Why the storms?

If you peruse the previous blog post, you'll see that there was quite a bit of activity early this afternoon, especially north and west of Rockford itself. Why'd it happen? The warm, humid air had pushed to our east. You'd think that if we didn't get storms yesterday, we wouldn't get them today. (I won't go into why no storms fired yesterday... I'm still scratching my head on that one.)

Here's why. A trough (basically an elongated area of low pressure) pushed through, sweeping away most of the humidity... but it also ushered in plenty of cool air aloft. With enough warming from the sun this morning, plus a cold front sweeping through, there was enough energy and lift for storms to quickly develop.

The reason we had hail is pretty easy to figure out - the cold air aloft. But what about those strong winds? We were pretty dry at the surface before the storms with dewpoints in the upper 40s. When heavy rain started falling into that dry air, it began to evaporate. Evaporation is a cooling process (which is why you might feel cold when you get out of the shower), and cool air sinks. So, with such rapid cooling of the air coming out of the storm, it really began to plummet toward the ground and, upon impact, spread out quickly. Some of those winds even caused damage.