Thunderstorms come in all shapes, sizes, and horsepower. An isolated, stationary thunderstorm in the tropics may appear quite different than one embedded in a hundred-mile-long derecho barreling down from the great plains into the Midwest. Storm morphology is a critical and intricate aspect of meteorology.
What is a Severe Thunderstorm?
Who knows better than our pets? Let’s introduce the pet-storm index. On an ascending scale of 1-5, does your dog look at the window (1), start pacing (2), look for solace (3), hide under the bed (4), write out their last will and testament (5)? For cats it’d be: Does the cat sleep (1), eat (1), lick itself (1), acknowledge the outside world with a slight twitch of the eye (5)?
Sadly, they usually don’t let us bring pets into the office. Consequently, meteorologists with the National Weather Service require an alternative classification system. They define a severe thunderstorm as one capable of producing hail larger than one inch in diameter and/or having wind speeds exceeding 50 knots (58 mph).
To identify common shapes among the most severe storms, researchers at LSU and NIU analyzed 5,156 radar reflectivity images of tornadic storms in the southeast USA from 1996 to 2017. They centered each image on the location of the first tornado report and then statistically overlaid them to create a composite radar reflectivity image. This technique visualizes spatial patterns of storm organization and structure.
The authors then manually classified the storm composites into three groups:
Quasi-linear convective systems (QLCS): Storm systems that seem long (3:1 Length-to-width ratio and over 100km in length)
Cellular system: Storm systems circular in organization that fit entirely within a 100x100km box.
Tropical system: Storm systems that occurred near hurricane tracks (per the HURDAT database).
Composite radar reflectivity images of the three storm categories. (Haberlie, Ashley & Karpinski, 2020)
Notice the elongation of storms along a southwest-to-northeast axis. This is normal in thunderstorms. As the authors put it, “[the NW] is often an area where cold or dry air has undercut warm or moist air, and instability has decreased.” (p. 13). And the SE areas often experience capping inversions. Typically the atmosphere cools as altitude increases, but sometimes it actually increases. This inversion layer acts as a cap or lid that suppresses the vertical movement of air needed for storm formation.
How storm shapes differ by region. (Haberlie, Ashley & Karpinski, 2020)
The researchers propose that this technique could enhance automated storm identification systems, potentially enabling faster detection of severe storms. This particular database could assist in training machine-learning technologies to recognize spatial characteristics of different storm types. Additionally, they suggest that this method could help improve weather communication and public education about storms.
And now for something completely different
The Moon will cast a shadow over North America next month, in case you haven’t heard. Here is a map showing the statistical history of clouds on that day across the eclipse region. Meteorologists and weather enthusiasts know it’s a fool’s game to predict cloud cover this far out (even though some are trying). However, I want to highlight a tool that has proven quite accurate for hourly predictions up to three days in advance: the Clear Sky Chart. It has been essential to amateur astronomers for decades. It provides hourly forecasts for cloud cover, seeing (image stability), and transparency - all in an ingeniously simple-to-read visualization. Start checking about 3 days out. And make whatever sacrifices are needed to your deity of choice.
We acknowledge additional contributions from Dr. Alicia Klees.