Isolated Showers in the Distance: What Causes the Difference Between Spotty and Widespread?


How many of you have seen a storm in the distance and just prayed for it to hit your field or lawn, only to have it miss a mile down the road? Or been that lucky one that your neighbors were jealous of? What causes storms to be so isolated and judgmental? And to the contrary, what makes them widespread?

My colleague asked me similar questions and I wrote down some answers for her. I hope you understand the ramblings of a weather nerd like myself, and perhaps learn about the conditions that separate isolated showers from widespread ones.

The difference between isolated pop-up showers and organized widespread showers is largely about focus. When we see pop-up showers during a summer day, it’s largely because there is good heat and moisture near the surface. I like to think of a pot of boiling water as a good visual. As the sun (stovetop) heats the atmosphere (pot of water), you start to see bubbles rise (like bubbly cumulus clouds forming overhead midday). When you heat up the water (atmosphere) enough to get a rapid boil (like in the afternoon), you likely get enough for rain to fall. But that’s heating evenly over a large area. The bubbles are very localized and not widespread. The whole pot of water does not just magically turn to gas and jump out of your pot. There is no focus, it is seemingly random.

Organized events usually have a feature that is driving them, and it could be myriad things like a front, upper-level disturbance, terrain, or something else. There are lots of reasons, but these features focus the heat and moisture to a smaller area. If it is moving, like a front, then you get an organized area of widespread showers moving through.

To use another analogy, the feature picks up the heat and moisture as it moves through like a push broom on your garage floor gathering scattered dust bunnies, concentrating them into a line of dirt. Or showers, in this analogy.

But even these features could produce spotty rain, too, right? Fronts do not always have widespread showers. We have seen that a lot across the northern tier of the country this June.

So, two main questions come into play. First, “Is the feature strong and is it consistent?” With the analogy: “Is my push broom solid, or do I have gaps in the bristles?”

And the second question: “Is there enough heat and moisture to produce the rain across the feature?” Or, “Are there enough dust bunnies to collect to form a line of dirt?”

If the answer is “Oh heck, yes!” to both, then you likely see a widespread event. If it is “Yeah, but” you will likely see scattered showers. If it’s “Eh, maybe?” you may only see isolated showers.

Now this is just a simple explanation, and weather is much more complicated than this. Thank goodness or I would not have a job. But it could give you an idea on why that little smudge of gray in the distance is bypassing you again. Or, why you should always leave space to put your car in the garage.

John Baranick can be reached at john.baranick@dtn.com



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