Ridges, Inversions, and East Winds

For our high school senior portraits in the Garfield High School yearbook, we had the option of putting a quote next to our name. I quoted the great John Ruskin, a multi-talented, leading English art critic of the Victorian Era. Ruskin wrote the following; I could only include the bolded portion due to space constraints (our senior class was quite large).

“Summer is delicious, rain is refreshing, wind braces up, snow is exhilarating; there is no such thing as bad weather, only different kinds of good weather.” – John Ruskin

But though Ruskin and I share a sense of intrinsic optimism, I think he’s mistaken when he says there’s “only types of good weather.” And I believe that next Tuesday through at least next Saturday will have universally unpleasant weather, no matter your climatological preferences. No, we won’t see anything as horrendous as the fires and smoke we saw in early/mid-September, but we will see a biting, intense, incessant “East Wind” through the Western Columbia River Gorge and greater Portland metro area, as well as inversions and fog/degrading air quality in the stagnant airmasses away from the Columbia River Gorge.

This is quite a long post, so to summarize, here’s what I’m thinking for the next 10+ days:

• We’ll see a moderate front Sunday night/Monday morning that will bring us a brief shot of rain/wind/mountain snow, but a ridge will build overhead in its wake and persist through this coming weekend and potentially into early next week
• The combination of warm, calm weather aloft from this ridge and a net radiation deficit at the surface will lead to the development of temperature inversions at the surface. These temperature inversions will be most pronounced east of the Cascades but will also form in the Willamette Valley and Puget Sound lowlands, and they will trap moisture and pollutants near the surface. These inversions should begin to form Tuesday and persist through Friday/Saturday
• With the possible exception of Monday night/Tuesday morning, the Portland metro area should not see fog. Instead, it will see persistent easterly winds through the Columbia River Gorge, with the strongest winds in the eastern Metro and Western Columbia River Gorge (Troutdale, Corbett, Rooster Rock, Crown Point, etc.)
• A few systems may begin to impact the area early next week, but they are expected to be relatively weak with the bulk of their energy going into BC or the Alaskan Panhandle.

Current Conditions:

Yesterday was actually a pretty nice day for late November! A weak shortwave trough and associated cold front moved inland and temporarily flattened the ridge that had brought us stagnant air and fog the past several days.

Despite being weak and bringing little in the way of precipitation or wind, this front still had a very textbook structure, with relatively strong and gradually increasing southwesterly winds ahead of the cold front and a sharp decrease in wind speed and shift to NW winds right after the cold front passage.

Photogenic cold front approaching the Pacific NW on the HRRR today. Note the wind shift and decrease right behind the frontal boundary. #wawx #orwx pic.twitter.com/0Vx0eBXfEr

— Charlie Phillips (@GeoduckChuck) November 27, 2020

This front was stunning on satellite. If you look closely, you can see the “rope cloud” – the very thin line of whiter, higher clouds on the leading edge of the larger cloud band that marks the exact location of the front.

Today, the weather is a little more gloomy. Some persistent low clouds have been giving Portland and parts of the Willamette Valley a light drizzle through much of the morning and early afternoon, though the precipitation is beginning to taper off. But at the ski resorts, you’ll find sunny skies with hardly any clouds in sight!

Forecast:

The next 10 days – and potentially the next 15 – will feature extremely strong ridging over the Western US. It is not unusual to have extended periods of ridging during some of our stormiest times of the year, but it is a bit more unusual to have them during a moderate/strong La Nina like the one we are currently seeing.

The below graphic shows the average 500mb height anomalies from the European ensemble mean for the next 15 days. If that graphic doesn’t scream “ridge!”, I don’t know what does. This is particularly true when you consider that the Euro actually has the least ridging compared to the GFS and Canadian models.

I mean, just look at the “spaghetti plots” from the GFS ensembles – outside of the Sunday evening/Monday morning front, the Pacific NW looks high and dry for the next 10 days, and potentially the next 15..

By now, you’re probably thinking, “well, what’s so awful about a ridge of high pressure in December? I’d rather have that than rain, wind, and snow.” But these wintertime ridges have two nasty side effects: inversions and poor air quality at low levels, and brutal “East Winds” through the Columbia River Gorge. And it’s all due to how little sunshine we receive in the winter.

Upper-Level Ridges, Radiation Imbalances, And Inversions

Upper-level-ridges are simply areas of relatively warm air and higher “heights” along a constant pressure surface (since warm air is less dense than cold air), while troughs are areas of relatively cool air and lower heights. We are most accustomed to viewing ridges and troughs as height lines along a constant pressure surface (ex: 500mb), and when we do this, ridges generally appear as the poleward perturbations of the upper level flow where warm, subtropical air is pushing poleward, while troughs are the equatorward perturbations in the jet stream that bring polar air to lower latitudes. There are exceptions – troughs can sometimes get “cut off” from the main westerly flow and ridges can force the jet stream to “split” into two sections – but this is typical pattern of upper-level ridges & troughs in the northern hemisphere.

