Wisconsin's Severe Weather Awareness Week

Spring has sprung in Wisconsin, and while we might see warmer temperatures and more sunshine, it also marks the start of the severe weather season.

To keep everyone safe and knowledgeable, I have put together a brief overview of the types of severe weather we might experience in the spring and summer, as well as ways to keep you and your family safe.

Photographed by Benjamin Overesch: Stormy skies over eastern Milwaukee, April 15th, around 6:15 p.m.

Severe Thunderstorms and Tornadoes

The National Weather Service classifies a thunderstorm as severe if it meets one of the following criteria:

Hail of 1-inch in diameter or larger
Winds of 58 mph or greater

In Wisconsin, severe thunderstorms can occur at any time of the year, day or night. Most commonly, they increase their frequency during the spring and summer months, and intensify the most from late afternoon to late evening.

Thunderstorms can come in all different shapes and sizes, but there are two basic types of Wisconsin severe thunderstorms:

Quasilinear Convective System (QLCS):

QLCS is a type of thunderstorm that looks like a long, “bowed” line of storms (see Figure 1).

Figure 1: Reflectivity radar showing two separate quasilinear convective systems with the “bowed out” features moving through NE Wisconsin, May 21, 2024. Credit: NOAA

These mainly produce:

strong winds, potentially “straight-line” of 60-100 mph
large hail of 1-inch or greater diameter
weak tornadoes

When a “notch” forms in a QLCS line, this signals winds going in opposite directions, indicating rotation in the atmosphere: a signature of tornadoes (see Figure 2).

Figure 2: This is a velocity radar showing a “notch” in the same storm line. Winds going toward the radar (green) and winds going away from the radar (red). When they meet, that indicates rotation. Credit: NOAA

Supercells:

Supercell thunderstorms are single storms that only persist for a couple of hours. These likely produce:

larger hail, nearing 4-inch or greater diameter, or softball size
strong, long-track tornadoes

On a reflectivity radar, these exhibit a “hook” shape (see Figure 3).

Figure 3: Photographed by Benjamin Overesch; This is Radar Omega’s reflectivity radar from April 14, 2026, showing a hook echo on a supercell going through Dane County WI.

Unlike the QLCS, a supercell can develop its own pockets of winds moving in opposite directions (see Figure 4).

Again, when this happens in close contact, this indicates rotation in the atmosphere. Tornadoes can then touch down at any time!

Figure 4: Photograph by Benjamin Overesch; This is Radar Omega’s velocity radar of the same storm over Dane County as Figure 3, showing winds going away from the radar (red) and winds going toward the radar (green).

In Wisconsin, damaging winds are more likely than damaging hailstorms. According to meteorologist Kristen Kirchhaine from TMJ4, 54% of severe weather events in Wisconsin are wind events, while only 34% are large hail events.

However, even non-severe weather such as heavy rainfall and cloud-to-ground lightning can be dangerous as well.

Safety

Now that we have reviewed the types of severe weather that can affect Wisconsin, you need to know how to respond. First, severe weather alerts come in two types: watches and warnings.

Watches:

Watches tell the public conditions in and around the watch area favor the development of severe weather, such as floods, wind, severe thunderstorms and tornadoes.

Storm watches are issued well in advance of development. Use this time to review safety plans and monitor local forecasts via Television or a NOAA weather radio regularly.

Warnings:

Warnings mean the severe weather has been observed on radar or by law enforcement. This is when you should act immediately!

In the case of a flood, when encountering flooded roadways, “turn around — don’t drown.”

For storms, if you can hear thunder, you are close enough to get struck by lightning, so “when thunder roars, go indoors.” To protect yourself in a severe thunderstorm, move to a sturdy shelter and stay away from electrical equipment, plumbing and windows.

With tornadoes, take shelter on the lowest level of a house or building, ideally in a basement or interior room like a bathroom. Take your weather radio with you and something to cover your head to protect yourself from flying debris.

If in a mobile home, a vehicle, or outdoors, move to the closest substantial shelter. If otherwise unable to find substantial shelter, get down and cover your head, or abandon your car and take shelter in a ditch or ravine.

I hope this article has highlighted the modes of severe weather that can affect Wisconsin during the spring and summer seasons. More importantly, now you know how to keep yourself and loved ones safe when these events occur.

This season, severe weather is already off to an active start for the state, as we had a quasilinear convective system move through just this week, and I am sure this system won’t be the last one of the season. Stay safe and enjoy the summer in Wisconsin.

Sources:

B, J. (n.d.). Doppler radar. RADAR: Oakfield, WI tornado radar. https://cimss.ssec.wisc.edu/oakfield/radar.htm
Kirchhaine, K. (2021, April 13). Severe weather awareness week: What is the most common severe weather threat in Wisconsin?. TMJ4 News. https://www.tmj4.com/weather/weather-blog/severe-weather-awareness-week-what-is-the-most-common-severe-weather-threat-in-wisconsin
US Department of Commerce, N. (2015a, July 23). What constitutes a severe thunderstorm?. National Weather Service. https://www.weather.gov/bmx/outreach_svr
US Department of Commerce, N. (2015b, September 8). Severe thunderstorm safety rules. National Weather Service. https://www.weather.gov/bmx/sps_svrsafetyrules
US Department of Commerce, N. (2016, September 18). Severe weather safety tips. National Weather Service. https://www.weather.gov/ama/SEVERESAFETYTIPS
US Department of Commerce, N. (2024a, May 14). Understand tornado alerts. National Weather Service. https://www.weather.gov/safety/tornado-ww
US Department of Commerce, N. (2024b, June 26). Preliminary severe storm summary for May 21, 2024. National Weather Service. https://www.weather.gov/grb/052124_SevereStormSummary1

Wisconsin’s Severe Weather Awareness Week