Ahhh!  Nothing says winter in the Pacific Northwest quite like cold valley rain, heavy Cascade snow…and occasional teases of snow at lower elevations.  Last night (Monday) was the first time this season we saw sticking snow at the lowest elevations in northwest Oregon/southwest Washington.  A heavy band of showers moved through the metro area around 9-10pm, dumping nearly an inch of snow in some neighborhoods in Washington County (i.e. Hillsboro and Beaverton).  My street in Orchards, WA saw a brief stint of flakes around 10:30pm, but nothing sticking.  Nonetheless, even for people who didn’t get any accumulation, it was the first taste of snowy weather for the 2021-22 season!

The lowlands of NW Oregon / SW Washington are in a very marginal climate for winter snowfall; we typically see sticking snow only once or twice per year – and some years, not at all.  The reason is that we live next to a massive region of relatively warm water, known as the Pacific Ocean.  As long as surface winds are coming off the 55-degree ocean, it’s nearly impossible to get significant snow at the lowest elevations west of the Cascades.  Nonetheless, there are actually a few different ways for us to get snow, in the rare instances where it happens.

1.  MARITIME SUBARCTIC AIR

The Pacific Ocean is warm, but not all the airmasses that pass over the ocean are.  Frigid arctic air from Siberia spills into the northwest corner of the Pacific on a regular basis, helping to power the fearsome winter low-pressure systems responsible for our rain and wind storms.  Sometimes arctic air also spills out from Alaska or British Columbia, and makes a quick turn toward the coasts of Oregon and Washington.  Even with the warm ocean moderating the lower levels of these cold airmasses, it’s still possible to get sufficient cold for low-elevation snow here – provided the source of the arctic air is frigid enough and the air doesn’t spend too much time over the water.  But the mid- to upper-level airmass has to be very cold for this to happen; usually you need 850mb temps close to -8 Celsius and the 500mb elevation has to drop below 5200 meters (520 dam).  Oh…and there actually has to be PRECIPITATION around when the coldest air arrives.

This setup doesn’t happen often in our area; only once every few years do we get significant accumulations (say 1-3″) from a “pure” maritime subarctic pattern.  The textbook example of this snow pattern was in early March 1951, when nearly 8″ fell on Portland over the course of a few days  thanks to a persistent cold and deep trough with onshore flow.

2.  HEAVY PRECIPITATION (DYNAMIC COOLING)

If we can’t get to -8 at the 850mb level, there is another way to overcome the snow-killing effects of the warm Pacific Ocean.  All else equal, heavy precipitation tends to cool the air column it is falling through, via a process called dynamic cooling.  The melting of snowflakes into raindrops absorbs heat energy from the surrounding air.  If heavy snow and rain are falling, this cooling effect can be quite significant.  It’s possible to see snow in the lowlands even if the 850mb temps are -6 to -5 – or in the case of very heavy and prolonged precip, it can happen with warmer 850s still.  This is what happened Monday night; the airmass was around -5.5C.  It “should” have only supported snow to about the 1,250′ level.  But showers were heavy, dragging the snow down to 500′ or lower.  Another excellent dynamic-cooling snow event happened in parts of Portland and Gresham on the evening of March 8, 2019.

3.  COOL/COLD EASTERLY FLOW

This is by far the easiest and most common way to get snow in the northern Willamette Valley and in Clark County.  The most surefire way to stop the moderating effects of the ocean, is to have surface winds coming off the land instead!  East wind accomplishes just that.  In the winter months, an east wind through the Columbia River Gorge will bring in colder air from the east side of the mountains.  This has two main effects.  First of all, the lower levels of the atmosphere are usually a little colder when there is easterly flow, so you don’t need a -8 airmass like you do when winds are coming from the west or south.  Second, humidity is usually lower with an east wind.  And when precipitation falls into a pocket of dry air near the surface, the process of evaporative cooling (wet bulbing) takes place.  This can drop the temperature a few degrees further.

We don’t need a really strong and cold east wind to get snow in Portland and Vancouver.  In fact, too much dry wind can actually eat up a lot of the snow totals.

Tomorrow morning is a good example of a weak and cool easterly flow event.  The WRF-GFS shows 925mb temps (2500′ elevation) just barely below freezing over the metro area:

You might think, “freezing levels barely below 2,500 feet, how does that translate to snow at the surface?”  In short, it’s because the lowest layers are expected to be nearly as cold as the 925s.  This, by the way, is called an isothermal air column; the temperature is largely the same at different elevations.  Winds will be from the east most of the morning, keeping the lower levels quite cool.  Here is the map for 10am:

The east wind will not extend much south of Oregon City, etc; this will prevent places like Salem and Eugene from seeing any white surfaces tomorrow.  But light accumulations are expected by models for Multnomah, Washington & Clark Counties:

Take  these snow maps with a grain of salt.  They cannot predict exact accumulation.  A spot on the valley floor with heavy showers could receive more snow than a hilltop with only light precip, for example.  Nonetheless, this is the big picture for tomorrow:

Precipitation moves in around 4-5am, at the same time that east wind begins to stir at the surface.  Temps will be in the mid-30s most places.  There could be some minor evaporative cooling as the rain/snow mix gets started, and this would turn things to all snow.  But temps will be marginal in the lowlands for sticking; intensity of snowfall is key as to whether you’ll pick up a quick inch in these locations.

By about 11am or noon, wind should switch to southerly and everyone just gets rain.  This marginal snow event will fade into memory quickly.

That’s about it for tonight.  Keep watching the long-range maps…there might still be more excitement for Christmas and beyond!  -Karl