Severe weather is common here in the Midwest, especially this time of year. June is Wisconsin's peak month for tornadoes, followed closely by July, May and August. Despite the large number of severe weather events that take place everyday across the United States, all the ingredients must line up in a very specific way. It's amazing severe weather outbreaks occur at all.
Moisture needs to be in place at low levels of the atmosphere. For us, that means high dew points in the 60s or even the low 70s. The higher the dew point temperature, the more moisture available for cloud and storm development. High dew points also make it uncomfortable for us. Anytime the dew point gets above 60 degrees, many people start to notice the stickiness of the air. Dew points above 65 degrees mean that nearly everyone is uncomfortable. For storms to develop, the low-level moisture needs to be lifted upwards, and the faster the better.
One way to lift moisture is by a front, either warm or cold. Cold air moves through like a snowplow and lifts the lighter, less dense warm air up and out of the way. That explosive lift can cause clouds to rapidly form and build skyward in a hurry. A warm front lifts the air gently, but storms can still develop. Warm air rides up and over cooler air below it, so the development of storms tends to be less explosive. Even if no fronts are present, high-moisture air can be heated by the Sun and rise up from the Earth's surface. As the air rises, it cools down to the dew point temperature and water vapor condenses into water droplets. These tiny droplets form clouds and storms.
Just the warmth from the Sun can be enough to get air to rise quickly, but two other events can lead to explosive storm development.
One is cold air aloft. Warm air will continue to rise as long as it is warmer than the surrounding air. With a cold pocket of air above 5,000 feet or so, warm air parcels keep on going up and up until they begin to condense out water. Once the condensation begins, air parcels can keep going up as clouds build higher. Another method to get air parcels to rise quickly is through the actions of the jet stream. This fast ribbon of air moving overhead above 20,000 feet rapidly moves air out of the way. Nature compensates for this by allowing air from below to rise up and take the place of the exiting air.
Severe weather forecasters look for all or some of these ingredients to come together over one area of the country. High dew points, plenty of warming sunshine, and a high-level jet stream roaring overhead may be enough to get things cooking. Sometimes a hot and humid day results in very little cloud cover and no severe weather. At other times a cold front may sweep through the area with little more than a few light showers. Not enough ingredients will lead to a lack of severe weather.
So during this time of year, when strong storms are popping up all over the country, think about nature's cookbook and the mix of items needed for tornadoes, hail, and high winds. It's amazing all of these come together at all!
Watch Vince Condella on Fox 6 Sunday through Thursday at 5, 6 , 9 and 10 p.m. You can see the FOX 6 weather forecast around the clock at www.fox6milwaukee.com
