About Illinois' Tornado Risk (Springfield, Fulton: moving, car, property)

[Lookout Kid]

^^Why is there seemingly a very thin buffer zone on the western shore of Lake Michigan? Is it just a graphical error, or are we somehow in a magical protected zone?

[SJaye]

Originally Posted by Lookout Kid

^^Why is there seemingly a very thin buffer zone on the western shore of Lake Michigan? Is it just a graphical error, or are we somehow in a magical protected zone?

That is actually a graphical error. Here is a map of Illinois tornadoes, 1950-2004.

http://www.isws.illinois.edu/atmos/s...-1950-2004.gif

On this map, you will not only see that there have been tornadoes that have reached the Lake Michigan shoreline, but you will also get a feel for just how uniform the tornado risk is across the state of Illinois.

[linicx]

The most debated areas in central Illinois are Springfield in Sangamon County, Bloomington in McLean County and Peoria in Peoria County. Maybe Peoria has just been lucky The former two counties are mostly flat and surrounded by open fields, Peoria is neither flat nor surrounded by open fields. Two F-2 tornadoes struck Springfield the same day in 2006 - the worst storm year in IL history. Click on any county for details.

National Weather Service, Lincoln IL -- Tornado Climatology of Central and Southeast Illinois

[SJaye]

With the exception of Peoria County, the differences in the numbers by county seen in this climatology page for central Illinois area function of two things:

1. County size, bigger county= more tornadoes
2. Storm spotting, counties with large towns, and interstate highways have more people to observe the storm and therefore the storm is less likely to go undetected.

If you look at the right side of the table, you will see the counties ranked by number of tornadoes per 100 square miles. The top 5 are Logan (Lincoln/ I-55), McLean (Bloomington/ I-55), Sangamon (Springfield/ I-55), Taxwell, Woodford (I-74). The next five are Macon, Champaign, Piatt, Dougless, and Coles, all along either I-72, I-57, or both. You can see the effects of having these highways on the likelihood of a storm being spotted, and higher frequencies for these places.

Peoria is an interesting question, why have they seemingly been so lucky as to not experience nearly as many tornadoes as surrounding counties.

[linicx]

The topography of central Illinois from Marseilles to Arthur, and from Iowa to Indiana, is almost completely flat and covered in corn and beans with a few scattered ranches. The four largest cities outside of the Chicago and East St. Louis MSA (Peoria, B-N. Springfield and Champaign) are carved out of the prairie - Peoria being the exception. The single thing common to the four cities is Interstate routes that either pass through, intersect or bypass closely to one of the cities. The only difference between I-55, I-74 and I-72 is the plat. I-74 is platted NW between Morton and The Quad Cities. US 150 is platted from the Quad Cities through Peoria, E. Peoria, Morton, and into B-N and then eastward. . The majority of Peoria County tornadoes seem to spawn in the open fields NW of the City of Peoria between Brimfield and Knox County and most the twisters that cross US150 follow a NE path.

I've never seen a F0 path in Central Illinois that moved due Eest or West and did not deviate. Maybe a microburst like the one that struck Walnut Grove, MO last month can travel that path.


You never did explain why the OKC tornado stayed on the ground for several hours or why it took that long to travel 40 miles. Does size and intensity make the difference. .

[SJaye]

Originally Posted by linicx

I've never seen a F0 path in Central Illinois that moved due Eest or West and did not deviate. Maybe a microburst like the one that struck Walnut Grove, MO last month can travel that path.

Northeast is definitely the preferred direction for a tornado to move

Originally Posted by linicx

You never did explain why the OKC tornado stayed on the ground for several hours or why it took that long to travel 40 miles. Does size and intensity make the difference. .

There is a longer scientific explanation for this, but wind shear (which is the difference between the wind at the surface and the wind at a higher level of the atmosphere) basically allows a thunderstorm's updraft to become separated from a thunderstorm's downdraft. With the right combination of moisture (or Convective Available Potential Energy) and wind sheer, what is known as a supercell thunderstorm forms. Most tornadoes come from supercell thunderstorms. There is a delicate balance between the updraft and the downdraft that must be maintained for a tornado to be present. Basically, as soon as a tornado has formed, the downdraft area is essentially trying to rip it apart. The Oklahoma tornado of May 3, 1999 was an exceptional storm that happened to maintain this balance for a long time, which is rare. The mesoscale weather conditions lined up exceptionally favorably for tornado development and maintinence.

[linicx]

The mesoscale coincidence certainly ripped OKC apart.

Another thing I see in Peoria tornadic activity I think is peculiar is that the twisters that form in Tazewell County around Morton never follow 1-74 across the river. They may move into Fondulac Township that fronts the river, but to date none have crossed into Peoria.

What causes a microburst and are they usually short lived? .

How large is a supercell and why does it spawn smaller twisters?

[SJaye]

Originally Posted by linicx

What causes a microburst and are they usually short lived? .

A microburst is a strong downburst usually caused by a layer of dry air aloft. This dry air cools rapidly when it starts to rain, and therefore descends quite fast when it forms. They are usually quite short-lived. When air descends rapidly to the ground, it usually leads to very strong straight line winds at the surface due to basic fluid dynamics (the air hits the ground and must go somewhere).

Originally Posted by linicx

How large is a supercell and why does it spawn smaller twisters?

A supercell is usually (in Illinois) 1/4 to 1/2 the size of the average county. Parts of it spawn heavy rain, ligtning, hail, straight line winds, and tornadoes if conditions are right. Tornadoes form within a supercell because a solenoidal rotation of air is tilted upward by an updraft.

[linicx]

I've decided some things have no explanation. Illinois tornadoes is one of them. The one thing no one asked, SJaye, is if you are a storm chaser, storm spotter or the weatherman? You know a lot of about nasty storms, that's for certain.

[SJaye]

There are definitely still things about tornadoes that even the scientists do not know- thus the reason for VORTEX 2. I am not the weatherman on any news channel, but I am a certified storm spotter, and have been storm chasing before.