It may not seem like it, but soon we'll be trading in the recent snow for springtime showers and thunderstorms. And with the recent severe weather in the Plains the past couple of days, I wanted to discuss a little more on how thunderstorms form, and the frequency of tornadoes in Illinois.
Just like the interstate you drive on to get you where you need to go, the jet stream acts like an interstate for storm systems. Since the jet stream flows from west to east, storms typically track from west to east. While there are a lot of factors that contribute to the development and strength of thunderstorms - like lift in the atmosphere, moisture and instability - the strength of the wind within the jet stream is a major factor.
Higher wind speeds within the jet stream are called 'jet streaks'. And it's those jet streaks that can help enhance thunderstorm development. Going back to the interstate thought, when you come up to a construction zone on the highway, you slow down. When you get out of the construction zone, you speed up. Winds do pretty much the same as they round the ridges and troughs in the jet stream. As winds get closer to the dip, or trough, in the jet stream they tend to slow down. As winds round the base of the trough and come out on the other side, they speed up and spread out. This causes 'divergence' in the atmosphere, which causes lift throughout the atmosphere. Where that lift occurs, is where low pressure systems develop and thunderstorms form, and intensify, at the surface. Now, there also needs to be moisture (higher dew points) and instability, but if all comes together just at the right moment, severe weather can develop.
Since we're getting closer to severe weather season in northern Illinois and southern Wisconsin, I thought we should take a look at the frequency of tornadoes by month, as well as time of day. Tornadoes can occur any month of the year - remember the January 2008 and November 2011 tornadoes? But, the months that experience the highest frequency of tornadoes throughout Illinois are April, May and June. This is as the jet stream shifts north, pulling warmer air with it.
Tornadoes can also occur any time of the day,
but most frequently we see them late in the afternoon and early evening. The reason: by that time we've maximized the amount of heating and instability (two ingredients needed for thunderstorms). However, tornadoes can occur any time of day and it's the ones that form during the 'off hours' of the early morning or overnight, that tend to be the most dangerous because people are either not aware that severe weather is occurring, or they're fast asleep and don't hear the warning. That's why it's so important to have a safety place in your house, know what to do when severe weather occurs and most importantly, have a way to receive warnings. Something that will wake you up in the middle of the night.
Warmer weather will come, I promise you this. And as we transition into the warmer months, let's work to become better prepared for when severe weather occurs!
Click here for the complete report from the Illinois State Climatologist Office, Illinois State Water Survey
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