The holy grail of weather communication is simple: deliver a message about a severe weather threat in a way that people understand and are able to act on.
But that doesn’t always happen.
I can’t tell you how many times I’ve given a forecast mentioning storms, only to hear someone later say, “You didn’t say it would rain.” Even more concerning is when I hear, “I didn’t know the storms would be this bad.”
Those moments tell me something important: sometimes the message about severe weather isn’t fully understood.
Over the next several weeks, I’d like to spend some time going back to the basics and building a clearer understanding of what severe weather and severe weather forecasts actually mean. My hope is that by the end of this little journey, we’ll all learn something and be better prepared the next time storms threaten.
What is severe weather?
You might be surprised to learn that severe weather is defined by just three elements:
- Hail: At least 1 inch in diameter (about the size of a quarter)
- Wind gusts: 58 mph or stronger
- A tornado
You may also notice something missing from that list. Lightning and flooding are not included in the official definition of severe weather. We’ll talk more about that in a future edition.
The history behind severe weather warnings:
When forecasters talk about a severe weather threat, it means one or more of those three elements could occur. Interestingly, severe weather warnings originally weren’t created for the public, they were designed for aviation.
In the early 1950s, hail warnings were issued for ¾-inch hail. A 1952 study found that hailstones of that size could cause significant damage to aircraft traveling between 200 and 300 mph.
In 2010, the threshold changed to 1-inch hail. Engineering studies showed hail that size can damage roofs, siding, and vehicles. A quarter-size hailstone falling at roughly 100 mph can cause real property damage.
Despite how destructive hail can be, hail-related deaths are extremely rare, only eight have been recorded in the United States over the past 70 years. So, when you see a hail warning, it’s primarily about protecting property.
Why 58 mph winds?
The history of wind warnings is also interesting. Originally, the National Weather Service issued severe thunderstorm warnings for winds of 75 mph or greater, while aviation used a separate warning threshold of 50 mph.
In 1962, the Air Force raised its warning threshold to 58 mph. Having two different standards was confusing, so in 1970 the National Weather Service unified the criteria. Since then, 58 mph has been the official benchmark for severe thunderstorm wind warnings.
At that speed, winds are strong enough to damage trees, which can then fall onto homes, vehicles, or people.
These wind warnings refer to straight-line winds, meaning the wind moves in one general direction. Tornado winds, on the other hand, rotate and spin.
The purpose of severe weather warnings:
The mission of the National Weather Service is simple: to save lives and protect property.
So, when you see a severe weather outlook, it means the atmosphere could produce storms capable of causing damage or posing a danger to people.
Wednesday’s severe weather threat:
Our next potential severe weather event for Houston and Southeast Texas arrives Wednesday.
Now that we understand what “severe weather” actually means, let’s take a look at the setup and what hazards could develop.
First, it’s important to know where this forecast comes from. The outlook you’re seeing is issued by the Storm Prediction Center in Norman, Oklahoma. The SPC is part of the National Weather Service, which operates within National Oceanic and Atmospheric Administration. Their job is to monitor and forecast severe weather across the United States.
The Storm Prediction Center issues outlooks for severe weather up to eight days in advance, giving forecasters and the public an early look at when and where storms could become dangerous.
For Houston on Wednesday, the SPC has placed our area in a marginal risk for severe weather and a slight risk north and east of the Loop.
Instead of just describing what a marginal and slight risks means, I think it’s more helpful to show you the SPC graphic that explains the different threat levels and what each one represents.
Specifically, Wednesday’s threats include all three severe weather elements. Damaging winds, large hail, and tornadoes are all possible. You may hear someone say the greatest risk is for damaging winds because that probability is 15%, which is represented by the yellow shaded area below.
One important thing to remember with this forecast, and really with any forecast, is that these graphics show what is possible, not necessarily what will happen. For severe weather to develop, several ingredients have to come together at the same time. If one or two of those ingredients are missing, the storms will not become as strong.
It’s also helpful to understand what these probabilities mean. For example, the hail risk on Wednesday is 5% across all of Southeast Texas. That doesn’t mean everyone will see hail. In fact, only a small number of cities or neighborhoods may experience it.
This is plenty of information for now. In the next articles, I’ll break down a few more important topics, including lightning, flood outlooks and warnings, and the difference between a watch and a warning.
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