Cyclone over Lake Michigan: Flying into O'Hare is always interesting!

Cyclone over Lake Michigan on September 26, 2011
Image from GOES NASA

Last Monday I was returning from the west coast, my flight delayed nearly an hour and a half by "weather in Chicago."  When we finally did depart and get over to the Chicago area four hours later, the landing was a prolonged bumpy ride, as was the puddle jumper down to Urbana. Described by The Capital Weather Gang as a "lumbering, sprawling cyclone," this weather system dominated the mid-West for three days from September 25-27.

This storm is a classic example of the mid-latitude cyclones that dominate the weather in the U.S. Air circulates counterclockwise around a low pressure core (in the Northern Hemisphere). Warm air pushing north and cold air pushing south get wrapped in bands around the center of the cyclone. Air in the low-pressure center rises to form towering clouds, and the comma-shaped tail consists of warm moist air (clouds) and colder dry air (clear areas). In the mid-west, cold air eventually "wins out and wraps completely around a storm," forming a so-called "cold core" storm.  It gets cut off from the jet stream and so, in this case, stalled near Chicago.  The Earth Observatory WWW site which featured this storm has a great animation of the storm from GEOS, here.

These cyclones develop when a trough in the jet stream interacts with a surface frontal zone. The essential low-ressure system forms if there is wind shear (winds increasing with height in the atmosphere) and convection (thermal instability). Three factors lead to formation of the low pressure at the surface: diverging airflow at high altitudes; warm, moist air flowing in at low and mid-levels; and latent heat release.  The storm typically has four stages, all of which can be seen in the video above.  First, a leaf cloud forms on the east side of the trough, a zone of deep and thick clouds. Within the leaf cloud, air is rotating and as the system develops, the clouds develop into a comma shape, which takes on various forms depending on the eastward motion. As the storm develops, the low-pressure circulation gets cut off from the jet stream and without the momentum from the jet stream, the system loses its ability to deepen.  The cold front overtakes the warm front and the system becomes "occluded." After this, the storm weakens as the upper-level winds tear it apart. The comma head may lag behind and continue to rotate, a phenomenon seen on the video above. The discussion and schematics here are very helpful in understanding the process of "cyclogenesis."