The period from Friday, January 12th through the 19th saw a major intrusion of arctic air into much of the Pacific Northwest, except for the southernmost locations like Medford. Now the cold air is gone, but the aftermath of the storms linger.

I remember thinking back in November that we probably wouldn’t get a major cold wave this season…or if we did, that it would be in and out within 3-4 days. Instead, PDX has been below 40° F for more than a week now, and stayed below 32° for just a hair over 5 days. This included the coldest day since December 1990, plus three rounds of frozen precipitation. Schools and facilities endured several days of closure and numerous events were canceled.

Without crunching any numbers, I’d say that this was the worst event since I moved to the metro area in 2018 in terms of overall disruption and misery. The impacts were widespread and prolonged: impassable roads, closures, cancellations, brutal wind chills, widespread power outages, and major tree/building damage. We’ve had a few other impactful winter events in the last 6 seasons, but nothing on quite the same scale and duration. But how did we end up with such a nasty mess, in an El Niño year no less? Let’s look back at the pattern responsible for all our frustrations.

=====

CHRONOLOGY

Like most of our cold and snow/ice scenarios, it was brought on by a blast of frigid arctic air dropping out of Western Canada. Around Tuesday-Wednesday of last week a lobe of the polar vortex was dislodged for its usual resting place in far northern Canada. Strong ridging developing over Alaska, helped to steer the arctic air southward…and by the night of Thursday the 11th it had spilled into the Columbia Basin and Puget Sound.

Interestingly, this was not a “deep” blast of arctic air, where the front continues south into Southern Oregon and Northern California (like Decembers 2009 & 2013). In deep arctic blasts the entire atmosphere over Western Oregon and SW Washington gets extremely cold, with airmass temperatures of -12°C or colder over Portland and Salem. Instead it was a shallow blast: the coldest air is confined to the Columbia Basin & Gorge, and sometimes northwestern Washington. Bitter air comes through the Gorge and freezes Portland, and spreads out into the Willamette Valley and Clark County. But the cold air is relatively shallow, and temperatures at the 5,000’ level never get too cold. Now, we can still get snow in the western valleys if the middle atmosphere is cool (i.e. 0 to -5°C at the 850mb elevation). But quite often warmer air will slip in at the middle levels, giving us sleet pellets or freezing rain instead of snow. This is, in fact, what happened for most of the cold wave this time.

After the initial arctic front came through Portland on Friday, it dropped temperatures from 41° in the wee hours to as low as 21° by late evening, as east wind intensified. That 21° became Saturday’s high temp, as things got even colder. Precipitation arrived between 7 and 8 am Saturday, initially in the form of snow for Portland and Vancouver. The forecast was to see 2-6” of snow, depending how much precip arrived.

But there was a plot twist. Warmer air in the middle levels (between 4,000 and 6,000 feet) came further north than expected, around the east side of the incoming low pressure system. This changed the snow to sleet over most of the metro area, even as temperatures at the surface remained very cold – as low as 15° at PDX and 12° in parts of the east metro! I never would have imagined that we would see sleet in the low teens, and with subzero wind chills no less. That goes to show just how much of a clash between cold and mild air there was. The same airmass responsible for turning snow into sleet, probably would have produced surface temps in the low 50s had the cold air not been in place below.

Eventually the sleet turned back to snow in the evening before precip died out. The Portland NWS office in Parkrose reported 1.6” of white stuff (snow + sleet) while my neighborhood saw about 2.0”. Spots in northern Clark County got 4-6” total white. Then Sunday was a tranquil day with low clouds and very little wind, and Monday actually saw sunshine even as the east wind returned to parts of the metro.

On Tuesday afternoon around 3 pm, freezing rain moved in from the south. This was originally supposed to be the “silver thaw” that ended with Portland and Vancouver jumping into the 40s Wednesday. But another plot twist: the south wind never made it beyond Lake Oswego and Tigard. The central and east side of the city lost the east wind for a day, causing temps to creep up into the mid 30s and begin a slow thaw. But yet another bout of freezing rain began midday Thursday as cold east wind ramped up again. Finally on Friday, east wind died down enough that milder air was able to slip into most places. But by then, the east metro looked like a war zone with all the carnage brought by two waves of ice accumulation.

TEMPS

Probably the most impressive facet of this event, at least for Portland, were the frigid temps. The high of 21°F Saturday was the coldest daily high temperature since December 1990! Even more dramatic was that it occurred during the wee morning hours, while the daily low of 15° happened in the afternoon under historic wind chills.

Areas away from the strong wind saw their coldest temps on Monday night. The station in Orchards bottomed out at 13° early Tuesday morning, with the help of clear skies and white surfaces. The weather station in Dallesport (DLS) reached -2°F, the first sub-zero temperature there since December 2013.

