few flurries…none of what I’m posting will happen.

Snow?  Ice Pellets?  Freezing Rain?

During at least the first half of the event the atmosphere overhead should be cold enough to support snow all the way down. For sure that’s the case in the metro area; I’m less confident on pure snow from Salem south to Eugene (thinner cold layer above).  Models are showing a warm layer (above freezing) pushing north up the valley during the afternoon/evening hours as part of the cold airmass is eroded away west of the Cascades.  I see a signature on the models I’ve seen before with the warm “nose” pushing north into the metro area late in the day mainly east/south sides away from the Columbia River.  So if trends remain the same, snow should change to ice pellets and/or freezing rain first down around Marion County and then into Clackamas County (Silverton/Molalla/Oregon City/Estacada etc..).  The rest of us in the metro area would stay in snow longer…possibly through the entire event.  Remember during the last east wind event in mid December that models eroded the cool air higher up too quickly…the precipitation stayed as snow longer than we expected.  That might be the case tomorrow.

Regardless of the timing and positioning of that warmer layer tomorrow afternoon/evening, it’s pretty clear that by late tomorrow night ALL areas west of the Cascades will have a warm layer higher up and the snow chance will have ended except in the Gorge/Mountains/Eastern Oregon.

With east wind continuing tomorrow night the cold air isn’t going anywhere.  Whatever you see outside your window tomorrow evening should still be there for the Monday AM commute.  Welcome to January!

Beyond tomorrow, one very clear message is that there is no weather system to wipe out the cold air in the Gorge through at least Wednesday/Thursday.  It’ll ever so slowly modify, but we’re starting pretty cold this time and it’ll take several days to warm up above freezing.

Additional precipitation possibly Monday night or Tuesday/Tuesday night will fall as freezing rain in the Gorge and POSSIBLY parts of the metro area too.

It might be a long 72 hours ahead for the meteorologists!

Chief Meteorologist Mark Nelsen


A Perfect Example of “Fake Cold”

January 2, 2016

7:45pm Saturday…

I can’t even tell you who came up with the phrase “Fake Cold”, but the last 4 days have been the perfect example!

Of course FAKE COLD is not a real meteorological phrase but a term we use here in our area, in the lower elevations of the Pacific Northwest.

What is it?  Probably better to define a “regular cold” airmass or modified arctic airmass.  That’s when cold (usually dry too) air comes down from the north or northeast (out of Canada).  It settles down into the Pacific Northwest, we forecast highs in the 30s, or colder, under sunny skies west of the Cascades.  Of course it would typically be 5-10 degrees colder east of the Cascades in the valleys.  For example, last New Year’s we had a cold near-arctic airmass descend south over us…with 850mb temps down around -8 to -10 over our area…that’s cold!  We ended up with high temperatures similar to what we’ve seen the past two days; mid-upper 30s.

MarkTemp_SinceMidnightPDX_Bars_Hot

FAKE COLD is when a cold airmass didn’t mainly come from Canada but is generated by air sitting over the valleys/basins of our region in winter.  Generally it’s the Columbia Basin in Eastern OR/WA.  Less often it can be in the Willamette Valley.  It works best with snow-covered ground, but you see the same weaker effect without snow cover.  Usually the airmass cools over several days; caused by long winter nights when more heat leaving the lower atmosphere than is taken in during the daytime.  In this case there is no mild wind to bring in warmer air so the lowest part of the atmosphere gradually cools.  Take a look at the 5pm temps at PDX for the past 4 days…all of them windy and well-mixed.

WED-38
THU-35
FRI-33
SAT-30

About 8 degrees cooling in the past 72 hours.

In the Gorge at 3,500′ (3 Corner Rock)

W-20
T-15
F-12
S-10

About 10 degrees cooling there in 3 days.

Same thing at The Dalles, 9-10 degrees cooling.

W-34
T-31
F-28
S-25

Interesting that all the well-mixed locations have cooled the same 8-10 degrees don’t you think?  Meanwhile the 850mb temperature over Salem at the beginning of this episode didn’t drop below -4.  That would typically give us high temps in the mid 40s this time of year, which is what we started out with on that first east wind day back on Wednesday.  Today we barely hit 36!  Troutdale only PEAKED at 33.

So that is FAKE COLD…an airmass that started just a little on the cool side and has now become similar to a weak arctic blast.   Dewpoints have also been steadily dropping.

Uh-oh, now we’re all primed for snow & ice…if/when the moisture shows up…

More on that in the next hour.

Chief Meteorologist Mark Nelsen