The difference in “low” and “no” threat

A few weeks ago, a Moderate Risk (and a nearby High Risk) of severe storms got this region worked up into a lather about severe weather.  The governor issued a State of Emergency.  Schools closed in droves, and life ground to a halt waiting on storms that for many communities never came.

We did end up with some hail, but the expected tornadoes never came. Good for us.

Fast forward to this past Saturday. Slight Risk, Severe Thunderstorm Watch, no closures, no states of emergency.

Storms were much worse Saturday than the day of the Moderate Risk, so you might be asking yourself 'what gives?'

#1 - Severe thunderstorm risk categories from the Storm Prediction Center are guidance, but they refer to the NUMBER of intense storms - not necessarily the intensity.  This is a bad way to make a decision on 'how bad it's going to be at my house.'

#2 - Thunderstorms operate on scales much smaller than our data network and smaller than our best model resolution.  The largest tornado on Saturday was only 720 yards wide.  That's about 0.66 kilometers.  Our best-resolution model guidance puts out data in 3-kilometer blocks (3km x 3km): much too large to resolve the two individual supercells that rolled over North Alabama and Southern Tennessee Saturday afternoon.

#3 - 'When it comes to storms, expect the unexpected.'  That's a quote I've heard repeated over and over throughout my career; it was coined by a meteorologist in Texas: Al Moller.  Meteorologists readily admit that we cannot know it all about how individual storms will behave; that's something you must understand when digesting forecast information.  Any given storm on any given day can become your worst nightmare.

Take the Skyline tornado for example.  It was an EF-0 with winds at 85 miles per hour.  If Saturday had been a "High Risk" day, I imagine the scoffers would have said "oh, big deal.  An EF-0.  You got us all worked up over nothing."

Samuel Arnold would disagree strongly with you on that.  Imagine if this tree had come down while his child was sleeping in that bunk bed.  We go from a day with some strong storms to a day with serious injuries or even worse.

The bottom line?

If you've lived here more than a few Spring tornado seasons you know there's always at least one day when the storms overachieve the general expectations.

If you live here a few more years, you'll see it again.  There are unknowns in the atmosphere that have to be considered when you hear "severe storms are possible."

No, I can't tell you a tornado will hit your house the day before.  I can say the risk is low, or high, or medium, or there's no risk at all.

Dr. David Cleveland, a Cardiothoracic Surgeon at UAB/Children's of Alabama, summed risk up in the best words I've ever heard the day before he operated on my youngest son:

"There's a small but very real risk that something could go wrong."

To apply that to Saturday: don't cancel your plans, but understand that a single intense storm can be damaging or even deadly.

It's the peak of severe weather season.  Don't let your guard down just because there's no high risk.

-Jason
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