The title reads like an Onion article, but it’s true. A massive ridge of high pressure will move into western North America today and stay there for the foreseeable future, putting an abrupt end to our normally stormy weather. In the summer, such a ridge would give us extreme warmth, and this ridge will indeed send freezing levels skyrocketing to over 10,000 feet. Unfortunately, because the surface emits more radiation at this time of the year than it receives from the sun, an inversion will form over the Willamette Valley and Western Washington lowlands, resulting in cooler-than-normal nights and trapping moisture and pollutants near the surface. Ugh!
Our first order of business is naming this ridge. Similar ridges have been called “Ridgodzilla” or “Ridgmageddon” by Cliff Mass, one of my atmospheric science professors/advisors at the UW (check out his awesome weather blog!). The ‘zilla’/’mageddon’/’pocalyse’ suffix was coined by blogger Greg Swan in 2007 and has been applied to many meteorological phenomena, including every single Eastern Seaboard snowstorm since. “Death Ridge” is my umbrella term for all stagnant, wintertime ridges that create extremely strong surface inversions, and since our ridge will be the deathiest of Death Ridges, it needs its own name. One of the members in the PDX WX Analysis Facebook group dubbed this ridge “Ridgy McRidgeface,” which I thought was absolutely fantastic. Let’s just call him by his nickname – McRidge.
McRidge is currently centered around 130 degrees west ahead of a very deep (~964 mb) and weakening low pressure system that made landfall near Kodiak Island early this morning.
Before I go further, I want to talk a little bit about the relationship between giant lows like the one above and Death Ridges that have dramatic effects on weather over the North American continent. The Gulf of Alaska, Aleutian Islands, and Bering Sea are home to some of the most ferocious low pressure systems in the world due to (1) a strong jet stream, (2) ample moisture from the Western Pacific, and (3) global atmospheric circulation patterns that make this area ripe for storm development, such as the existence of Hadley and Ferrel cells (I’ll have a page on this later… it is outside the scope of this blog). Such strong low pressure systems give rise to a phenomenon known as downstream development, meaning that they affect areas downstream of the storm’s movement (for example, areas to the east of the storm if the storm is traveling to the east). The general pattern is to form a ridge directly downstream of the storm, then a trough downstream of the ridge, then another ridge downstream of the trough, and so on.
The animation of tropopause height below from professor Greg Hakim of the University of Washington shows how the formation of an upper-level disturbance affects downstream development. Because we have cold-core cyclones in the mid-latitudes and cold air is denser than warm air, cold air lowers the tropopause height and is correlated with troughs, and warm air increases the tropopause height and is associated with ridges.
A prime example of downstream development is the Bering Sea Cyclone of 2014, which was born out of the ghostly remains of Supertyphoon Nuri. After it had reached peak intensity as a 921 mb cyclone and was beginning to degrade, it formed a large ridge ahead of it in the Eastern Pacific, which then formed a large trough over much of the country and sent the dreaded “polar vortex” swinging through the Midwest into the Eastern Seaboard. This polar vortex was responsible for one of Buffalo’s greatest lake effect snowstorms in recorded history. Check out my previous blog posts about Supertyphoon Nuri/The Bering Sea Cyclone, the polar vortex, and the resulting lake effect snowstorm for more.
McRidge will move slightly east tonight and continue to amplify. By Tuesday morning, McRidge stretches all the way north into the Yukon.
But McRidge is just getting started. By Wednesday evening, McRidge will move ever so slightly eastward and amplify dramatically. By this point, freezing levels will range from 10-12,000 feet, which is more typical of summer than winter.
Ensembles say that McRidge will remain over the area and retain its strength for at least the next 10 days, potentially longer.
In fact, the most recent GFS operational run still has it centered over the area for hour 384. I believe it is a cardinal sin to show images from operational models more than 10 days out (this is due to chaos theory – small perturbations in the initial conditions have a huge effect down the road, and this is not as big of a problem with ensembles because you can take the mean of many different runs), so I’ll just put a link to it here.
Inversions and Fog
With such low sun angles and such warm temperatures aloft, a strong inversion will develop over Puget Sound and the Willamette Valley. Inversions are so-called because the temperature profile in them increases with height, which is the opposite of what is normal in the troposphere. Inversions are very stable because air expands/cools as it rises, meaning that rising air becomes cooler (and therefore more dense) in an environment that becomes less dense with height due to increasing temperature, therefore trapping that cool air the surface. Now’s a good time to say hi and make friends with the air around you, because it will be trapped over the region for the foreseeable future. Thankfully, current forecasts show less fog that you’d normally expect with such a strong inversion, meaning our high temperatures will make it up into the mid 40s and perhaps even flirt with 50 on sunnier days. Mornings should be cooler and foggier, particularly as we go into next week and in some of the more sheltered regions such as Olympia.
Columbia Gorge Winds
Even though our weather is extremely boring when a massive ridge is directly overhead, the entrance (and sometimes exit) of these ridges can generate some extremely strong winds for the Columbia River Gorge. This is because the Columbia Basin is a more effective radiator of heat than Western Oregon/Washington in the winter and is the first to develop a strong surface cold pool and inversion. Because cold air is denser than warm air, the formation of a cold layer near the surface while the pressure aloft remains constant increases the pressure at the surface, and this helps create an extremely strong cross-Cascade pressure gradient. Wind flows from high to low pressure through gaps in terrain like the Columbia River Gorge and accelerates at the exit regions of these gaps. During periods like this with cold, dense air, there is also a significant contribution in exit-region wind speed from the depth of cold air collapsing as the height of the terrain guiding the cold air decreases.
Crown Point, Oregon is an exposed headland 20 miles east of Portland notorious for its incredibly strong easterly winds during these cold pool setups. I’m hoping to go midday Wednesday and experience some hurricane-force gusts on the steps leading up to “Vista House,” a tourist stop on the point that opened to the public in 1918. Winds accelerate over the steps, and gusts of 100 mph are not unheard of during the most intense easterly gap flow events. This morning’s WRF-GFS has KTTD-KDLS pressure gradients around -8 to -9 mb, which is enough for some serious wind.
Cold Weather for the Eastern U.S.
Downstream of our massive ridge, we’ll see a large trough over the eastern half of the country that promises to bring frigid temperatures and potentially heavy lake-effect snow. By Friday, a weak low pressure system is expected to form over the Upper Midwest, bringing a reinforcing shot of frigid air into the lower 48. High temperatures in the coldest places (like International Falls) could have trouble making it to 0 degrees on Friday.
As this system moves east, it will drag cold, northeasterly winds in its wake. This is a classic pattern for lake-effect snow, so I wouldn’t be surprised to see some lake-effect headlines next weekend.
To sum everything up – the weather is gonna be pretty darn boring over the next 10 days and possibly longer. But we’ll get a pretty epic easterly wind event Wednesday/Thursday as a consolation prize. And I hope you learned a little bit about the importance of downstream development!
Thanks for reading, and have a great week!
Vertical Structure of Low Pressure Systems. (n.d.). Retrieved December 04, 2017, from http://meteorologytraining.tpub.com/14312/css/14312_75.htm
Hakim, G. (n.d.). Downstream Development. Retrieved December 04, 2017, from https://atmos.washington.edu/~hakim/442/eady_dd.html