Why was there no tornado warning for Lenawee County tornado?

NWS confirms EF-1 tornado in Riga Township

By now you’ve heard the news that a tornado touched down Sunday evening in Lenawee County.

Specifically, it touched down just northwest of Riga Township and traveled east-northeast for 2.7 miles before lifting. The funnel was 125 yards wide at the ground, and its speed reached 90 miles per hour, which rates it an EF-1 on the 0-to-5 Enhanced Fujita Scale.

So if no tornado warning was issued, was that a mistake by the National Weather Service? The answer is short and decisive: no.

Let’s examine what happened Sunday evening.  As you know, severe storms moved across the area multiple times Sunday evening and overnight.  At various times, some of the storms showed rotation on radar.

However, it is very important to remember that not all storms with rotation produce tornadoes.  In fact, most severe thunderstorms don’t drop twisters.

Determining if a severe thunderstorm is about to develop a tornado is frequently difficult.  We look at velocity products on radar and not only determine if there’s a circulation strengthening and tightening in the radar’s lowest scan, but also if we see that happening aloft.  And even if we do see this, that circulation still might not extend down to the surface.

Also, the fact that this tornado occurred so far from the radar in White Lake made it more difficult to see that lowest level circulation, since the radar beam leaves the radar at a slight angle and that, combined with the curvature of the earth, puts the beam higher and higher above the ground the farther and farther you get from the radar.

Related: What to do before, during and after a storm

Furthermore, just because a new radar scan comes in suddenly showing promising rotation, the NWS doesn’t just jump on that and automatically issue a tornado warning.  Rather, they (and we) like to confirm this rotation on the next radar scan, as well as examine that circulation aloft to reduce false alarms.

In the case of Sunday evening, the tornado that damaged those five homes and barns and caused other tree damage was a very brief spin-up.  It was on the ground for only four minutes.  So, it came and went with a brief radar signature, and there was no confirming evidence, such as a visual sighting, to alert the NWS (or the Local4Casters) that a tornado has actually touched down.  It wasn’t until Monday when the NWS did their storm survey that it was determined that the damage was caused by rotating winds, and not straight-line thunderstorm winds.

Something else to keep in mind is that most tornado deaths are caused by the strongest two percent of tornadoes.  Since the strongest tornadoes are also larger, stronger circulations, we often see them more easily on radar and can warn you ten-to-twenty minutes before they touch down.  In fact, since these violent tornadoes are strong enough to lift debris in the air, a special Doppler radar product called correlation coefficient actually  highlights that debris…we call that radar signature a debris ball.  This product has actually been the confirmation I needed with a couple of past tornadoes to confidently say that a tornado was on the ground before there was any visual confirmation.

There’s one other very important thing to consider:  even though we were under a severe thunderstorm watch Sunday evening, a severe thunderstorm watch does not mean that tornadoes are not possible.  Rather, it means that the risk is low enough as to not warrant a tornado watch.  All of my messaging on Sunday mentioned tornadoes as a possibility, with the greater threat being severe wind gusts.  So, despite the type of watch we are under, you always need to be prepared for any severe possibility, including tornadoes.

The bottom line with the Riga Township tornado is that it was a smaller, brief spin-up twister that could not have been warned for.

Related: Paul Gross explains: Why do sirens sound if there’s no tornado warning?