A friend who teaches middle school science asked me to send him something about Friday's severe weather threat he could discuss with his students. I thought it would also make a good blog post, so here you go.
While severe weather is possible Friday, it is far from certainty. There are several things needed to create an environment supportive of thunderstorms becoming severe thunderstorms.
I often describe it like baking a cake. You have many ingredients needed to make one. However, if one ingredient isn't there, it doesn't come out of the oven looking like a cake.
Some of the ingredients needed for severe weather: fuel for thunderstorms, a trigger/lift in the atmosphere, instability, turning of the winds through the atmospheric column.
FUEL: Fuel for thunderstorms is already present. Moisture. For that, we look at dew point temperatures. In basic terms, the dew point is a measure of the moisture in the air. You may have noticed Wednesday and today it feels a little more humid outside. Dew points are in the 50°s. This is usual moisture for May leading in to June. Dew points are projected to be in the middle to upper 50°s Friday ahead of the weather system. We can put a check mark next to this ingredient.
INSTABILITY: Instability is needed to keep things churned up in the atmosphere. It adds some explosion to the atmosphere as air/moisture is lifted. When it is cloudy, there can be instability, but it is dramatically held back versus having sunshine to "bake the atmosphere". That is why you often hear meteorologists say "If we see sunshine today, that is a bad thing if you don't like severe weather." I've seen a number of potential severe weather days bust due to cloud cover.
Winds at the surface are projected to be out of the south/southeast (160°) much of Friday. Go up to 5,000ft and the winds will be out of the south-southwest (200°). Go up to 15,000ft and the winds will be more southwest (230°). Winds at 30,000 will be out of the southwest (250°). If you step back and look at the directions in a 3D sense, you can see there is turning of the winds . This sets up an environment in which a parcel of air will begin to rotate. Thunderstorms that rotate often become severe thunderstorms.
I like to use 500mb divergence because a thunderstorm is like a car engine. For a car engine to work, you need to have air flowing in, but also need exhaust flowing out. If you cut off the exhaust, it kills the engine. By having that void 15,000 ft up, it acts as the exhaust, to help pull air out of the thunderstorm, giving it somewhere to go.
There will be areas of divergence present over Indiana Friday, however, it is not perfectly aligned with some of the other ingredients.
So we now know about the ingredients. What we have to watch for at this point, is whether the ingredients will be lined up and interact all at the same time. At this point, I'm not convinced they are going to be. It might be slightly out of line. But since the ingredients are all there, that is why we are talking about the potential for severe weather Friday. We won't know for sure it will happen until a few hours before it happens.
BOTTOM LINE: Potential does exist for severe weather in central Indiana Friday but it is not certain. It is an afternoon/night to keep an eye on the weather and be ready to act if a warning is issued. Right now I have the Freak-Out-Meter at a 3 out of 10 for central Indiana.
Be sure to follow me on Twitter (@johndissauer) for the latest on the situation.