A senior researcher familiar with the emerging science of weather extremes tells me by email:
“This is another example of a very amplified or wavy pattern in the jet stream causing unusual and slow-moving weather patterns…”
“Through the first half of September there has been a very strong northward bulge (or ridge) in the jet over western Canada that created the easterly flow south of the ridge that brought moist air from the Gulf of Mexico up the east side of the Rockies where it condensed into rain — lots of it in Boulder.”
”I can’t say it’s connected to Arctic change,….”
“…but it’s another example of the kinds of wavy jet stream patterns that we expect to see happen more frequently in the future. Just amazing photos and videos coming out of this story!”
Here’s how it happened: A blocking pattern has set up over the western United States, drawing a conveyor belt of tropical moisture north from coastal Mexico. Blocking patterns form when the jet stream slows to a crawl, and weather patterns get stuck in place. When all that warm, wet air hit the Rocky Mountains, it had nowhere to go but up, pushed further skyward by the mountains themselves. By some measurements, the atmosphere at the time of the heaviest rains was the among most soaked it has ever been in Colorado.
Gauge measurements show floodwaters in Colorado have now exceeded the legendary Big Thompson Canyon Flood of 1976, the flood of record for the region. In downtown Boulder, a meandering creek expanded 40-fold in just a few hours. More rain is on the way this weekend.
Earlier this summer, similarly devastating floods have hit Las Vegas, Nevada, and Calgary, Canada, and the basic cause has been the same: tropical moisture making its way to places it shouldn’t be, in amounts rarely seen before. This is not a fluke event, nor something that’s going to go away.
Why it will keep happening: Blocking patterns are fertile ground for extreme weather. A blocking pattern near Greenland was also to blame for steering Superstorm Sandy toward the east coast of the United States last fall. Persistent high pressure this year in the western United States has led to what is (so far) California’s driest year on record. That, in turn, fueled last month’s massive Rim Fire in Yosemite National Park, which grew to a size larger than New York City.
This map of US rainfall over the past 30 days shows record flooding in between expanses of drought—all of it extreme weather. Map from NOAA
According to press accounts last night and this morning, 1,200 remain missing. Heavy rains over the weekend kept rescue choppers from flying.
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