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Making sense of the weather forecast

April 27, 2015
Gilbert Sebenste

NIU staff meteorologist Gilbert Sebenste

With the tornadoes that took place on April 9, more people are wondering what their threat of severe weather is in the forecast. For many people, a 50 percent chance of thunderstorms just won’t do.

Will they be severe? And if so, what is the coverage of these expected to be?

NIU Weather and the National Weather Service Storm Prediction Center now use a six-tier risk category system. The lowest risk is just for the “everyday” thunderstorms. The highest risk is for a major and potentially historic severe weather event. Let’s go through each category and I’ll show you what each one means, and how they are used.


  1. General thunderstorms:  No severe thunderstorms are expected. They will produce lightning, of course. And if you are outdoors hiking, biking, playing or watching sports, this IS a red flag. Half of all people who die from lightning-sustained injuries are golfers, but bicyclists, joggers and bystanders to sports round out most of the rest of the 50 percent. This means that if you are sitting watch your child play baseball or softball, football or other outdoor sports, there is a risk of injury or death from lightning during the time period.
  2. Marginal: This means that, in addition to all of the above, a few isolated thunderstorms may reach severe status, producing hail up to quarter size, 40 to 60 MPH winds, or an isolated tornado.
  3. Slight: This means that scattered severe thunderstorms are possible. A few tornadoes occur, but some of the winds can go up to 80 MPH, or even higher, in a few thunderstorms. Hail up to golf ball size is possible.
  4. Enhanced: This is the risk DeKalb was in on April 9, 2015. This means numerous severe thunderstorms are possible, and a few of the storms will be high-end severe. That is, hail up to baseball size, damaging winds of 60-100 MPH, and a few tornadoes, possibly long-lived and very damaging.
  5. Moderate: Widespread severe thunderstorms are likely. Strong tornadoes and widespread wind damage are likely; large hail up to softball size could also occur.
  6. High: Widespread severe thunderstorms are all but a given. Long-lived, widespread, and particularly damaging severe thunderstorms are expected. This could either be 1) a tornado outbreak, or 2) a long-lived, particularly severe squall line of severe thunderstorms, producing a wide swath of wind damage, with winds of 80-120 MPH.

Tornado photo courtesy Walker Ashley

You can see these in the forecast found on the NIU Weather website by clicking on our weekday or weekend forecasts in addition to the Storm Prediction Center website.

By learning these basic rules, you’ll have an idea of how bad things could be on any given day. Remember, however, that current conditions and forecasts can change quickly, so check back during the day to these sites in case the risks have changed for the better, or worse.

Have a safe severe weather season!


Gilbert Sebenste is the staff meteorologist at Northern Illinois University. NIU weather resources may be found on the NIU weather page,