Huge Illinois hail stone provides incentive to heed NWS products

Infographic from National Weather Service Chicago weather forecast office showing huge hail stone (4.75 inch) that fell in Minooka, Illinois, June 10, 2015
Infographic from National Weather Service Chicago weather forecast office.

Softball-size pieces of ice fell from the sky in northern Illinois June 10, at around 7:20 p.m. One resident provided the National Weather Service’s Chicago weather forecast office (WFO) with a photo of a hail stone that was 4.75 inches across its largest dimension. It’s the second-largest hail stone on record in the entire state of Illinois. I surely would not want to be caught outside when stuff like that starts dropping!

Yellow polygon surrounds the area covered by a severe thunderstorm warning that the Chicago National Weather Service issued for the Minooka, IL area at 7:05 p.m. CDT.
Yellow polygon surrounds the area covered by a severe thunderstorm warning that the Chicago National Weather Service issued for the Minooka, IL area at 7:05 p.m. CDT.

This demonstrates why there should be no such thing as “only” a severe thunderstorm warning (a phrase I often hear from people who are less weather-aware). At least 10 minutes before that ice rock fell on Minooka, the Chicago WFO issued a severe thunderstorm warning that covered that area.

It also shows why “large hail” is one of the three threats that the NWS Storm Prediction Center includes in its convective outlooks. The  SPC convective outlook that was in effect at the time this huge hail stone fell indicated that the area had a slight risk of hail of one inch or more in diameter.

Hail probability map from the SPC convective outlook that was in effect at the time the huge hail stone fell on Minooka, IL.
Hail probability map from the SPC convective outlook that was in effect at the time the huge hail stone fell on Minooka, IL.

Before you saw this huge hail stone, you might have thought a “slight” risk was nothing to worry about. After all, it technically means that there’s “only” a 15 percent probability of large hail within 25 miles of where you’re standing. That seems like a small probability in a large area.  No reason to be concerned, right?

But consider this: The “normal” probability of one-inch or larger hail falling within 25 miles of the center of Minooka (or anywhere in Illinois) at this time of year is one percent, according to climatology data from the National Severe Storms Laboratory. That means that the “slight risk” in the convective outlook for the evening of June 10 indicated a probability that was 15 times greater than normal!

Want to be more weather-aware now? Below are some helpful links:

  • Storm Prediction Center convective outlooks depict the severe weather risk levels everywhere in the continental U.S. for today and each of the next seven days.
  • Storm Prediction Center mesoscale discussions indicate when the SPC is thinking about issuing a severe thunderstorm watch or tornado watch.
  • Hazardous Weather Outlooks (HWOs) issued by your local weather forecast office indicate whether local meteorologists expect severe weather in your area and whether they think SKYWARN storm spotters will need to activate. To find the HWO for your area, go to weather.gov, type your ZIP code in the box near the upper left-hand corner, click the “Go” button, wait for a new page to load and then look for a link that reads, “Hazardous Weather Outlook” in a section near the top labeled, “Hazardous Weather Conditions.”
  • Watches and warnings. If either are in effect for your area, they’ll also be listed in the “Hazardous Weather Conditions” section of your local forecast page (see “Hazardous Weather Outlooks,” above). Remember, a watch means conditions are favorable for severe weather, making it likely. A warning means that severe weather is happening now.

 

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