Pop-up thunderstorms are possible this weekend… but what are they


Frequently during the summertime months, we see the development of short-lived thunderstorms. Meteorologists call these airmass thunderstorms, or more commonly known as pop-up or garden variety thunderstorms.

When these develop they will cause brief downpours, lightning, and occasionally gusty winds. These short-lived storms don’t last very long due to a missing key ingredient for thunderstorms. The main four ingredients for severe weather is known as SLIM.
Shear: ——-The changing of wind speed and wind direction as you go to higher altitudes
Lift: ——— A mechanism that allows air to rise (ie. a front or area of low pressure)
Instability: — When air at the surface is warmer than air aloft, causing air to rise
Moisture: — Clouds are made of water and need moisture to form

Shear is the lacking ingredient in airmass thunderstorms. We will dive into why that plays an important role in the longevity of thunderstorms.

But first, let’s talk a little about how thunderstorms form in the first place.

Firstly, we will start with our chart here. The red line shows the profile of the temperature from the surface to the top of the troposphere. Think of this as a vertical slice of the atmosphere. We get this kind of data from weather balloons launched twice a day from the National Weather Service across the country.

So, as the day progresses the temperature gets warmer due to the sun. At the surface, this forms little pockets of warmer air.
Think of this like when you are going to boil water. When you turn the burner on bubbles form at the bottom of the pot. When a boil is reached the bubbles begin to rise in the pot because they are now warmer than the surrounding water. The atmosphere works the exact same way!

The growing dotted line is the temperature profile these bubbles would have as they rise through the atmosphere. In this case, the dotted line or air bubble is warmer than its environment.

Once the air bubbles are warmer than the surrounding air, they begin to rise. Naturally, as you go up in the atmosphere the temperature cools down. This causes the warm moist air bubble to cool down and begin to condense. Just like a cold can of soda in the summer the air bubble forms water droplets, in turn, creates the clouds that we see.

As long as the air bubble is warmer than the air around it, it will continue to rise. This creates clouds called cumulonimbus clouds or thunderstorm clouds. The rising air is known as an updraft.

That little spike along the temperature line is known as a temperature inversion, where warmer air is higher up. Meteorologists also refer to that as a cap. This can cause the air bubbles to stop rising and keeps storms from firing. In our case here, the dotted line is warmer than the surrounding air, so the air bubble will keep rising.

Eventually, the air bubble will keep rising and cool so much it will be the same temperature as the environment around it. This stops the air bubble from rising and forms the top of the cumulonimbus cloud. The air will fan out forming the anvil of the cumulonimbus cloud. At this point, the air is no longer rising and will continue to cool and begin to sink.

When the air begins to sink it forms what’s known as the downdraft. The rain-cooled air falls rapidly out of the thunderstorm. When it hits the ground it will spread out and cause gusty winds at the surface. This is commonly referred to as the outflow, or gust front.

So, with all that in mind, what is the reason wind shear is so important for thunderstorms.

In the case of airmass thunderstorms, there is no front that will spark thunderstorms. Instead, airmass storms form in the afternoon time frame when it is hot and humid out. This causes higher instability in isolated areas, the extra instability acts as the lifting mechanism for thunderstorms to develop. So, just like we explained before the pop-up storms develop like any other thunderstorm.

Without wind shear or faster winds aloft, the sinking air (downdraft) falls on top of the warm rising air (updraft). This will choke off the developing thunderstorm causing it to fall apart within a matter of minutes. Causing a quick and heavy downpour keeping these storms from becoming severe.

When wind shear is added into the mix, this will tilt the updraft. The now tilted thunderstorm keeps the updraft and downdraft separated from each other. This lets the storm live longer by allowing the warm rising air to continue to fuel the storm. The rain-cooled downdraft acts like exhaust that is pushed out from the storm. The downdraft usually pushes out ahead of thunderstorms, and because the air is cooler and denser it acts as a lifting mechanism or a mini cold front. This allows for more storms to develop around it. This is why when one small storm develops, more thunderstorms around it can form.

So, as we head into the end of the week and weekend we are looking at hot and humid conditions. Due to the extra heat and moisture, we will likely see added instability allowing for a few isolated thunderstorms to develop in the afternoon hours. So there are no solid chances of rain, but a stray thundershower can’t be ruled out over the next few days.

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