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Just in time for severe weather, learn why summer storms usually bring straight-line winds

We have seen quite a few damaging, straight-line wind events in recent days around Northeast Kansas. You might wonder: Why is it that thunderstorms in the summer often producing more “straight-line” wind damage, rather than tornadoes? Part of that answer can be explained in a number called, the “Bulk Richardson Number,” or BRN.

Note, before I delve into this topic, there are exceptions to the BRN; it is just a number. However, meteorological observations find the general rules I talk about to fit most thunderstorm scenarios. In fact, I spent quite a bit of time working on this topic surrounding the Barneveld, Wisconsin EF-5 Tornado of 1984.

The BRN is a measure of heat and humidity, versus the stronger winds higher up in the sky. Stronger winds high up are needed to balance out the heat and humidity closer to the ground. When winds higher up are strong enough to balance out the heat and humidity closer to the ground, you often get twisting thunderstorms, called “supercells”.

Supercells often form when BRN numbers are smaller (don’t worry about the actual numbers). These storms are capable of producing twisting winds, i.e. tornadoes. Yes, you can get straight-line winds in supercell thunderstorms, too (that's a different topic). However, the storms that produce straight-line wind events we often seen during the summer are somewhat different from the storms that produce straight-line winds in the spring. Summer straight-line wind storms often occur when the winds higher up are not all that strong.

During the summertime, we often lose strong winds higher up, here in Northeast Kansas. We certainly do not lose the summertime heat and humidity. So, the atmosphere is definitely primed with energy for thunderstorms (from the heat and humidity). When we get a kick, like a cold front, we often get thunderstorms. Unlike spring thunderstorms, these storms usually don’t twist (unless we get just the right combo of heat and humidity and stronger winds higher up, like we often do in the spring).

BRN numbers are often very high in Northeast Kansas during the summer because the heat and humidity is high, but winds higher up are often quite weak. High BRN numbers usually produce thunderstorms capable of strong, straight-line wind events. These storms are often seen during the overnight and early morning hours.

Of course there are exceptions to this rule. Sometimes we get tornadoes to form when BRN numbers are really high (usually owed to the turbulent nature of an atmosphere that is really hot and humid). Sometimes we don’t get rotating thunderstorms when BRN are numbers are smaller. In general, this rule works: when winds are not very strong higher up, we usually don’t get rotating thunderstorms. This is why we often don’t see too many tornadoes in Northeast Kansas once we get into July; rather, we see thunderstorms that pose more of a threat from straight-line winds.

Straight-line wind thunderstorms versus twisting thunderstorms owe much to that correct combo of heat, humidity and winds higher up. Tuesday afternoon and evening we will have very high heat and humidity, along with a cold front coming in. We will likely see thunderstorms form by Tuesday evening (most likely after 4 p.m.). At this point, much of the stronger winds higher up will stay mostly to the north of Northeast Kansas. This combination will likely facilitate more of a straight-line wind event in Northeast Kansas, should severe storms ensue. Again, we cannot rule out that isolated tornado; but, at this point, that better conditions for twisting thunderstorms, capable of producing tornadoes, look to stay up in Iowa. Stay tuned, as we know, weather elements can change quite quickly.




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