Ridge "Rollers"/Ring of Fire

The "Ring of Fire" (ROF)  is a weather pattern that happens during the summer months and its quite interesting too.

During the summer months a large area of high pressure often stagnates across the Southeast US. This high is an extension of the Bermuda-Azores high which is a semi-permanent weather feature throughout the year. This high is at its greatest extent and strength during the summer and early fall months and is often the reason why prolonged hot spells (heat waves) and droughts occur across the US during the summertime.

In the Northern Hemisphere a high pressure system has a wind flow in a clockwise direction. The air flows out from the center of the high. Aloft, the air sinks over the center of the high. This sinking is called subsidence. Air that sinks is very stable. Subsidence leads to CAPping ( CAP) in the atmosphere. As one moves away from the center of the high the amount of subsidence and CAPping decrease.
  
 Here is a 3D picture of the airflow in and around a high pressure system:

"Image courtesy of Windows to the Universe"

Here is what a  "typical" summer weather map might look like at a height of about 18,000 feet above the earth's surface:

Note the large high over the Southeast U.S. The two lines to its north and northwest is a "disturbance" in the jetstream flow.

Now keep in mind that the air flows in a clockwise direction around high pressure areas in the Northern Hemisphere. What can happen sometimes during the summer months is disturbances aloft called Ridge Rollers  rotate around the the stalled high, sometimes originating south of the high, moving from east to west along the high's southern perimeter (from Florida to Texas) then turn northward around its western flank along or east of the Rockies to the Northern Plains then eastward across the Great Lakes to the Northeast or New England States. As these disturbances move north and east around the western and northern perimeter of the high they interact with (disturbances in) the jetstream producing thunderstorms or on occasion an organized mesoscale convective system (MCS).

  

The map below illustrates this: The purple arrow is both the wind flow around the high and also the path that a disturbance might take. The the large orange arrow is the the jetstream.

Now if all of the necessary elements come together in both time and place these thunderstorm clusters can become quite intense. If the MCS does become severe and begins to move rapidly around the northern perimeter of the high (forward speed often 50 knots or greater) it can evolve into a derecho (pronounced deh-REY-cho). A derecho is often accompanied by extremely high straightline winds (sometimes a few tornadoes too) and some large hail. However the primary threat from a derecho is the strong and damaging straightline thunderstorm winds. Often times the derecho will affect many states over a relatively short time period ( in a period less than 24 hours). Derechos often form in a ROF or "Ridge Roller" weather pattern and often in conjunction with heat waves Derechos and Heat Waves. Derecho or not, this sort of a weather pattern can often result in successive days of severe weather in the same general geographic areas due to the passage of these MCS'. Here are a few examples:

• Severe Weather: 17-23 July 2010
• Heat wave ending severe events of 23-25 July 2010
• Derecho Series of July 1995

Last night and once again tonight a series of RRs continue to cause severe weather across the High Plains into the Western Great Lakes States. Here are the severe weather reports for Friday June 21st and so far for today the 22nd.

References: SPC: About Derechos

Ridge Roolers: Mesoscale Disturbances on the Periphery of Cutoff Anticyclones by Thomas J. Galarneau, Jr. and Lance F. Bosart