Here’s a view of a ridge/trough in three dimensions.

In addition to having cooler air by definition, upper-level troughs are often associated with clouds and precipitation since they are associated with rising air. Ridges, on the other hand, are associated with sinking air, creating clear skies and further warming the atmosphere as these parcels experience adiabatic warming as they sink.

Radiation Budgets

We’ve established that ridging is associated with warm air and clear skies. But counterintuitively, these very characteristics can lead to a strong temperature inversion, with much colder air at the surface and often a persistent layer of fog/smog. This is due to the Earth’s surface having a net radiation deficit – in other words, it radiates more energy out to space than it receives from the sun and other sources.

The above diagram breaks down the major components of incoming and outgoing radiation as they relate to the Earth’s energy budget. The numbers represent average values over the entire globe; note how the total incoming radiation (341.3 W/m² ) is slightly less than the total outgoing radiation (238.5 + 101.9 –> 340.4 W/m²), with the Earth’s receiving a  absorbing a 0.9 W/m² surplus. This radiation surplus is why the Earth is experiencing global warming.

But even though there is a slight radiation surplus right now over the earth as a whole, there is a huge radiation deficit over the Pacific NW now that the days are so short and the sun is so low in the sky. This is particularly true when we have clear skies at night, as the amount of infrared radiation that escapes from the Earth to the atmosphere at night far outweighs any incoming solar radiation received during a crystal clear late November day.

Because the ground is far more effective at absorbing and emitting heat than the atmosphere, it experiences the most dramatic cooling when a stagnant wintertime ridge of high pressure is overhead. As an example, take a look at the forecast “sounding” at Olympia this Wednesday, when a strong ridge will be in place. At 7am, this sounding has sub-freezing surface temperatures and very high relative humidities, while temperatures rise all the way to the mid/upper 40s by the 2,500-3,000′ level.

Inversions are a very stable pattern because the temperature profile (increasing with height) resists the vertical displacement of air. As a result, they tend to trap moisture and pollutants at the surface, creating a persistent layer of fog or smog.

In some locations (such as London in 1952), these smogs can be deadly; in the Pacific Northwest, they are generally just a nuisance, but they can make for dangerous driving conditions and freezing fog, and the air quality can cause health problems in some sensitive individuals.

The ‘East Wind’

The inversions that build in the Willamette Valley and Puget Sound lowlands – while substantial, pale to those that form over the Columbia Basin. In fact, the inversions over the Columbia Basin are so strong that they form a very persistent “cold pool” that can enshroud lower elevations in fog and low stratus clouds for days on end. And because this cold air is so dense, it is also associated with locally higher pressure. And with higher pressure east of the Cascades and lower pressure west of the Cascades, we’ll get a persistent, easterly wind as long as the ridge of high pressure – and the inversions that result – stay in place.

And unlike the catastrophic firestorms on September 7-8 2020, the easterly winds will primarily be channeled through the Columbia River Gorge due to their higher density. This will cause the winds to be more localized but more intense, with notorious locations like the steps of Vista House at Crown Point receiving 60, 80, even 100 mph gusts! Back in January 2014, Crown Point measured an incredible 122 mph gust – I doubt it will experience 100+mph gusts this time around, but 80+ mph gusts and windchills in the teens are highly likely.

 

Forecast

A front will give us a nice shot of rain late Sunday night/early Monday morning to begin the long workweek. It won’t be anything out of the ordinary for late November, but it may very well be the wettest system until mid-December.

Beginning Tuesday, a ridge of high pressure develops over the area. The strength and position of this ridge will vary somewhat through the weekend, but overall it looks to remain in place and squash any storms that dare approach it out of existence.

One of those audacious systems will attempt to move inland Thursday. Unfortunately, this front’s attempt to punch through the ridge is like David trying to fight Goliath without a slingshot. This front doesn’t stand a chance.

With this ridge overhead, a deep cold pool will develop over the Columbia Basin, increasing offshore gradients and easterly winds through the Columbia River Gorge. The below image shows the pressure gradients at 4pm Wednesday – look at all those isobars stacked up against the Cascades!

This cold pool and easterly winds should gradually weaken Friday through the weekend, but they won’t go away entirely. Some of the inversions may begin to weaken west of the Cascades, but they are expected to become even more pronounced east of the Cascades. For example, notice how much warmer the temperatures are in the Idaho Rockies than the Treasure Valley region of Southern Idaho in the image below.

Our first real chance at rain returning to the area is likely Tuesday/Wednesday next week, as some models weaken this ridge and allow systems passing to the north to clip our area. However, other models keep the ridge in place through next weekend, resulting in more abnormally dry weather. We *may*not see a return to our typical, unsettled, winter La Nina pattern until the second half of December.

Thanks for reading, and prepare for those east winds and inversions this coming week!
Charlie