Weather Watch: Thunderstorm rollout
Convective horrors—how they form, and how to avoid them
In truth, convective storms can happen any time of year, depending on the alignment of the three main variables that, together, trigger thunderstorms: atmospheric instability, adequate moisture, and a lifting mechanism. Those variables really start to perk up in the warmer months of the year, beginning in April and May. Climatologically speaking, May is the month with the highest number of tornadic thunderstorms. In May 2017, the National Weather Service recorded 299 tornadoes in the United States. In July and August, there were 83 and 145, respectively.
Pulse thunderstorms
Also known as air mass thunderstorms, “pulse” is a more apt description because these storms grow and die in single cycles. Pulse thunderstorms begin when localized surface heating in a moisture-laden environment creates thermals that quickly rise. They will continue to rise as long as they remain warmer than the ambient air surrounding them. As this heated parcel rises, it cools to the dew point, condensation occurs, and cumulus clouds form. This is the cumulus stage, characterized by updrafts.
In the mature stage, cloud tops reach an altitude where ambient temperatures match cloud-top temperatures, and the storm cell ceases to grow. But rainfall, updrafts, and downdrafts now predominate. And the cloud tops can be so cold they generate ice crystals that rise and fall within the storm, growing into hail as successive rounds of rain freeze to the hail nuclei. Positive and negative electrical charges can also build up among the churning clouds, creating lightning.
Isolated pulse storms are easy to circumnavigate, as long as you can maintain visual separation.Finally, during the storm’s dissipating stage, downdrafts predominate, bringing hail to the surface. The downdrafts also prevent any further inflows, which halts the feed of warm, rising air and shuts down the storm.
Typically, pulse storms last about an hour, and happen when winds are light, so their structure is vertical. When isolated, they are easy to circumnavigate—as long as you can maintain visual separation. But if the sky becomes congested with multicell cumulonimbus clouds, weaving your way around pulse storms can be risky. Gaps between clouds can close. Air that appears clear behind one storm can yield another that was hidden, or in its early cumulus stage. If flying on top of a growing thunderstorm, its tops may rise to your altitude. Without the ability to outclimb them, you could be drawn into the tops, where turbulence and icing can be the worst. The best tactic: Give cumulus buildups a wide berth, or perform a 180-degree turn if the way is clear behind you. ATC can help with avoidance, as can datalink weather radar imagery. Just remember that datalink imagery can be several critical minutes old, thereby painting a false picture.
Outflow boundaries
When a thunderstorm’s cold downdrafts hit the surface, they spread out and radiate high-speed winds. That means wind shear, but it also means that these outflows can make low-level boundaries that lift the air ahead of them. A new cycle of thunderstorms can result.
How can you identify outflow boundaries? You may see dirt and debris blown ahead of them. Boundaries that have turned convective will show up as thin, curved slices of precipitation on a datalink weather display, if you have one. Again, remember the time lag. The best avoidance technique? Circumnavigate by a wide margin or make the time-honored 180-degree turn.
Frontal thunderstorms
Fronts are all about rising air ahead of a surface temperature boundary, and we’ve already seen how that can be a setup for convection. The problem with frontal weather is that it can cover such a wide area. Cold fronts happen when advancing cold air wedges itself beneath a warmer air mass ahead of it. Warm fronts occur when warmer, lighter air rides up and over denser, colder air. As a general rule, cold fronts are more violent because they tend to move faster, the temperature and pressure differences across the frontal boundary can be greater, and squall lines can precede them. Comparatively narrow bands of tropopause-busting cloud tops, precipitation, turbulence, and all-around violent weather at the surface and aloft mark many warm-season cold fronts.
Imagine a thunderstorm surrounded by winds that strengthen and change direction with altitude, causing the storm to tilt as it grows. Inflows are free to enter this rotating storm, outflows near the surface can’t shut off the circulation and this thunderstorm takes on a life of its own.Warm fronts, on the other hand, move more slowly, have more widespread cloud coverage, bring lower ceilings for longer periods of time, and their clouds can include a mix of stratus layers with embedded thunderstorms—which is perhaps the most dangerous aspect of warm-front thunderstorms. A pilot traversing a warm front may be flying between layers or, if on an IFR flight plan, in clouds, without any difficulty—until he or she stumbles into a thunderstorm obscured by clouds.
Avoid and escape are always the safest tactics, but with warm fronts there’s an added complication. Because convection and instrument meteorological conditions are often so widespread, airports with good visual meteorological conditions may be far away. That means it could take a long time, and a lot of fuel, to divert. See warm-front weather loaded with watch boxes and convective sigmets? The best option may be to wait a day and reevaluate. Come to think of it, that’s a good rule for any convective weather situation.
Supercells
Remember how those vertical pulse storms’ final outflows prevent any more air from feeding and perpetuating the storm cell? Well, imagine a thunderstorm surrounded by winds that strengthen and change direction with altitude, causing the storm to tilt as it grows. Inflows are free to enter this rotating storm, outflows near the surface can’t shut off the circulation, and this thunderstorm takes on a life of its own. A central circulation core—a mesocyclone—is one feed for the supercell. Others enter the storm complex at altitude. Now the entire complex rotates, generating shelf and wall clouds near the surface, rain and hail shafts, anvil clouds at altitude, cloud tops so high that they can reach 50,000 feet—and tornadoes. That’s a supercell, and like all the other types of convective storms, avoidance should be obvious.
Mesoscale convective system (MCS)
These large thunderstorm complexes often form ahead of warm fronts in the Midwest but can occur anywhere east of the Rockies. They’re usually ringed by a perimeter of cold air. Outflows from the central portions of the complex turn this cold air into squall lines and derechos—which are regional-scale windstorms with the characteristics of either squall lines, bow echoes as identified on radar, and even supercells. Avoiding these monsters can mean deviations lasting hundreds of nautical miles, because MCSs and their cousins—multicell convective complexes (MCCs)—are big.
How big? Satellite measurements define an MCC’s characteristics, which include an area of 54,000 square nautical miles of cloud cover with minus-32-degree Celsius tops, and minus-52-degree-Celsius cloud top coverage of 27,000 square nautical miles. These conditions must persist for at least six hours.
Not too long ago, pilots had minimal sources of up-to-date information concerning thunderstorm location, movement, or intensity. Today, we’re blessed with all manner of near-real-time Doppler radar imagery; scads of websites dealing with convective weather; and growing, world-class inflight datalink services. This must certainly explain why the number of thunderstorm-related fatalities has steadily dropped. What hasn’t changed are the human factors affecting weather-related decision making: get-home-itis; the sense of invulnerability; a macho attitude;and impulsive, spur-of-the-moment decisions. All the education in the world may not alter the hard cases with these profiles.
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