Severe storms are more likely to form when there is vertical wind shear (the picture above is on page 156 in the ClassNotes).  Wind shear (Point 1) is changing wind direction and/or wind speed with distance.  In the case shown above, the wind speed is increasing with increasing altitude, this is vertical wind shear.

A thunderstorm that forms in this kind of an environment will move (at perhaps the average of the speeds at the top and bottom of the cloud) (Point 2).  The thunderstorm will move to the right more rapidly than the air at the ground which is where the updraft begins.  Rising air that is situated at the front bottom edge of the thunderstorm will find itself at the back edge of the storm when it reaches the top of the cloud. 

This produces a tilted updraft (Point 3).  The downdraft is situated at the back of the cloud.  The updraft is continually moving to the right and staying away from the downdraft.  The updraft and downdraft coexist and do not "get in each others way."  If you remember in air mass thunderstorms, the downdraft gets in the way of the updraft and leads to dissipation of the storm.

Sometimes the tilted updraft will begin to rotate.  A rotating updraft is called a mesocyclone (Point 4).  Meso refers to medium size (thunderstorm size) and cyclone means winds spinning around low pressure (tornadoes are sometimes called cyclones).  Low pressure in the core of the mesocyclone creates an inward pointing pressure gradient force needed to keep the updraft winds spinning in circular path.

The cloud that extends below the cloud base and surrounds the mesocyclone is called a wall cloud (Point 5).  The largest and strongest tornadoes will generally come from the wall cloud. 

Note (Point 6) that a tilted updraft also provides a way of keeping growing hailstones inside the cloud.  Hailstones get carried up toward the top of the cloud where they begin to fall.  But they then fall back into the strong core of the updraft and get carried back up toward the top of the cloud.

An X-ray image of a person doesn't usually show the entire body, often just the bones and skeleton inside.

This map, created by Alex Matus and from a Wikipedia article on tornado climatology (https://en.wikipedia.org/wiki/Tornado_climatology) shows the average frequency of tornado occurrence in the US.	This map from the National Centers for Environmental Information (https://www.ncdc.noaa.gov/climate-information/extreme-events/us-tornado-climatology) shows the average number of tornadoes by state.

Tornadoes begin in and descend from a thunderstorm.  You would usually see a funnel cloud dropping from the base of the thunderstorm.  Spinning winds will probably be present between the cloud and ground before the tornado cloud becomes visible.  The spinning winds can stir up dust at ground level.  The spinning winds might also be strong enough at this point to produce some minor damage. 
In Stage 2, moist air moves horizontally toward the low pressure in the core of the tornado.  This sideways moving air will expand and cool just as rising air does (see figure below).  Once the air cools enough (to the dew point temperature) a cloud will form. 

Tornadoes can go from Stage 2 to Stage 3 (this is what the strongest tornadoes do) or directly from stage 2 to stage 4 or 5.  Note a strong tornado is usually vertical and thick as shown in Stage 3.  "Wedge tornadoes" actually appear wider than they are tall.

Here is video of the Laverne Oklahoma tornado that was shown in class and that shows the initial dust swirl stage up to the mature stage very well. 

The thunderstorm and the top of the tornado will move faster than the surface winds and the bottom of the tornado.  This will tilt and stretch the tornado.  The rope like appearance in Stage 5 is usually a sign of a weakening (though still a dangerous) tornado.

EF2 Damage roof is gone, but all walls still standing	EF4 Damage only the strong reinforced concrete basement walls (part of the wall was below ground) are left standing.  It doesn't look like there would have been anywhere in this building that would have provided protection from a tornado this strong.	EF5 Damage complete destruction of the structure

	One of the better examples that I've seen of very different levels of damage in close proximity.  This is damage from an EF4 tornado that hit Northwood ND on Aug. 26, 2007.  (National Weather Service photo, source click on the Track Segments and Photos link)

Several levels of damage (EF1 to about EF3) are visible in the photograph above.   Here's another example with level F0 through F5 damage all found in a few block area.

It was puzzling initially how some homes could be nearly destroyed while a home nearby or in between was left with only light damage.  One possible explanation is shown below.

Sketch of multiple vortices in a large tornado and the damage pattern they could leave on the ground	An actual aerial survey of tornado damage.  This was an EF 4 tornado that hit Washington Illinois on Nov. 17, 2013 (here is YouTube video of the tornado and the damage left behind).  Photo by Zbigniew Bzdak for the Chicago Tribune (source)

 

Some big strong tornadoes may have smaller more intense "suction vortices" that spin around the center of the tornado (they would be hard to see because of all the dust in the tornado cloud.  You can see what appear to be multiple vortices in the first minute or so of the Hesston KS tornado video shown in Tuesday's class.   Tornado researchers have actually seen the damage pattern shown above scratched into the ground by the multiple vortices in a strong tornado.