Most impressive was the duration. PDX stayed below 32° for 122 consecutive hours, from midday Friday the 12th until early afternoon on Wednesday the 17th. And they were below 40° for 180 hours (7 ½ days). That is easily the longest freeze since at least January 2017.

 

WIND

Perhaps the most historic element of this cold wave were the strong winds, and the brutal wind chills that came with them. At the same time that temperatures were in the teens across the metro area Saturday, a very powerful east wind was whipping through the Gorge and spilling down the west slopes of Mt. Hood near Sandy, OR. The wind was largely confined to areas south of the Columbia River, where peak gusts of 40-60 mph were fairly widespread. In my Orchards neighborhood there was little more than a light breeze, though I did take a drive about 3 miles south on Saturday morning. Blowing snow, 30-35 mph gusts, and windchills near 0°F made it a very brief excursion though!

There was also a 64 mph gust near Sandy, presumably caused by a downslope wind from Mt. Hood. This unusually strong east wind was due to a historic pressure gradient in the Columbia River Gorge; up to -17.7 millibars from PDX to Dallesport! I don’t think I ever remember seeing an easterly gradient larger than -14 before. (In this case, negative gradients are easterly and positive gradients give westerly wind. The terms “up/down” and “large/small” refer to absolute value, in this context!)

The wind died down Sunday and returned Monday/Tuesday, though not quite as strong. A third surge of wind Thursday was responsible for bringing freezing rain back into the Metro. Speaking of which….

FROZEN PRECIP

This cold wave gave Portland and Vancouver all three varieties of wintry precipitation: first a mix of snow and sleet on Saturday 1/13, then two separate freezing rain episodes. The first episode occurred late Tuesday into Wednesday morning…and the second episode was from midday Thursday until late evening. In the southern Willamette Valley though, the cold air was much shallower- and this meant that even Saturday’s precip was mostly freezing rain.

Snow/sleet accumulations ranged from about 1” to 6-8”, with the highest totals in northern Clark County. The Tuesday ice storm brought about 0.2-0.4” glaze, making the entire metro area shut down.

The Thursday ice storm affected only the east and central metro, plus the West Hills. In some places, temps were a bit too marginal for major glazing on the roads (needs to be 29°F or colder for that), but ½-¾” ice on trees and power lines still caused big problems.

Of course the Willamette Valley near Eugene was the worst for ice due to the freezing rain with the Saturday storm…with 1-2” glaze wreaking havoc to the power grid; as of Tuesday the 23rd there were still over a thousand homes in the McKenzie area that needed restoration.

OUTAGES / ROADS

I was actually quite lucky in my Orchards neighborhood. I got very little east wind, and only one wave of freezing rain on Tuesday since Thursday stayed slightly above freezing. But east Portland, Gresham, Corbett, Troutdale and Washougal were less lucky. Schools were canceled for the entire 3rd week of January in much of the area, as multiple waves of frozen precip and damaging winds made daily life come to a halt.

There were more than 24,000 outages in the Eugene-Springfield metro, according to Eugene Water & Electric Board. At least 160,000 customers lost power in the Portland metro area, according to PGE. The combo of ice and frigid wind made those outages more miserable than, say, outages from a brief southerly wind storm.

The slow warmup at the end made everything more difficult, by allowing another wave of ice Thursday followed by very slow thawing of affected roads; many places remained impassable well into Saturday the 20th.

FINAL THOUGHTS

1. Saturday’s sleet instead of snow. This is an inevitable problem that comes with local forecasting. Often there will be a “tongue” of warm southerly flow that overrides low-level cold air in the Willamette Valley, leading to pellets or freezing rain instead of snow. But in this case the mid-level south wind was stronger than expected, allowing the warm air to punch further north. Was this at least partially due to the low-level air being so cold (i.e. an “equal but opposite reaction”)? I don’t know enough about atmospheric dynamics to conjecture this. But in any instance it allowed many of us to see sleet with surface temps in the teens.

2.  The slow warmup. This part was probably most annoying to forecasters. We knew from past experience that models can underestimate cold air’s stubbornness; at least in part due to resolution problems near the Gorge. But this went above and beyond normal discrepancies. South wind was originally slated to arrive at PDX on Wednesday the 17th. It finally arrived SIX days later, scouring out the dying remnants of cool air. As someone who grew up in The Dalles, I’m familiar with the problem of slow thaw, yet never imagined I’d see something similar west of the Cascades. This probably has to do with the fact that the surface air was unusually cold to begin with. It didn’t help to have widespread snow cover in the Columbia Basin either…because that reinforces high pressure and allows cold easterly gradients to persist longer.

 

That about sums everything up. We are now back to the very mild pattern that dominated December and early January…this Sunday and Monday we could touch 60° for the first time in nearly 2 months!

Karl