How Do Severe Storms Develop?

Have you ever wondered what it takes for severe thunderstorms to develop? Well look no more. Here are the primary ingredients required for severe storms.

Instability – An unstable airmass is one where there are warm temperatures at the surface and colder temperatures aloft. The warmer the air at the surface and the colder the air aloft, typically the greater the instability. 

Wind Shear – While instability is required for thunderstorms, they may not become severe unless there is enough wind shear. Wind shear is wind changing direction and/or speed with height. This allows storms to tilt and rotate allowing them to last much longer than ordinary thunderstorms do. 

Lift – You typically need something to force air to rise. While instability will force air to rise, you will often need something to get the parcel of warm & moist air to start rising. This can come in the form of mountains, fronts, and boundaries. However, seeing as Central Illinois is void of mountains, cold fronts, warm fronts and outflow boundaries can act as our lifting mechanisms.

Moisture – If an approaching area of low pressure is the engine, then low-level moisture is the fuel for these storms. For storms to become severe, we need moisture. This moisture often comes from the Gulf of Mexico. The more moisture we have, the greater the instability will be giving us a greater possibility of severe storms.

How Do Tornadoes Develop?

While pinpointing exactly when and where a tornado will touchdown, we can (with reasonable accuracy) forecast days in advance when conditions are right for them to develop. Once we find that the atmosphere will be unstable enough for thunderstorms, we’ll look at the amount of wind shear that’s in place to see if any of these storms can become severe. 

In order for tornadoes to form, you need to have a significant amount of wind shear. This is wind changing direction and or speed with height. In the typical tornado set up, you’ll have southeasterly winds at the surface, a fast-moving low-level jet a few thousand feet off of the ground moving in from the south with the jet stream moving from the west at 25-35 thousand feet. 

Due to the change in wind direction and wind speed with height, air will begin to rotate horizontally. As storms begin to develop, these horizontal rolling tubes of air will be absorbed by a thunderstorm’s updraft allowing that storm to take on rotation. As the storm’s rotation tightens, the base of the storm will lower eventually leading to the development of a tornado.

Illinois Tornado Stats

While tornadoes are possible anytime of year, they are more common in late Spring and early summer across Illinois. On average, Illinois sees 12 tornadoes in April, 15 tornadoes in May and 10 tornadoes in June. The reason tornadoes are more likely this time of year is due to the Jet Stream retreating north for the Summer months. This allows warm moist air from the Gulf of Mexico to interact with cool dry air coming from the north. With the Jet Stream positioned over Illinois from April through June, it allows severe storms to develop. 

Once summer arrives, the Jet Stream is typically positioned across the northern U.S. and southern Canada.This will keep the bulk of the wind shear north of Illinois lowering the overall risk for tornadoes. You will notice that there is a slight increase in tornadic activity in November. This is often seen as Illinois’ “Second Severe Weather Season” as the jet stream begins to drop south for the winter. While November is typically seen as a colder month and a time of year we start to talk about snow, tornadoes are still very possible…such as the Washington tornado on November 17th, 2013.

We’ve gone through and broken down the number of tornadoes to strike Central Illinois by county between 1950 and 2014. Smaller counties such as Putnam, Menard, and Cass have seen less than ten tornadoes in this 64 year period. However, larger counties such as McLean County have seen significantly more. Tazewell, Logan and McLean Counties have seen the most tornadoes with McLean county seeing over 100 tornadoes between 1950  and 2014.

How Does Hail Form?

Hail forms when powerful updrafts send water droplets to the top of a thunderstorm where temperatures are below freezing. When these water droplets move in to an environment where they become super-cooled or freeze in to ice pellets. As these supercooled water droplets and ice pellets run in to one another they will grow in to larger and larger hailstones. The hail will remain in the storm until it becomes too heavy and falls to the ground. 

The size of the hail is determined by the strength of a thunderstorm’s updraft. In order for a thunderstorm to produce hail that is the size of quarters or one in in diameter, the storm would need to have an updraft speed of at least 49 MPH. Storms that produce hail the size of hen eggs, or two inches in diameter, would need an updraft speed of 69 MPH. In the event that a thunderstorm produces 4.5 inch diameter (Softball Size) hail, the storm would have an updraft speed more than 100 MPH!

Hail Size Comparison Chart

0.50″ – Marble/Mothball

0.75″ – Penny

0.88″ – Nickel

1.00″ – Quarter

1.25″ – Half Dollar

1.50″ – Walnut, Ping Pong Ball

1.75″ – Golf Ball

2.00″ – Hen Egg

2.50″ – Tennis Ball

2.75″ – Baseball

3.00″ – Tea Cup

4.00″ – Softball

4.50″ – Grapefruit

Storm Preparedness

It’s important to plan ahead in the event of a severe weather outbreak. Even though severe weather may not be on the mind when the weather is cold, it’s the perfect time to have a plan in place for when the time comes. here are some tips to keep you and your family safe. 

Know Your Risk

Make sure you understand what types of severe weather could potentially impact your area. 

  • Lightning – When thunder roars, go indoors. If you hear thunder you are close enough to be struck by lightning
  • Hail
  • Damaging Winds
  • Tornadoes
  • Flash Flooding – Turn around don’t, drown. Most deaths that occur during flash flooding events can be avoided by simply avoiding flooded roadways. While the water may appear shallow, the condition of the road below the water may be poor and could lead to your vehicle being washed away.

Know the Difference Between a Watch and a Warning

  • A watch simply means that conditions are right for severe weather to occur. These watches are typically issued hours before storms have even developed and typically are very large and cover numerous counties and even states. When a watch is issued, make sure you have emergency plans in place and emergency kits ready to go.
  • A warning will be issued if an event is occurring, imminent or likely. This is the time to act!!! Seek shelter immediately in a sturdy building and avoid exterior walls and windows. It’s also good practice to stay on the lowest floor of the building until the warning has ended. 

Build An Emergency Kit

  • 3 day supply of water, one gallon of water per person per day (Family of 4 = 12 Gallons of Water)
  • At least 3 day supply of non-perishable food
  • Can opener for canned food
  • NOAA all hazards weather radio with extra batteries (Weather Radio Programming Link)
  • Flashlight with extra batteries
  • First aid kit
  • Whistle to signal for help
  • Infant  formula and diapers, if you have an infant
  • Moist towelettes, garbage bags and plastic ties for personal sanitation
  • Wrench or pliers to turn off utilities
  • Have a map and compass to help guide you to predetermined safe places and shelters that should be circled on the map. It’s important to note that streets may become unrecognizable after a significant tornado so make sure you are able to navigate without the aide of street signs and landmarks.

For additional weather safety tips visit the following links…


Red Cross

National Weather Service

Weather-Ready Nation

WMBD and WYZZ are Weather-Ready Nation Ambassadors and will be partnering with Midland Radio Corporation, Walgreens and Hy-Vee to sell weather radios across Central Illinois. Stay tuned for upcoming events where Your Local Weather Authority and members from the NWS along with Emergency Managers will team up to program Midland Weather Radios.