Weather Questions with Martz: Why do thunderstorms split before they reach my house?

Don Story sent me this question on Twitter yesterday: “Hi Chris, maybe you can answer a question for us Central Floridians. I live near Summerfield and a group of us at the pool are avid radar watchers. A lot of times when it looks like I can’t miss from a thunderstorm it suddenly splits and goes around us.”

This is a great question and is actually one of the most frequently asked questions I receive from local farmers. Of course, there’s no rest for the weary when many of them aren’t satisfied with the answer I provide. Such was the case two Saturday mornings ago at our local diner, when an elderly gentleman asked me this same question rather abrasively. He kept insisting that I was wrong, so I don’t really know why he asked me the question in the first place.

Unfortunately, one of the realities of telling the truth is that it can mean telling others what they don’t want to hear, which leads us to our first explanation for this common and seemingly apparent phenomenon—that is confirmation bias.

Reason #1: Confirmation Bias

Let’s face it. If the morning forecast called for afternoon thunderstorms and you’ve been watching the radar avidly, it can be frustrating watching a promising storm fall apart or split before it reaches your location. This is especially true on hot summer days when rain-cooled air provides relief from scorching heat, and is a necessity for farmers, particularly during prolonged periods of drought, which can make or break a plentiful harvest.

In such instances, individuals often want approaching storms to hit them. As a kid, you may recall this not occurring because you didn’t stare at radar imagery and notice how often storms seem to miss your location. This tendency to interpret information in a manner that supports your beliefs is known as confirmation bias.

But, there is some element of truth, but not in the way you might expect…

Reason #2: The Meteorology

Indeed, local features such as the topography of the land (e.g., mountains and hills), in addition to the proximity of large, cool bodies of water, such as lakes and rivers, certainly do affect the behavior and motion storms.

In Don’s case, Summerfield, Florida is in the heart of the state and borders Lake Weir, though it’s to the east of town, so more stable air over the lake is not likely to affect all too much the evolution of storms that normally move in from the west off the Gulf of Mexico. Florida is also very flat with its highest natural point only 345 feet above sea-level, but that’s Britton Hill near the Alabama border. In a general sense, however, a storm hitting one location, A, and missing another, B, in a straight, forward trajectory from its starting point has less to do with geographic features and more to do with atmospheric conditions and the life cycle of thunderstorms.

There are three common types of thunderstorms—your typical run-of-the-mill single cell storms, quasi-linear convective systems (squall lines), and supercells that are known to produce tornadoes. The latter two are most commonly associated with outbreaks of severe weather, so let’s discard them. By and large, the most frequent type of thunderstorm people experience are single cell storms, which are localized bursts of convection and occur nearly every summer afternoon.

These storms are often referred to as pop-up thunderstorms as there is no focal point of formation, as would be the case with dynamic lifting mechanisms such as frontal wedging, orographic lift or low-level convergence coupled with upper-level divergence in synoptic-scale extratropical cyclones. Instead, they form as positively buoyant air rises through a deep unstable layer and cools to saturation. At peak intensity, any given storm will be able to maintain its strength granted the rates of condensation equal that of evaporation.

However, a moisture-rich environment with enough instability to accelerate air upwards to from deep convective clouds isn’t uniform throughout the atmosphere. If a storm traverses over the terrain into a region that’s drier, cloud droplets will begin to evaporate as the vapor pressure of the air decreases while the high saturation vapor pressure of cloud droplets, due to their curvature, remains constant. Resultantly, the storm will die off in a matter of 30 minutes to an hour.

Image credit here.

Over time, evidence shows that precipitation amounts are fairly uniform over any given region. So, while half the time, a storm may miss your location, other times it won’t. Nature is a pretty good balancing act.


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Categories: Weather, Weather Questions with Martz

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