By now, many of us have heard of the term “bomb cyclone.” Just like “polar vortex,” “bomb cyclone”/”bombogenesis”/”meteorological bomb” (and many other bomb-related derivatives) has actually been in the meteorological literature for decades, first entering the vernacular in the Bergen School in the 40s and 50s to describe the explosively-developing cyclones over the North Atlantic. Over the past couple years, the “bomb cyclone” designation has been reserved for strong Nor’Easters on the other side of the country, so it’s nice to see a West Coast storm worthy of such an honor.
Credit: The Oregonian
To quote the Medford NWS forecast discussion from 1532 this afternoon, this cyclone is “unprecedented in its strength and track.” While Southern Oregon and Northern California have seen higher winds (ex: the Columbus Day Storm), this storm will approach from the northwest, which is highly unusual for a meteorological bomb in the Northern Hemisphere, let alone the Pacific Northwest (storms tend to approach from the SW and curl northward as they undergo bombogenesis). The northwest approach of this storm means it will be affiliated with a much cooler airmass than most for our major storms, and heavy snow will fall to low elevations, with some flakes potentially even reaching sea-level in the Southern Willamette Valley. A storm on January 1, 2004 followed a similar track, bringing a blizzard to southern Oregon and even a few inches of snow to the Portland metro area. This storm will be warmer than the 2004 storm, but it will be much larger and more intense, and it will likely be the most disruptive weather event Southern OR/Northern CA has seen in decades. If there is a silver lining, it is that it the worst of the storm will occur Tuesday night and early Wednesday. If the storm was just a day later, I bet a lot of Thanksgiving hosts would be eating leftover turkey through Christmas!
We’ve got a ton to talk about with this system. Let’s start with the current conditions, proceed to the forecast of the storm itself as in tracks inland, and finally talk about the impacts – including the ones for my compadres in the Portland metro area.
Current Conditions:
We currently have a rather unique setup, with several discrete features coming together to create a favorable environment for bombogenesis. We have an “atmospheric river” of moisture sandwiched between the counterclockwise circulation around the upper-level low near 175W and the ridge of high pressure over the Eastern Pacific, an extremely strong jet stream due to the large temperature gradients in the upper atmosphere between these upper-level highs/lows, and a strengthening shortwave trough over the Gulf of Alaska. This shortwave trough is affiliated with an “open wave” of low pressure at the surface, and this wave will mature into an intense extratropical cyclone as it taps into the extremely strong jet stream and subtropical moisture affiliated with it.
Credit: UW
Although this is a rather unusual pattern, similar patterns have also given rise to intense, fast-moving extratropical cyclones. For example, take the November 17, 2015 storm (a storm that ravaged Eastern Washington in particular – I am doing a writeup on it for Wolf Read’s The Storm King website and hope to have it completed within a few weeks) – note the juxtaposed deep upper-level-low and strong Pacific high. The colors show the “total precipitable water” in the atmosphere, which is the amount of liquid water you would have if you condensed all of the water vapor in a given column of the atmosphere.
Credit: WeatherTogether models
Compare that to our current setup, and you’ll see that while the patterns are slightly different (our current one is more amplified and doesn’t have an atmospheric river pointed directly at the Pacific Northwest), they both share many features, such as the juxtaposed upper-level low and high over the Pacific, the sharp shortwave trough over the Gulf of Alaska, an extremely strong jet stream, and a deep trough over the Western US. I’ll definitely be keeping a watch out for this type of pattern in the future!
Credit: WeatherTogether models
The atmospheric river and circulations (counterclockwise around the low, clockwise around the high) can be seen in the precipitable water graphic below.
And the surface low can already be seen on the incoming 00Z WRF run as a 1022mb open wave between 140 and 150 degrees west. Note the huge temperature gradient (i.e. stationary front) along the 50 degree latitude line – extratropical storms derive their energy from baroclinicity (horizontal temperature gradients) in the atmosphere, so the sharp temperature gradient, coupled with favorable upper-level dynamics (due the strong jet and shortwave trough over the Gulf of Alaska) and ample moisture from the atmospheric river bodes well for explosive cyclogenesis.
Forecast:
Tor Bergeron, a Swedish meteorologist of the Bergen School famous for describing the mechanism by which precipitation is formed in clouds (known as the Bergeron Process), defined a cyclogenic bomb as an extratropical cyclone that deepens at least 24mb within 24 hours. This storm will easily pass the “bomb test,” as it is predicted to deepen to a mid-970s cyclone before making landfall between Cape Blanco and Brookings Tuesday evening. If we assume (based on the WRF above) that the low was ~1022 mb at 4pm and will make landfall around 4pm tomorrow as a mid 970s mb cyclone, that’s a drop of 45-50 mb in 24 hours, doubling Bergeron’s threshhold.
