Monday, August 24, 2015

Thinking About Cutoff Lows

Interesting to see the weather models argue with one another. This week, an Aleutian cutoff low will descend off the Oregon coast. The question is: exactly where will it descend? Far west of the 140 degree longitude (bad), or closer in (good)? Forecasting the behavior of cutoff lows is weather's greatest challenge, particularly when the low doesn't even exist yet, yet the possibility of rain in the Pacific Northwest and Northern California depends on the answer to that question.

A cutoff low is a more-or-less self-contained rotating low pressure system often seen in mid-latitudes. Rotating flow like you see in hurricanes (just not nearly so powerful). Flow is momentarily diverted away from the jet stream to initiate the cutoff low, then resumes its flow. The spun-off cutoff low wanders around for awhile, then later, it encounters the jet stream again and is reabsorbed. Cutoff lows are responsible for a large amount of mid-latitude weather, but they are squirrelly. Asymmetric cutoff lows being reabsorbed into the jet stream can generate huge amounts of rain, but symmetric cutoff lows might not generate any rain at all. Forecasting what you'll actually get is a big challenge.

As Jerry notes, the formation of a cutoff low is analogous to the way an oxbow lake forms when the channel of a river cuts across the neck of a meander. It's a good analogy! And the oxbow lake eventually gets reabsorbed by the meandering river.

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