The rite of summer across Washington-Baltimore involves stagnant episodes of air mass weather, characterized by oppressive heat and humidity and “pop-up” thunderstorms.
Why do pop-up cells sometimes briefly turn severe, and why are these “pulse-type severe thunderstorms”, as they are called, among the most difficult of all summer thunderstorms to watch and warn for?
In the absence of a large-scale thunderstorm trigger such as a cold front, individual thunderstorms erupt in an isolated or scattered manner.
When wind shear (winds that increase with altitude) is also lacking, most thunderstorm cells are short-lived – dissipating after 30 minutes or so – and fail to organize into larger clusters or lines. These cells provide more benefit then harm, including a brief, heavy shower, refreshing breezes and a bit of lightning.
However, isolated pop-up cells occasionally unleash a brief volley of violent weather, including hail and damaging wind gusts, frequent lightning and a cloudburst.
For example, on Friday, July 19, an isolated, pulse-type storm erupted quickly over Rockville, Md. At 5:09 p.m, the National Weather Service (NWS) forecast office in Sterling issued a severe thunderstorm warning for quarter-size hail and wind gusts exceeding 60 mph. Pepco reported 3,700 utility outages. The storm weakened as it drifted into eastern Montgomery County, and dissipated by 6 pm. The region was not under a severe thunderstorm watch.
The watch and warning problem
“Pulse” storms are often too random and isolated to issue watches for
Watches are issued when the Storm Prediction Center (SPC), in coordination with NWS Sterling, foresees the potential for widespread severe thunderstorms. These storms typically evolve into organized clusters, bow echoes or supercells, and – given an unstable air mass – wind shear is the critical determinant of storm type.
Additionally, wind shear promotes storm longevity – meaning storms eventually reach an intense and self-sustaining state during which they unleash various forms of severe weather.
There are days, however, when the air mass may reach exceptionally unstable levels, but winds aloft are very weak. Organized thunderstorm systems do not develop in these circumstances, but an isolated cell can reach severe limits, by virtue of extreme instability alone.
In the situation described, it is very difficult to pinpoint where individual, severe cells will develop. In fact, SPC would have to blanket unacceptably large regions with severe thunderstorm watches, in the hopes of casting a wide enough “net” of coverage. During these exceptionally unstable days, the probability that any location will experience a pulse-type severe cell is around 5 percent. In other words, too low to justify a large watch box, and most of the region would experience yet another “false alarm”.
This is part of the forecaster’s dilemma.
Pulse-type severe storms may develop and decay too quickly to provide timely warnings
The other communication challenge involves the short lifecycle of a pulse-type storm, and brief interval over which it produces damaging weather. Pulse-type cells rarely last more than 30-45 minutes, and the timeframe for severe weather is on the order of 5-15 minutes. To see why, let’s examine the diagram.
The top panel depicts stages in the lifecycle of an ordinary, pop-up type cell, every five minutes. The cloud outline is shown, and colors indicate rain intensity, similar to what a radar operator observes. Red arrows depict the cloud updraft, while blue arrows show the downdraft. Essentially, a buoyant plume or bubble of rapidly rising air – the updraft – ascends during the first 15 minutes of the storm. Large amounts of water vapor are drawn inward and upward, condensing into a mass of rain.
After 15-20 minutes, the rain falls out. The drag of the descending rain mass creates the downdraft, which is further enhanced by evaporative cooling (evaporation chills the air, increasing its density). As the downdraft plummets straight down through the updraft, it snuffs out its buoyancy. Without a self-sustaining updraft, the storm’s final minutes are literally spent raining itself out.
So, to recap: The pop-up thunderstorm has an early growth phase, followed by a collapse phase. Via the downdraft, the storm sows the seeds of its own destruction. Total cloud lifetime is about 30 minutes.
Now, the events in a pulse-type storm’s lifecycle follow a similar script (bottom panel), with an important exception: The cloud updraft, during its growth phase, is much more intense, because of an extremely unstable (hot and humid) air mass.
On July 19, the Rockville storm cloud contained rising air that was 10 °F warmer than its surroundings, which is truly exceptional (this value is typically 3 to 5 °F). With tremendous buoyancy, the updraft rockets upwards, approaching 50-60 mph. It sucks copious amounts of water vapor into the cloud.
These cells stand out on radar because of their unusually tall and narrow appearance, perhaps topping 60,000 feet, and high intensity. The storm quickly begins to collapse and enters a brief, severe phase. The cascade of descending rain is so dense it creates a blinding cloudburst.
If sufficient ice mass develops aloft, small hailstones (marble to quarter-size) accompany the torrent. Lightning may be nearly continuous. The downdraft may accelerate to high velocity. As it spreads along the surface, it creates a fan-shaped microburst with gusts to 60-80 mph. That’s enough to snap tree limbs and down electrical lines.
For a brief (5-10 min) period, a pulse of severe weather is unleashed, but the damage is highly localized. The cells are only 3-5 miles in diameter.
The NWS forecaster may have little time to react to the swift growth of the cloud, and its equally swift collapse. A typical radar “sweep” or volume scan takes 8-10 minutes. Allowing for scan time, recognition of the event, and the time it takes to disseminate a warning to the public, the game is already over!
Every attempt is made to recognize pulse-type storms, but their fleeting and isolated nature makes them very difficult to successfully warn against. Combined with the difficulty of issuing credible watches, pulse storms pose a dual, summertime challenge for forecasters.
Jeffrey Halverson is a Professor in the Department of Geography and Environmental Systems at the University of Maryland, Baltimore County.