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1220PM CST UPDATE: Several large, violent tornadoes have already been reported in Illinois. There are pictures of horrific damage to homes and businesses coming in from the Peoria IL area. Tornado warnings stretch the entire length of the state of Illinois right now, and the tornado threat will continue for as long as the storms stay discrete (individual) before they merge into a line later this afternoon.

The tornado watches today are a PARTICULARLY DANGEROUS SITUATION (PDS), signaling the likelihood of numerous violent, long-lived tornadoes, some of which have already occurred.

I've updated the images below to reflect the expanded high risk area.

A rare "HIGH RISK" severe thunderstorm outbreak is expected across the Midwestern United States during the day on Sunday, bringing with it the possibility of several violent, long-lived tornadoes, widespread damaging winds, and hail up to the size of golf balls.

The following shows the Storm Prediction Center's severe weather forecast as of Midnight CST. The risk zones are likely going to change with subsequent forecasts, so always check with the Storm Prediction Center's website for the latest information.

Only a couple of high risk severe weather outbreaks occur each year. This is the second high risk issued this year, and the first high risk issued in the month of November since 2005.

Here's the overall severe weather outlook as of NOON CST. The pink signifies a high risk for severe thunderstorms; the red a moderate risk; the yellow a slight risk; the green a risk for non-severe thunderstorms.

Here are the tornado probabilities as of NOON CST. The percentages correlate to the % chance that a tornado will occur within 25 miles of any point within the shaded area. A 15% shading means that there is a 15% chance that a tornado will occur within 25 miles of any point within the shaded area.

The black hatching indicates the risk for violent, long-lived tornadoes that could be EF-2 or stronger on the Enhanced Fujita Scale.

2% is elevated, and 5% warrants concern. Any tornado probability above 5% is serious, and anything above 15% is extremely dangerous.

Here are the damaging wind probabilities as of NOON CST. As with tornadoes, the % refers to the probability of damaging winds (>60 MPH or a report of wind damage) within 25 miles of any point within the shaded area. A 15% probability means that there is a 15% chance of one report of damaging winds within 25 miles of any point within the shaded area.

The black hatching indicates the possibility of thunderstorm wind gusts exceeding 75 MPH.

15% typically warrants concern, and that large 45% purple shaded region signals a high risk for wind damage. See the discussion below the orange squiggle for details.

Here's the hail risk as of NOON CST. 15% warrants concern. A few golfball sized hail reports aren't out of the question during the day on Sunday, but it shouldn't be as big of a problem as the tornadoes and wind. (But it's always a problem if it's your car or your head that gets dinged up.)

Jump the squiggle for a brief discussion on why this is happening and what to do in order to prepare for the severe weather.

As the above forecasts suggest scream, the ingredients are coming together for a very significant severe weather event during the day on Sunday. A strengthening area of low pressure is on track to move over central Wisconsin by mid-afternoon, dragging a cold front through the country to its south and sucking a large amount of warm, moist, unstable air northward from the Gulf of Mexico.

Here is the North American Model's (NAM) forecast surface chart for 18z (Noon CST) Sunday, showing the low pressure sitting over Wisconsin. The sharp cold front to its south (and the unstable air out ahead of it) will serve as the focus for most of the severe weather during the mid-afternoon hours.

This is only half the story. The other half of the story is the massive amount of wind shear that is expected to occur over the region. Wind shear is the change in direction and speed of wind with height. Higher wind shear is conducive to severe weather, and the right combination of instability and wind shear can produce strong tornadoes.

Here is the NAM's wind/height forecast for 300 millibars (roughly 30,000 feet) for 18z (Noon CST) on Sunday, showing the incredibly strong jet stream that is associated with the system. Not only will the jet strengthen the low pressure, but it will help the thunderstorms strengthen well beyond severe limits. If you look carefully, there is a 140 knot (161 MPH) jet streak over Kansas. This system means business.

The result is wind shear that is between 50 and 70 knots. Today's severe weather is expected to come as a "one-two punch" so to speak. The first threat will be supercells, followed a few hours later by a strong line of thunderstorms.

The supercells will carry the risk of violent, long-lived tornadoes, large hail, and damaging winds. Once the supercells coalesce into a line of severe thunderstorms, the threat will gradually change over to damaging winds (with isolated tornadoes) as the evening progresses.

