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Severe weather to slam U.S. midsection, tornadoes possible

A dangerous severe weather outbreak across the country's midsection is becoming more likely for Saturday, with strong tornadoes possible and some which may stay on the ground for a long time. As a result, NOAA's Storm Prediction Center has issued a moderate risk, the second highest designation, ranking it as a 4 on a scale of 1 to 5.

A strong upper-level low-pressure system, with a vigorous jet stream over the Rockies, is ejecting eastward into the Plains States. At the same time, record heat is building across Texas and the Southeast with many cities maxing out in the 80s and 90s, with around 100 record highs in jeopardy through the weekend. 

Over the next 24 hours, these ingredients will combine to fuel an explosive atmosphere, especially for Missouri, Illinois and eastern Iowa. 

There will be two rounds of severe weather. The second, later Saturday, will be the most significant.

The first round will begin Friday night with severe storms developing on the west side of the risk area in eastern Kansas and progressing east into the western Ohio Valley by Saturday morning. The main threat Friday night is large hail with a few pockets of damaging winds and isolated tornadoes. Cities in the risk area include Wichita, Kansas; Kansas City and St Louis, Missouri; Springfield, Illinois and Indianapolis, Indiana.

Expect the first round of storms to dissipate and move east Saturday morning, allowing skies to clear out behind the first batch of storms. As the sun returns and southerly winds pump warm and moist Gulf of Mexico air far northward towards the Great Lakes, the second round of storms will fire up in very unstable air on Saturday afternoon.

This time the storms will have the assistance of a 150 mph jet stream above, lots of twisting and turning in the atmosphere called wind shear and a convergence of fronts right near where Iowa, Illinois and Missouri come together. The orange and red risk areas below from the Storm Prediction Center shows the bullseye for the greatest risk.

While all severe weather hazards are on the table, unlike many severe weather outbreaks, this time strong tornadoes appear likely. In addition, the Storm Prediction Center warns of long-track tornadoes - tornadoes that stay on the ground for an abnormally long time. The greatest threat is for cities like Quincy and Peoria, Illinois, and Davenport, Iowa, but severe weather may include Milwaukee, Chicago and the Tennessee Valley area.

Meteorologists use something called severe weather indices, essentially mathematical algorithms, which take into account the amount of instability and spin in the atmosphere. The higher the numbers, the greater the risk. In the case of tomorrow the numbers go beyond the traditional scale for both the Significant Tornado Parameter and the Supercell Composite.

This will make for a potentially dangerous situation on Saturday, so pay close attention to any warnings that are issued.

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