Ag Weather Forum

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

John Baranick
By  John Baranick , DTN Meteorologist
A rain shower is passing by this field without a drop. (DTN photo by Pam Smith)

This was the year I decided that I would volunteer to coach my son's little league team. I will not bore you with the complexities of teaching baseball to 7- and 8-year-old kids. Rather, I'll note an experience that I'm sure many of you have had before regarding changing weather and one a colleague questioned me about today that I thought could use a little explanation.

It was team picture day on the evening of June 18 at the local middle school. On the way there, I noticed a big thunderstorm off in the distance. Being the weather nerd that I am, usually, I would check my phone, which has DTN's RadarScope app on it, to check out the radar. How far away is it? How strong might it be? Where is it moving? Are there other storms around, too? All of these are important questions. But, also being a little late to the appointment, I decided to head inside instead. After about 15 minutes of getting our brand-new shirts and hats and taking pictures with the rest of his team, we left. As we got outside the building, I immediately looked toward the sky and noted how the anvil-shaped cloud had grown so large that it was wiping out the sun. So I turned to my phone ...

Whew! That cell was large and had a severe thunderstorm warning issued by the National Weather Service. But it was to our south and moving east. We just missed it, I thought. But of course, I did not notice the small speck growing off to our west.

On the way home, which is about 7 miles away, my son noticed a rain shaft in the distance. It is hard to tell how far away clouds and storms are. I am a terrible judge of distance. But I thought it could be close to home. We saw a few lightning flashes and I was getting excited. Our area, like most of southern Minnesota, needed rain. Bad. Upon entering the driveway, it started raining those big, fat drops. We parked the car outside and made it into our stuffed garage just in time before the hail started to fall.

And it was large. I measured 1.25 inches on a couple of them, which is enough to verify the severe thunderstorm warning that the National Weather Service placed on the storm prior to me getting into the driveway. My roof and my minivan took a beating, but no damage thankfully. But it made me wonder ... I was ultimately 7 to 10 miles away from the storm. I could see it. It looked close, or far, or I could not tell. But it was enough to make me think we might just miss out on the needed rainfall.

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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