This region of 50-70 knot wind shear is conducive for the development of potent supercells. Wind shear causes air to rotate horizontally. Think of the cardboard tube that comes in the middle of a roll of paper towels. Hold it horizontally and start rotating it -- that's essentially what's happening with the wind in the atmosphere on a day like this.

The horizontal tube of rotation caused by vertical wind shear (winds increasing in speed with height).
When a strong updraft comes in contact with one of these horizontal tubes of rotation, it can start to bend it upwards and break it into two vertical tubes of rotation -- one spinning clockwise, the other counterclockwise. The one that's spinning counterclockwise almost always wins out because we live in the Northern Hemisphere, and this tube gets absorbed into the updraft and forms a rotating updraft. The existing vertical wind shear tips over this updraft and orients it diagonally through the storm, preventing the updraft from "choking" on itself.
The horizontal tube of rotation is titled vertically by the updraft. The resulting vertical rotation is the engine that drives the supercell.
The result is called a supercell, a thunderstorm that is much stronger than a "normal" storm and can survive for hours and travel hundreds of miles before collapsing. These storms are capable of producing violent, long-lived tornadoes, very large hail, and damaging wind gusts.

The setup tomorrow strongly favors supercells forming early in the afternoon, accompanied by the risk for violent, long-lived tornadoes across the moderate and high risk areas.

Over the years, research meteorologists have developed mathematical formulas to pump out indices to give us a general idea of what the atmosphere is expected to do.  One of these indices is called "helicity," which takes into account both speed shear (wind changing speed with height) and directional shear (wind changing direction with height) to determine how much twisting and turning of the wind is occurring in the atmosphere.

Helicity values between 300 and 400 are generally favorable for supercells to form. The higher the helicity value, the higher the risk for strong supercells and strong tornadoes. Models show helicity values for parts of Illinois and Indiana reaching between 350-400 during the early afternoon hours.

Another index is called the "Energy Helicity Index," or EHI for short. This takes helicity and adds the amount of instability (energy) available in the atmosphere to calculate where tornadic thunderstorms are most likely to form.

EHI values greater than 1 are concerning, and the EHI values in parts of the high risk zone are between 2 and 3. But as a word of caution, don't focus on exactly where the model shows the EHI values -- it's just a model forecast. The SPC has a much larger area under a risk for strong tornadoes, so always follow their forecast.

As the afternoon and evening wear on, these supercells will start to merge with each other and begin to form a line of severe thunderstorms. As this occurs, the tornado/hail threat will start to wane and the threat for damaging winds will increase greatly.

This line of severe thunderstorms is expected to sweep across Indiana, Ohio, Michigan, and Kentucky into the evening and nighttime hours. The main risk will be wind gusts potentially exceeding 75 MPH, as well as the risk for some isolated, smaller spin-up tornadoes.

One of the biggest dangers with this kind of severe weather outbreak is that not many people are paying attention to severe weather this time of year. People don't realize that there are two distinct severe weather seasons -- one in the spring and another in the fall. The biggest issue will be getting the word out. If you live in the area or know anybody who does, please call/text/email/message them and let them know of the risk.

Here are some links to help you keep track of the weather. Stay safe.


National Weather Service Main Page
National Weather Service -- Chicago Area
National Weather Service -- Central Illinois
National Weather Service -- Indianapolis Area
National Weather Service -- Northern Indiana
National Weather Service -- Paducah KY Area
National Weather Service -- Louisville KY Area
National Weather Service -- Grand Rapids MI Area
National Weather Service -- Detroit MI Area
National Weather Service -- Wilmington OH Area
National Weather Service -- Pittsburgh PA Area

Storm Prediction Center Main Page
Storm Prediction Center -- Current Severe Weather Watches
Storm Prediction Center -- Convective (Severe Weather) Outlooks
Storm Prediction Center -- Mesoscale Discussions
Storm Prediction Center -- Storm Reports
Storm Prediction Center -- Mesoscale Analysis Pages

Wunderground's Detailed Radar (click the + nearest to you to see your local radar)

NOAA Weather Models

iMapWeather Radio App for iPhone/iPod Touch (costs $9.99 but well worth it)

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