The following images show the progression of the cyclone in 3-hour intervals from 4am to 7pm Tuesday. Note the extremely strong winds (white = sustained winds over 55 knots!) to the west of the low center – this is known as the “bent-back-occlusion,” which is where the occluded front wraps around the low pressure center, creating extreme gradients and the strongest winds you’ll find in the entire storm.
By 7 am, the low has already deepened from 1002 to 994 mb. The bent-back occlusion begins to form.
At 10 am, the low continues to intensify and move east.
At 1pm, the storm’s powerful cold front lashes the Southern Oregon Coast, bringing high winds from Brookings to Bandon. These winds will likely be enhanced by the existence of a coastal low-level jet (CLLJ) – these coastal jets form when the terrain along the coast acts as a barrier to the low/mid-level flow and “traps” a fast, low-level jet along the coast. Coastal jets are responsible for some of the Coast’s greatest windstorms (such as the Great Coastal Gale of 2007), and I wouldn’t be surprised if Cape Blanco, an exposed headland with locally higher winds than the surrounding areas, had a triple digit gust either during this time or when the bent-back occlusion comes ashore late Tuesday afternoon.
At 4 pm, the low is just off the Southern Oregon Coast and is at maximum intensity. Portland will begin to see a surge in easterly winds through the Gorge during this time.
By 7pm, the low has moved onshore and is beginning to weaken rapidly as it encounters the Southern Oregon Coast Range, Siskoyous, and Southern Cascades. Strong east winds will persist for the Portland metro area through Tuesday night, along with strong northerlies through the Willamette Valley. 
We can also look at an ensemble of forecasts – many different forecasts made with slightly different parameters/initial conditions. The European ensemble members all show this low near the Southern Oregon coast as a ~965-975 mb cyclone – slightly stronger than the WRF is advertising. The fact that every single ensemble member shows the low near here gives us high confidence that it will make a Southern Oregon landfall and not unexpectedly make landfall further north.
With such strong winds off the coast, this storm will generate truly massive waves. NOAA’s WaveWatch iii model shows a small section of 13-14 meter (42-45 foot) waves in the bent-back occlusion at 4pm Tuesday. Don’t even think about going for a boat ride!
The upper-level charts give us some insight into why this storm will develop so explosively. Look at the temperature contrasts this thing is developing along, and how it is able to retain a subtropical connection while simultaneously bringing a shot of very cold air into the Pacific Northwest. This is a weather geek’s dream, and it promises to be a remarkable storm.
Impacts:
The biggest impact from this storm will be damaging winds throughout Southern OR/Northern CA, with extreme winds along the coast and offshore. Hurricane Force Wind Warnings are in effect for many offshore locations, and most inland spots have high wind warnings or wind advisories in effect. While the Portland Metro and Willamette Valley will definitely escape the brunt of the storm, strong East Winds through the Gorge and north winds through the Valley will ramp up Tuesday afternoon, peak Tuesday night, and decrease early Wednesday morning.
Heavy snowfall over the mountains will combine with these extreme winds to render all the passes of Southern OR and Northern CA, well, impassable. Don’t even think about trying to travel across the mountains from midday Tuesday to Wednesday morning. Snow levels will drop close to the Valley floor Tuesday night into Wednesday, and Eugene could pick up an inch or two of the white stuff.
Despite all of the snow, precipitation totals won’t be remarkable, with 1-2 inches for Southern OR/Northern CA, more over terrain. So there is no flooding threat from this system.
The NWS Western Region Headquarters Page is very colorful, with more watches and warnings than you can shake a stick at. Yet, through all of the chaos, nothing is currently in effect for NW OR/SW WA or even the Seattle metro area.
Will Portland see snow?
Let’s be honest… this is the question that’s really on our minds… and you might be mildly annoyed that I’m only getting to it now. Most models say no, and I agree… this storm is just headed too far south, and we’ll be dry by the time it is cold enough for snow.
However, it’s at least worth mentioning that the precipitation shields on the northern sides of these storms sometimes extend further north than modeled. In any event, I think that snow is unlikely, but if the low tracks a bit further north than modeled and the precipitation extends north into the Portland metro area, some wet flakes to fall in the Portland metro late Tuesday night/early Wednesday morning. I don’t expect any accumulations below 500 feet, and places from 500-1000 feet would be lucky to get a slush 1/2 inch.
I gotta get to bed… it’s late and work begins early for me! But thanks for sticking with me through the post.
Stay safe,
Charlie
