A deeper look at Tuesday night's possible tornadoes in SE Michigan

By Paul Gross - Meteorologist

DETROIT - We explained in great detail Tuesday afternoon why we were so concerned about the Tuesday night tornado threat, and you now know that at least one and possibly two tornadoes did indeed touch down and cause quite a bit of damage in northeast Monroe County and also in the Gibraltar area in Wayne County.  

UPDATE: NWS confirms EF-1 tornado touched down Tuesday in Michigan's Monroe County

The National Weather Service will conduct formal damage surveys later Wednesday and determine if they were indeed tornadoes. So allow me to now show you some of what Ben Bailey and I were seeing on radar in the Local 4 weather office as the event unfolded, and how this technology undoubtedly saved lives.

First of all, here’s the “usual” radar image we typically show you on TV as an average looking heavy thunderstorm moved across Monroe County.


This radar product is called Base Reflectivity -- all you get to see here is rain. Yes, the red colors indicate torrential downpours, but this gives you no additional information about what’s going on inside the storm.

Now take a look at this radar image:

This is called Storm Relative Velocity. You see, Doppler Radar not only sees those raindrops, it also sees the wind inside of the storm. How? Because the wind blows those raindrops around. 

So here’s what happens: The radar sends out a beam, and it knows the frequency of that beam. When the beam hits raindrops being blown toward the radar, the part of the beam reflected back to the radar has a higher frequency. Wind blowing away from the radar is reflected back to the radar with a lower frequency. The radar computes the difference in frequency between the beam that left the radar and the beam reflected back, and knows not only if it’s wind blowing toward or away from it, but also the wind speed. Now take a look again at the above image. The radar in White Lake is directed about 50 miles to the upper left of this image. Green colors are blowing toward the radar, and red colors are blowing away from the radar. At the time of this image, we were noticing some greens right next to some reds, which we call a couplet and, at 8:19 p.m., the National Weather Service issued a tornado warning for Monroe County.

It is important to understand that this warning was what is called a Radar Based Warning. Radar indicated solid rotation, but there was no reports of a tornado. Yet. 

Now take a look at these three radar images:

The official title of this product, used by meteorologists all around the country, is Correlation Coefficient (“CC”).  But that’s such a complicated term that we title this radar product “Debris Potential” when we show it to you on TV.

Here’s how it works: The radar also looks at the consistency of the targets that its beam is detecting.

For example, raindrops are relatively uniform in their shape and size. So are snowflakes. Hail starts to show some differences, and this product indicates that. But the biggest use for this product is during a tornado event, because tornadoes lift debris of very different shapes and sizes into the air, and this usually shows up as a little “ball” on the CC image. We meteorologists call that a Debris Ball. If you look at these three images, I’ve circled a Debris Ball that developed in association with the Monroe County tornado. Later, a weaker one developed near Gibraltar, where a brief tornado touchdown may have occurred.

The most important morel to this story is that the National Weather Service’s tornado warning was issued approximately ten minutes before the first Debris Ball occurred. That’s the power of Doppler Radar, and meteorologists who know how to interpret its data. The people in Monroe County affected by the first possible tornado had at least 10 MINUTES notice before the possible tornado touched down.

Ben and I were keenly aware that a number of people were upset about interrupting programming during NBC Premier Week, and we weren't happy about it, either. However, lives were in danger, and the Local4Casters will ALWAYS put your safety first. 

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