DETROIT – The number of people asking me what’s going on with these recent unexpected tornadoes has prompted me to write an article explaining this.
It all started on Aug. 21 when six tornadoes touched down in western Michigan. The area was only in the marginal risk area for severe weather that day, and was not even in the tornado outlook -- meaning there was less than a 2 percent chance for tornado formation.
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Then, last Wednesday, a few severe thunderstorms developed tornadoes across parts of northern Indiana and Ohio, including two that touched down just across the Detroit River in western Essex County, Ontario. Once again, this area was in the marginal risk, with no tornado outlook.
Read Paul Gross' latest forecast here.
Then there was Sunday's funnel clouds in Lenawee and Monroe Counties, on a day when the area was not in any severe weather risk outlook.
What’s going on?
Before getting into why these types of things happen, I first have to explain the large-scale ingredients needed to create super cells -- the special type of severe thunderstorm most likely to produce a tornado.
It all begins with rising air.
Setting things in motion is normally accomplished by an approaching trigger -- a cold front, warm front, trough, strong lake breeze or upper level disturbance. But to really get things moving, we need an unstable atmosphere, which allows parcels of heated air at the surface to begin rising violently aloft.
To develop instability, we need relatively warmer and humid air near the surface, with cooler and drier air aloft. When we have this vertical profile, warm surface parcels rising into the cooler air begin to accelerate, because the warm air parcel is relatively lighter than the surrounding air. It’s kind of like a helium balloon accelerating upward when you let go of it.
But tornadoes require more than just lift. We also need winds that veer (i.e., start turning clockwise) as you go up. For example, if surface winds are blowing from the south or southeast, and then from the southwest 5,000 feet aloft, and then from the west 10,000 feet aloft, this is very conducive for super cell development -- if you have enough instability. Wind speed is also important. Having the nose of an approaching piece of stronger wind aloft approaching the area also brings stronger winds into the overall mix.
VIDEO: Funnel clouds spotted in Monroe County, possible tornado touchdown
The temperature and moisture profile of the atmosphere is called thermodynamics, and the physical lifting of the air parcels is called dynamics. Meteorologists keenly monitor both to predict our severe weather potential.
Let’s say that instability is weak, but we have strong dynamics approaching. This may force those surface air parcels violently aloft despite the weak instability. Conversely, let’s say that we have weak dynamics (i.e., a weak front) approaching, high instability may be enough to generate super cell development.
A super cell develops when a large-scale, strong circulation develops in the parent thunderstorm cloud. This is not the tornado. Not every rotating super cell develops a tornado. In fact, one of the great mysteries in meteorology is what exactly causes the tornado to begin dropping down from the cloud.
However, we do know how the strong winds develop. It’s the result of an important law of physics called the "Conservation of Angular Momentum." I won’t bore you with the math, but here’s the gist of it: If an object is spinning, and you decrease the radius of that object, then its speed of rotation must increase. That’s how figure skaters spin so fast -- they start their spin with their arms extended, and then bring their arms (and even a leg) in. Their spin speed increases as their radius decreases.
That’s what happens with a tornado. Something causes the vortex in the cloud to narrow and extend down, and that narrowing increases the wind speed in the vortex. If you want to experience the Law of Conservation of Angular Momentum yourself, all you need is a chair that spins freely, and a couple of big books or bricks. Start spinning with your arms extended and the books or bricks in your hands, and then quickly bring them in to your chest. You'll feel an immediate acceleration.
This brings us to the past week, and in particular, Sunday’s funnel clouds. All three days began with no expectation that these large-scale ingredients would be in place, and especially without much turning of the wind aloft.
Tornadoes were not expected in these areas on any of the days. So how did tornadoes and funnel clouds develop? Sometimes, when we don’t have the required large-scale wind field, individual thunderstorms and their individual wind fields (wind rushing out ahead of the storm) interact on a very local level.
A thunderstorm may shift the direction of the wind flowing into a nearby thunderstorm, and create a period of low-level wind shear conducive to rotation. Sometimes, funnel clouds and even tornadoes result. Most of the time in this scenario, any funnel clouds or tornadoes are small, brief, and weak (that doesn’t mean that they aren’t dangerous or destructive), and we call them gustnadoes. The photos from Sunday’s funnel clouds in Lenawee and Monroe Counties clearly show very small, weak funnel clouds.
It is a natural tendency to blast meteorologists in these situations when people say they didn’t get any warning. But keep in mind that these small funnel clouds and gustnadoes are normally so small that we can’t even detect them on Doppler radar. Furthermore, they happen so fast that they are frequently gone before the National Weather Service even learns about them and can issue a warning.
Fortunately, the super cells that develop the highest end, most destructive tornadoes have large, easily seen circulations that give us anywhere from 10 to 20 minutes' notice before there’s even a tornado, and we can warn you about these well ahead of time.