What is exactly is a ‘scattered’ storm anyway?


Shelf cloud near Cullman (Photo: Randy Cone)

Two stories from my school days can put the daily summer forecast into perspective.

How quickly we forget!

I had Physics my senior year at Holly Pond. That means I had every math class offered in the 90s before taking physics, as did most of us in that class. Ms. Butler put the following equation on the board and asked us what it meant in an almost rhetorical way; she wasn’t very happy that none of us answered. I mean, NO ONE answered her.

y = mx + b

That’s Algebra 1 stuff, and her class sat there like a mule looking at a new gate. (By the way, that’s called slope-intercept form.)

What’s that got to do with the weather forecast?

Slope-intercept form isn’t the easiest thing you’ll ever learn, but once you learn it, you should at least be able to recognize it. Summer storms around Alabama are similar: once you know what they’re like and how chaotic they are, you shouldn’t be surprised when one sneaks up on you. A chance of a storm – a real one with torrential rain, strong winds and continuous lightning – means just that: a chance that you’ll get hit by one.

It’s not as complicated as Algebra or Physics, but you’d do well to remember how powerful these big boomers can be.

It’s not going to be perfect

In Seventh Grade, we all had to take twelve weeks of Literature, twelve weeks of ‘Ag,’ and twelve weeks of Home Economics. ‘Home Ec’ required some skills!

Mrs. Hill required we learn some sewing, and we had to do our best to use a sewing machine to follow a pattern: perfectly. She counted off points for every teeny-tiny error.

If you’re skilled at sewing, you’re saying: “big deal, so what?” If you’re like me, you start to sweat at the prospect of having to be perfect with a fast-moving needle near your fingertips!

What in the world does this have to do with summer storms?

Perfection is possible when you are the one controlling everything: the speed of the machine, the orientation of the pattern, the amount of practice, etc.

Perfection is NOT possible when you have no control (and can’t even truly get a good measurement of the entire atmosphere). We don’t even attempt to claim a perfectly accurate forecast: especially in the summertime.

The rain chance is one part confidence that rain/storms will develop and one part how much area gets covered by rainfall. We’re looking at it from a top-down view (maps) and trying to relate that to you with a number: a percentage chance that the rain happens at your house, office, farm, etc.

What we’re really giving you with that number is a PQPF: a probability quantitative precipitation forecast. It’s the chance of getting at least a given amount of rain over a space of time.

This is a PQPF map for 1 PM to 7 PM on Tuesday, July 7th

Remember these things about summer storms

  • They can absolutely be the worst weather you experience in a calendar year because they’re numerous, they’re an almost daily occurrence in June, July and August, and they’re powerful.
  • A 20% chance of a thunderstorm can get you the same amount of rain, wind, lightning and hail that a 70% chance of a thunderstorm can serve. It just won’t hit as many communities as the 70% day.
  • If we forecast a high of 90ºF and a 30% chance of thunderstorms, and your high temperature is 88ºF and you got two inches of rain in 90 minutes, that’s not a “busted” forecast.
  • If we forecast a high of 92ºF and a 60% chance of scattered thunderstorms and you get 94ºF and never even hear it thunder, that’s also not a “busted” forecast. Both scenarios are within expectations.

If you’ll remember what summer storms are capable of and how chaotic and spaced-out geographically they can be, you’ll have a better understanding of what to expect on a given day when there’s a chance of storms.

Looking for the rest of the forecast? It’s always online at WHNT.com/Weather and in the “Daily Forecast” section on Live Alert 19!

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