"Pop-up storms" is a pretty common term used by meteorologists all across the country, but do you know what it really means?
Large batches of rain and storms that cross an area are normally triggered by a large-scale feature in the atmosphere, such as a cold front, warm front, trough (line) of low pressure, area of surface low pressure, upper level disturbance, etc.
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Whether or not these triggers initiate a batch of rain or storms is also dependent upon how unstable the atmosphere is. The more unstable it is, the more weather we get; the less unstable it is, the less weather we get.
Here’s how that works:
Clouds and precipitation are caused by rising air. The more violently we can get parcels of air to rise, the more significant the resulting weather is. So, the “lighter” the air is, the easier it is for a trigger to get it rising. Hot, humid air is “lighter” than cooler, drier air, which is why thunderstorms are so much more common on hotter, steamier summer days.
Here’s an analogy to consider:
Let’s say I asked you and a friend to time yourselves going up a flight of stairs. BUT, I asked you to carry a pillow up those stairs, and I asked your friend to carry a couch. Who would be able to go up the stairs faster?
You would, of course, because you are much lighter carrying that pillow than your friend carrying the couch. That’s how it also works in the atmosphere. On days where it’s less unstable, it’s like trying to carry that couch up the stairs. Doing it by yourself, you probably couldn't even do it. However, let’s say I let you and your friend both carry that couch up the stairs -- you could make that happen. You and your friend working together are acting like a stronger atmospheric trigger -- I.e.: a stronger front, storm system, or upper level disturbance – which can overcome less instability and still force those parcels upwards despite the lower instability.
On hot, humid days, getting those parcels of air to rise quickly is like you running up the stairs with that pillow. It’s no problem. But what if there’s no trigger? What happens then? Well, if it’s unstable enough, individual parcels pulse upward. Their motion up can be initiated by topography, a lake breeze, or just an area that heats up more than areas around it. These parcels rise violently to form individual thunderstorms, but there is frequently no organized line or cluster of the storms -- they just “pop-up” randomly. That’s why we call them pop-up storms (sometimes also called "popcorn thunderstorms").
If the atmosphere is really unstable, then something we meteorologists always monitor is the downward core of air rushing out from the lead edge of a thunderstorm, because these little “outflow boundaries,” as they’re called, also can trigger additional storms. Sometimes, we can actually see these outflow boundaries on radar, and know ahead of time where some of the next storms will pop-up.
So now you know what pop-up storms are. They are just random, scattered thunderstorms, with no apparent geographic organization.
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