Answers: Extratropical Cyclone Winds

By Jack Williams ©2015

Q: My question is about your article in the November  AOPA Flight Training magazine on tropical cyclones.  I’m trying to reconcile two potentially different ideas: first, that a cyclone has swirling air (which I assume to mean the the air masses are rotating around the Low), and second, that the warm front and cold front are generally in the same place (3 o’clock and 6 o’clock, respectively as in the article’s example).  I would expect the fronts to rotate around the Low.  What am I missing here?  Thanks in advance and for your great articles.

Sam, Petaluma, California

A: It’s too late for me to add the couple of sentences to the Flight Training November article that would have kept your question from arising: During the storm’s life, the cold and warm fronts rotate partly around the storm’s center with the cold front generally moving faster than the warm front.

Forecast for Oct. 15, 7 a.m. ET. Storm center forms. NWS image.

Forecast for Oct. 15, 7 a.m. ET. Storm center forms. NWS image.

The National Weather Service’s online School for Weather Norwegian Cyclone Model page has images showing the birth and evolution of an extratropical cyclone showing this.  But, like most introductions to weather topics, this presentation is very simplified and the real weather rarely goes through a clear-cut sequence.

It’s called the “Norwegian cyclone model” because a group of atmospheric scientists working in Bergen, Norway during and after World War I developed it. Like all simplified, mental models of complex phenomena, it helps you make sense of events that aren’t obviously connected. But, real events rarely exactly match the idealized model.

The image above and the two below are National Weather Service forecasts of the evolution of an extratropical cyclone over three days that happens to come close to the idealized model shown in the NWS online school link above.

October 11, 7 a.m. ET forecast. Low moves to the northeast as cold front advances across the Southeast and warm from moves to the north. NWS image.

October 16, 7 a.m. ET forecast. Low moves into western North Carolina as the cold front advances across the Southeast and the warm from moves to the north. A secondary low has formed on the Coast -- this often happens. NWS image.

These images are from the NWS Hydrometeorological Prediction Center’s seven-day sea-level pressures and fronts Web page. This page is a good place to go for a general picture of what the weather is expected to do. When using it you have to keep in mind that the further in the future you go, the less likely the actual weather is to agree with the forecast.

While the winds generally follow a counterclockwise path around a low-pressure center they aren’t making a perfect circle or flowing at the same speeds. On upper air maps, the winds will be generally following the lines of equal pressure.  On surface maps, like those to the left, the winds will be flowing at a slight angle across the isobars (equal pressure lines). In the northern hemisphere the winds are pointed to the left of the isobars, which means  they are generally spiraling in a counterclockwise direction toward the low-pressure center.

The close together the isobars, the faster the winds will be blowing. In other words, a glance at a surface chart will give you a good idea of where to expect the fastest winds.

Forecast for October 11, 7 a.m. ET. An occluded fornt - the purple line - has formed near the storm center.

Forecast for October 17, 7 a.m. ET. The secondary low on the coast is now the storm center and the original low has faded away. An occluded front - the purple line - has formed near the storm center off the New England Coast. NWS image.

Near the end of the storm’s life an occluded front forms. Unlike the cold and warm fronts, which are boundaries between two air masses, an occluded front (also called an occlusion) is an area with boundaries between warm, cool, and cold air with one of the boundaries aloft. Some of an extratropical cyclone’s worst flying weather is often found in an occlusion.

An occluded front is often the first part to arrive with storms that come ashore on the U.S. West Coast.

I always enjoy hearing from pilots with questions about the weather because all pilots should be students of the weather. No matter how big, fast, expensive, and high-flying your airplane and no matter how many hours of pilot-in-command time you’ve logged, you  can’t escape the weather unless you’re piloting the Space Shuttle in orbit.

I recommend my latest book, The AMS Weather Book, as a good place to begin. It’s written for intelligent adults who don’t necessarily know much about science. The book’s Web site has information usually found a a book’s foot or end notes, which includes many likes to online sites as well as other books and articles.

To go further in learning more about the weather, I recommend a college-level text book. My favorite is Meteorology Today by C. Donald Ahrens. Like most college text books, it’s expensive. But, I’ve found that you can pick up a used copy of an earlier edition for reasonable prices.

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