Zonal Flow And Shortwaves: Why We Haven't Had The Usual Stormy November

This is an example of zonal flow, a flat west-to east
orientation of the jet stream across the U.S. That tiny
little dip in the flow over the Great Lakes is called
a short wave. We feel it as weak, fast moving little
storms, as opposed to the much bigger storms we're
accustomed to this time of year.

After that terrible windstorm in late October, we sure weren't looking forward to November, which has a stormy reputation both here in Vermont and in the nation as a whole 

The month will draw to a close in less than a week, and so far, almost nothing in terms of storms.  And nothing big is in the forecast for the next week.

Oh sure, there's been some rain and snow locally. It's been windy at times. And other parts of the nation had spells of bad weather. But there's really been nothing dramatic, like we've seen in most past Novembers.

What's going on?

Blame something called a zonal flow.

The jet stream usually meanders quite a lot especially in the colder months. The jet stream is a river of high speed, upper atmospheric air that circles the globe. It runs generally west to east in the northern hemisphere, but takes frequent big dives southward in some spots, and lifts dramatically northward in others. Those big excursions north are called ridges.

Near where there's these big dips, you get a storm. In the United States for instance cold air might slide southward along with the jet stream, and warm air, and moisture from the Atlantic Ocean and Gulf of Mexico ride northward where the jet stream curves to the north.

That clash is a recipe for the formation of storms. So you get lots of rain, snow, wind and sometimes severe weather.

This November, the portion of the jet stream that comes across the United States has been mostly just west to east, with no pronounced dips or curves to the north to speak of.  In meteorology circles, we call that a zonal flow.

The jet stream has been entering North America over the Pacific Northwest of the United States and British Columbia, Canada. That has brought repeated storms off the Pacific Ocean, which have caused flooding in that region.

The moisture from these storms gets wrung out over the mountains. The storms, now small, zip west to east across the northern United States. These are called short waves. They're basically just mini-dips in the jet stream, as opposed to the big dips that usually cause those big storms this time of year.

These shortwaves, being just part of a mini dip in the jet stream, are moisture starved, since there's no big dip in the jet stream to grab Gulf of Mexico moisture. They also move fast, so they don't last long in any one place.

Do remember that I said that the jet stream is a fast moving river of air. These shortwaves can grab some of that high speed air and bring it to the surface. That largely explains why it has been so windy this month, despite the lack of big storms.

Generally speaking, this month's zonal flow has not been perfectly west to east. The jet stream is a little further north in the western half of the nation, and that has allowed occasionally record heat to spread over the Southwest and southern Plains.

Here in the Northeast, the zonal flow jet stream has been coming our way at an angle that's more west-northwesterly. That orientation has allowed those short waves that zip on through to grab little pieces of cold Arctic air from Canada and spread it over the Northeast in brief bursts.  That's why November has been a wee bit cooler than normal this month in Vermont.

Zonal flows always end eventually. The jet stream will buckle, and we'll go back to big dips and ridges. My guess is that will happen by mid-December or even earlier. And, we'll go back to our normal stormy winter weather.