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.
TRIGGER: The trigger for this event will be a cold front projected to sweep through the state Friday night. As colder air is more dense, it remains close to the ground. Think of a cold front as a moving wedge. As it moves through the state, it will take warm, moist air and lift it as cold air undercuts it. We can put a check mark next to this ingredient.
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. While the dew points will be high by February standards, there is some question as to whether or not we can look at this as a true February air mass. There is some thought that we are in more of a late April air mass, and if the atmosphere is playing by April rules, dew points in the 60°s might be better for creating severe thunderstorms. So I can not give “Fuel” a complete check mark.
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.
One of the values we look at for instability is Convective Available Potential Energy, also known as CAPE. Typically, levels above 500 would be high enough to support thunderstorms, and often severe thunderstorms, in February.
TURNING OF THE WINDS: For this, you need to think of the atmosphere in 3D. What happens at the surface, where we live, is affected by what is going on overhead. When we forecast, we look at several layers of the atmosphere. The surface, 5,000ft above the surface, 15,000 feet above the surface, and 30,000 feet above the surface. All the layers play a part in what we get down at the ground.
Winds at the surface are projected to be out of the south (190°) much of Friday. Go up to 5,000ft and the winds will be out of the south-southwest (230°). Go up to 15,000ft and the winds will be more southwest (240°). 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.
One of the key things I look for when putting together a forecast for tornadoes and storm chasing, 500 millibar divergence. 500mb is approximately 15,000 feet above the ground. I look for areas at that level where the wind direction leaves a void (divergence). Its similar to creating a vaccum. When there’s a vacuum what happens? Air rushes to fill the “emptiness”. When there is a void at 15,000ft, and lift is in place, air rushes to fill the void. That air comes from lower levels. It aids pulling thunderstorms higher in to the atmosphere.
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 as aligned, as I would like to see it when forecasting tornadoes, 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 if 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. Right now, the hours between 4pm and 9pm is our window of opportunity for severe storms. So you should keep an eye on the weather and be ready to act if a warning is issued for your location.
Be sure to follow me on Twitter (@johndissauer) for the latest on the situation.