What makes a tornado? And why does the American Midwest have more than any other area of the world? UPDATED 5:00 Central time

Shawnee, Oklahoma tornado, May 19, 2013
photo by Brett Wright
from http://www.tornadotitans.com

The forecast map posted for May 20, 2013
on the tornado chaser's WWW site at
http://www.tornadotitans.com

Note added 5/22: Here is an excellent description in CNN news of tornado formation from a NOAA weather researcher, and here is an excellent technical discussion by Cliff Mass (on his May 21 post).

Update: A one-mile wide tornado, estimated already to be an EF-4 or EF-5, moved through Moore, Oklahoma, south of metropolitan Oklahoma City, destroying houses over a significant area along a two-mile wide track. Reports are that two elementary schools, Plaza Towers and Briarwood, were hit, and that the number of children taking shelter in those schools and currently unaccounted for is up to 75. Moore Medical Center had to be evacuated, with patients being evacuated to Normal Regional Hospital and Health Plex Hospital. There is a video of the tornado on this CNN website (at least until they replace it with something else). Moore was devastated on May 3, 1999 when 36 people died and over 8,000 homes were badly damaged or destroyed. The hook echo trace from that tornado is discussed at the end of this post. There is an incredible time-lapse photo of this tornado here, as long as it keeps being posted. It's absolutely frightening. It shows the strong rotation that occurs in these storms, discussed below. My heartfelt sympathy and prayers to the people affected by this storm.



NOTE: Between the time I started this and posted it, the forecast for storms in my area of central Illinois has gone from "strong severe storms predicted for 5:00" to "isolated thunderstorms." Forecasts from the commercial sites, such as weather.com or wunderground.com vary considerably. Here's the National Weather Service site--you can type in your own state and county or city.
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     I love meteorology and weather forecasts, but sometimes I find the jargon baffling. During tornado season (springtime, April-June in Illinois), we hear the words supercell, mesocyclone, updraft, instability, wind shear, dry line, hook echo, tornado warning, tornado watch, jet stream trough...  Here I'll try to put these all into context in a "primer" on tornados.
     The first ingredient needed for tornado formation is a storm, but not just any old storm. The storm has to be a so-called "supercell thunderstorm." A supercell is an organized thunderstorm that contains a very strong, rotating updraft--called a mesocyclone. Supercells usually exist apart from other storms because they grab up energy and moisture from miles around, hogging it to themselves, so to speak. Because of hoarding, supercells can dominate the local weather up to 20 miles away (but can also be buried within a squall line).

     Next,  another essential ingredient for supercell and tornado formation: wind shear. This simply means that there needs to be winds of differing strength between the ground and the upper atmosphere. In the figure with the red arrows here (from Wiki), imagine that the wind velocity is about zero on the ground, and "strong" at higher elevations. If you hold your palms together and move one relative to the other, you are "shearing" your hans. Wind shear sets the air near the ground spinning around a horizontal axis (the figure with the red arrows).  As such air starts to rise ("convect," like in a convection oven) in a thunderstorm, the spinning air is drawn upward, that is, the spin axis changes from horizontal toward vertical (the other  figure). If conditions aren't right to produce these rotating updrafts, no tornado danger!
     So, a mesocyclone forms within a thunderstorm when air rises and rotates about a vertical axis. They are typically 2-50 miles in diameter, but can be bigger or smaller.

The typical meteorologic setup for severe weather.
(This is generic, not specific to today's situation.)
From NOAA via Wiki.
(Note: it always bothers me that the
boundary of Tornado Alley is limited to the US in
most illustrations, like this one. Tornado Alley
extends up into southern Canada, and down
into Northern Mexico. 

Tornados form where three different types of air come together, illustrated to the left. First, there must be warm and humid air near the ground. Typically this hot humid air is driven by winds originating in the Gulf of Mexico (the green area). Second, there must be cool, dry air  being blown by westerlies from the northwestern states and the Rocky Mountains (the blue area). These two air masses are separated by an imaginary line that separates the warm moist air from the cool dry air, the "dry line." It occurs roughly where the 55 F line lies north-south across the continent (in the illustration to the left, it is the unlabeled dashed line in Texas and Oklahoma). In this illustration, tornados form along the dry line.
     In the illustration above, warm dry air is flowing from the south into Texas, Oklahoma and Arkansas. If the temperature is the same everywhere, as it tends to be on the east side of the dry line, the moist air is denser than the dry air, and so it will "wedge under" the  dry air. The dry air forms a "cap" that allows the warm air near the ground to get even warmer. As I write this (1:30 in the afternoon, with the severe storms originally predicted to hit in a few hours, the humidity is increasing noticeably, but it is not yet intolerable. The temperature is 88 F, the relative humidity is 35%, and the dew point is 57 F.  If that dew point gets up to about 70, trouble almost inevitably looms. The apparently low dew point now is probably one reason that the forecasted severe weather this afternoon is no longer in the predictions.) As the temperature increases in the warm moist air, the density decreases until it is less than that of the cap, and it wants to rise. That is, the situation becomes unstable.
     Next, something must change to allow the warm, humid air near the ground to punch through the hot dry cap. Note in the figure above that the jet stream (blue) forms a "trough" of cold air coming down from Canada, whereas the red line with dots, a so-called "warm front" separating the warm and cold air exists over Missouri and Kansas. The jet stream is always acting to push weather systems from west to east across the U.S. colliding at this time of year with the warm front. At this collision, a severe weather pattern can develop. Air aloft moving from west to east begins lifting the cap and warm air, allowing upward movement (updrafts) and, sometimes, explosive thunderstorms to develop as the warm air rushes upward. 
     Within a supercell, "hook echos" on radar are an indication of a tornado-producing supercell. The first hook echo documentation occurred on April 9, 1953 by the Illinois State Water Survey, now a part of the University of Illinois, my home! A tornado watch may be issued when conditions are right for a possible tornado, but a tornado warning usually implies that a tornado has been sited. The existence of a hook echo on radar is sufficient to prompt the National Weather Service to issue a tornado warning, even if a tornado has not been sighted on the ground.

So, for those residents of Texas, Oklahoma, Arkansas, Kansas, and Missouri still under the gun today, best wishes for your safety.

An example of the structure of a supercell
showing the hook echo part of the
mesocyclone on the lower left, and a tornado
within the hook.
NOAA image from here.  

      

Here are a few sites that I used to create this post. You can get more details there:

http://en.wikipedia.org/wiki/Hook_echo
http://www.wunderground.com/