What those lightning bolts in your thunderstorm icon mean
Meteorologist
Thursday, June 2, 2016, 1:10 PM - Have you ever checked your TWN app or gone online for the day’s forecast, and seen something like the image below?
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You probably notice the thunderstorm icon first – indicated by the lightning bolts and rain drops stemming from the cloud – and maybe decide to postpone those planned outdoor activities. But by the day’s end, much to your chagrin, no thunderstorms developed; not even a single drop of rain fell. What a terrible forecast!
For many Canadians, this situation will become increasingly common as we continue through spring and head into summer, when the risk for the development of thunderstorms steadily increases from now until its peak in July. But are these forecasts really "terrible" if a thunderstorm didn’t go right over your house?
It’s time to learn exactly what meteorologists at The Weather Network are trying to tell you when you see that infamous thunderstorm icon.
Lesson One: A thunderstorm icon is not a guarantee that you will experience a thunderstorm
While icons are great visual aids for quickly conveying the weather forecast, they can often be misinterpreted when it comes to thunderstorms.
Because thunderstorms are very localized (more on that in the section below) meteorologists employ the icon as a way to highlight the potential for thunderstorms to develop in your region, but by no means guarantees that you will see one. This is why the text under this icon reads, Risk of thunderstorms."
In order for this icon to appear, the meteorologist must have deemed the atmosphere to be sufficiently unstable – or favourable, in other words – for a thunderstorm to develop, and is therefore his or her way of giving you advanced notice that one could occur rather suddenly. After all, thunderstorms can seemingly come out of nowhere and pose a serious threat to your personal safety and property.
Another common misconception pertains to the relationship between the thunderstorm icon and its associated POP value (the probability of precipitation). Frequently, people think that the POP value represents how likely it is that a thunderstorm will develop (i.e. an 80% POP with a thunderstorm icon must mean there’s an 80% chance of thunderstorms).
This is not the case.
POP is an independent quantity that addresses the overall probability of any precipitation occurring over a given area at a given time – be it rain, hail, or snow. Certainly, a higher POP implies a greater likelihood of precipitation occurring – but it does not directly relate the specific probability of a thunderstorm occurring.
Lastly, it is worth mentioning that a lack of lightning bolts in your icon does not rule out hearing thunder. Oftentimes, thunderstorms can be embedded within widespread rain showers, or be heard over large distances.
Lesson Two: Thunderstorms are very localized
Unlike snow storms or widespread rain events that occur on a synoptic scale (a distance of hundreds of kilometres) thunderstorms occur on the mesoscale (a distance of ten to one hundred kilometres).
This means that it’s entirely possible for a thunderstorm to pass directly over you, while your friend or family member on the other side of town remains completely dry.
This fact alone makes it almost impossible for a meteorologist to specifically pinpoint which areas will see thunderstorms and which will remain dry, leading to the preference of using the icon to highlight thunderstorm potential over a general region. This potential is often shown as a risk map on our TV and web platforms, like the example below.
In addition to being incredibly localised, the exact timing of thunderstorm initiation is another difficult task for any meteorologist. This is why you’ll seldom here a phrase like, "Thunderstorms are set to impact Windsor, Ontario tomorrow night at 5 pm." Instead, a range of time is given, on the order of a few hours, to highlight the most likely window in which a storm could develop. This range of time usually coincides with the peak atmospheric instability on any given day, which on average is around the dinner-hour.
Lesson Three: Not all thunderstorms are equal
While it is always prudent (and encouraged) to take any thunderstorm risk seriously, there is in fact a range of thunderstorm severity.
According to Environment Canada – the governing body in charge of issuing severe thunderstorm watches and warnings – a thunderstorm is deemed severe when one or more of the following occur:
Large hail (>2 cm in diameter); damaging winds (gusts of 90 km/h or above); or heavy rain (50 mm/h or more in most of Canada; 25 mm/h or more in Pacific, Northern or Maritime provinces).
When none of the above conditions are met, thunderstorms will be classified as non-severe. This does not mean, however, that they are benign. One of the most dangerous aspects of any thunderstorm is their ability to produce frequent and intense lightning.
According to Environment Canada, lightning strikes kill up to 10 people per year, seriously injure up to 164 others, and ignite some 4000 forest fires. It is no wonder, then, why we often employ a familiar tagline: "When thunder roars, go indoors!"
It is worth clarifying that the forecast you receive online or on your TWN app will not indicate the severity of the thunderstorm expected, but extra caution should be taken when the text accompanying the icon reads "risk of strong thunderstorms."
Finally, there is no correlation between the loudness of thunder and the severity of the storm; the loudness more accurately reflects how close you were to a lightning strike. And as many know, the number of seconds between seeing a lightning strike and hearing thunder can give a good indication of how far away the storm is: five seconds between the two means the storm is roughly a mile (1.6 km) away.
So the next time you see those lightning bolts in your icon, keep in mind it’s our way of giving you a head’s up that some active weather may be coming your way. It may not last all day, and it may not happen at all, but better to err on the side of caution.
