Severe thunderstorms are rapidly firing up across the Upper Midwest this afternoon, with tornado watches in effect from central Missouri through the western shores of Lake Michigan. The largest cities under the risk for tornadoes this afternoon are Chicago and Milwaukee. Some of the tornadoes could be strong in the most well-organized supercells.

An enhanced risk for severe weather—a three on a scale from zero to five—is in place across a huge portion of the central United States this evening, and it does not look like a dolphin this time around, as about 750 of you pointed out yesterday.

The main threats include large hail (some up to the size of golf balls), damaging winds in excess of 60 MPH, and tornadoes. While hail and winds are dangerous in their own right, tornadoes are the threat we tend to focus on the most.

Tornadoes

There's a 10% risk for significant tornadoes across the southwestern Great Lakes, including most cities in central and northern Illinois. The same region is under a threat for 'significant' tornadoes, which are ones that produce damage equivalent to an EF-2 or stronger on the Enhanced Fujita Scale.

A 10% risk for tornadoes is different from a 10% chance of rain. A 2% risk is worthy of concern, let alone 10%, and these risks also correlate to climatological probabilities. The climatological risk for tornadoes across the region is about 0.40% today (meaning that tornadoes occurred within 25 miles of any point in the area 0.40% of the time on April 9 between 1982 and 2011), so today's risk is 25 times higher than normal (10% risk ÷ 0.40% climatology = 25x higher). The risk for significant tornadoes is 66 times higher than normal (10% risk ÷ 0.15% climatology = ~66x higher).

Watches

Here's an overview of severe weather watches as of about 4:21 PM EDT:

Counties under tornado watches are shaded in red, while counties under severe thunderstorm watches are shaded in blue. These watches will grow, shrink, and disappear through the evening as needed to keep up with the evolving threats, and future watches are likely in other areas that see the potential for a thumping. You can keep up with the latest watches by checking the Storm Prediction Center.

A severe thunderstorm or tornado watch means that conditions are favorable for the development of large hail, damaging winds, or tornadoes in any thunderstorms that develop over the next couple of hours. Watches are typically in effect for six hours at a time. Warnings, on the other hand, mean that the threat for severe weather is imminent and you need to take immediate action to ensure your safety.

Here's How It'll Happen

The area was socked in clouds for a good part of the morning, but the clouds started to break up, allowing the sun to shine in and warm the atmosphere enough to allow air to rapidly rise and begin producing thunderstorms. The wind shear (change in speed and direction with height) over the region is strong enough to permit any storms that develop to turn into supercells, which carry the risk of large hail and will be the likely culprits of any stronger tornadoes that form this afternoon.

The severe weather threat, as we see so many times, will come in two batches: discrete storms with a squall line behind them. The discrete (individual, standalone) storms carry the greatest threat for hail and tornadoes, while the squall line's predominant threat is damaging winds and possibly a tornado or two.

In this case, the squall line is forming along a cold front that extends from Des Moines through northeastern Oklahoma, and the line of clouds and storms is awesomely visible on the satellite image a few paragraphs up. Don't let your guard down if one storm blows through—this is a situation where you could see three or four different thunderstorms (each with their own set of hazards) before you're in the clear.

Friday

The threat shifts east tomorrow along the cold front, where thunderstorms and one or more squall lines will pose a threat for damaging winds and large hail for most major cities from Philadelphia to the Rio Grande River. Many of us have seen our first thunderstorm of the year—there was golf-ball-size hail near Raleigh, North Carolina last night—and this one should do it for those of you who haven't had the pleasure of listening to rumbling thunder yet.

As always, pay attention to your forecasts and know where to go and what to do if you're threatened by hazardous weather. The only time you should ever leave the safety of a sturdy building ahead of a tornado is if you live in a mobile or prefabricated home, in which case they are neither safe nor sturdy. If you find yourself in the path of a tornado while you're in a vehicle, don't get out and go into a ditch to ride out a tornado; instead, drive to the nearest safe building (like a school) or get away from the tornado by driving perpendicular to its motion.

[maps: author | watches/satellite: GREarth]

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