The events unfolding across the western United States this week are remarkable and include heavy snowfall along the Front Range of Colorado, severe downslope winds in Utah, and extreme fire behavior in the Pacific states. We'll get to some of the details of what happened in those areas eventually, but to start, let's talk about why this is such an exceptional event.
To do this, we must take a global perspective as everything starts with the highly anomalous large scale flow that has developed over the past few days. A critical player in all of this appears to be Typhoon Julian, which on August 30th was near the Philippines but on track to move northward and interact with the mid-latitude jet stream (see red arrow below).
Whenever there's a tropical cyclone recurving into the midlatitudes and interacting with the jet stream, I go on high alert. Although this doesn't happen all the time, such interactions often lead to a rapid amplification of the jet stream flow, with the flow becoming "wavier" as the ridges amplify and the troughs strengthen. This typically happens in domino-like fashion, with the ridge downstream of the typhoon amplifying, followed by the trough downstream of the ridge, and so on. This is a process known as
downstream development.
By September 2nd, the seeds were sown as the decaying Julian approached the Korean Peninsula and the downstream ridge off the coast of Japan began to amplify. This is the beginning of a period during which the jet stream pattern over the North Pacific and western North America would strongly amplify from a state that was already somewhat amplified.
By September 5, the pattern was highly amplified with strong ridges off the coast of Asia and over western North America and the adjoining eastern Pacific and a deep trough near the dateline. Much of the western U.S. was baking to a crisp after an already long hot, dry, summer.
However, Mother Nature was not finished. At low-levels beneath the upper-level trough near the dateline, a mid-latitude cyclone was developing.
Typhoon Julian was the left jab, but this midlatitude cyclone was the right hook. It moved northward into the Behring Sea, with warm moist air transported northward deep into Alaska and northwest Canada. Without getting into hard-core geophysical fluid dynamics, such a pattern favors the development of a strong upper-level ridge downstream, as occurred over southeast Alaska (black 500-mb contours below), which in turn strengthened the high-pressure system at low levels over western Canada.
At this point the door was open for further amplification of the upper-level flow and the push of an anticyclone (high pressure system) and cold air into the northern and Central Rockies. By 1200 UTC yesterday, cold air was surging down the east side of the Rockies, stimulating an upslope snowfall event along the front range, cold air was pushing across the Wasatch at the same time an upper-level trough was forming to drive strong northeasterly flow at crest level, and strong offshore winds were developing over the Pacific Northwest (and eventually parts of California) as high pressure pushed into the Columbia and western Great Basins.
That's an awful lot to pack into a blog post, but it often takes a series of events to drive the atmosphere into outlier mode, as has occurred over the past several days.
That's the large-scale setup. What has happened across the west is also dependent on the interaction of the large-scale flow with regional terrain features and variations in flow direction with height.
Let's focus on 1200 UTC (0600 MDT) yesterday morning when we were experiencing severe downslope winds, snow was just beginning to develop in Boulder, CO. At that time, high pressure was centered over southern Montana, but a pronounced high-pressure ridge extended southward to the east of the Colorado Rockies. That ridge was associated with a surge of cold air along the front range, lowering snow levels.
At the same time at 700 mb (about 10,000 feet above sea level), the flow over the northern front range was easterly and a precipitation system associated with the upper-level trough was moving into the area. This favored snowfall in that area. At the same time, strong easterly flow originating over southwest Wyoming was directed across the northern Wasatch, driving downslope winds along the Wasatch Front.
So, while the large-scale setup requires a global perspective, what happens locally over the west requires a more regional perspective and consideration of not only what is happening at low levels, but also aloft. Due to lack of time I won't get into it here, but the severity of the downslope wind storms along the Wasatch Front yesterday was probably also related to how the wind direction changed with height, which allowed for the trapping of mountain-wave energy at low levels.
I would be remiss not to also comment here about the terrible tragedy to our west as wildfires are being fanned by strong winds associated with this strong high-pressure system. So much is happening that I'm just going to embed a satellite loop below of the incredible fires in Oregon, which spread rapidly in strong offshore winds.
Due to lack of power and spotty cell service, I still haven't caught up on everything happening to our west, but the situation appears awful for many communities. I consider myself fortunate to have lost only some trees from this event.
Really nice analysis! A great reminder of how all scales can interact.
ReplyDeleteMaybe I will get that extra pallet of m.r.e's......
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