When we think about thunderstorms, we often imagine an idealized representation of an air mass or frontal storm. You know, cauliflower tops, anvil clouds, scary radar signatures, one-hour life spans, and the other generalities and warnings that go with the adverse conditions associated with all convective storms. But there are other types of thunderstorms that are very, very bad news for any pilot of any airplane who ventures into their vicinity. As we all know, any type of thunderstorm should be avoided at all costs. But the ones summarized below — while they don't get a lot of press — can be the most dangerous of all convective systems. I mention them here precisely because they aren't widely addressed, but pilots should understand how they form, their life cycles, and their characteristic behaviors.
Squall lines
Squall lines are most often found east of the Rockies, ahead of cold fronts and in the warm sector (the area between a warm front and a cold front where warm air resides) of typical low-pressure systems. They can precede cold fronts by anywhere from 50 to 200 nautical miles, and the line of storms can be solid or broken into segments. The weather in squall lines is typified by intense convection, violent wind shear — both horizontal and vertical — and extremely heavy rainfall.
The conditions that favor squall-line formation are the same as those that create "general" thunderstorms: an unstable air mass, a lifting force, and plentiful moisture. But why do these storms form in lines instead of individual clusters? The answer, in large part, is that low-level convergence (rather than localized heating of the Earth's surface) plays a big role as a lifting force. This convergence can be initiated by the outflow of winds from the advancing cold front or the squall line itself as it plows along in its formative stages, barging its way into the warm, moist air ahead of it. Sometimes remnant outflows from earlier nighttime thunderstorms can produce convergence in lines once the heat of the day sets in.
Squall lines can travel at speeds of 50 knots or more. Their trajectories usually follow the wind flow at 18,000 feet (the 500-millibar level on constant pressure charts).
As for the instability, squall lines seem to thrive in young, rapidly deepening surface low pressure systems — the kind that are overlaid by low-pressure troughs aloft, say, above the 18,000-foot level. However, researchers have found some signature features to squall-line vertical structure. Instead of the usual cold-air-atop-warm setup that produces garden-variety thunderstorms, there's a mid-level (in the area of 5,000 to 6,000 feet) layer of very dry air. Near the surface, a temperature inversion exists. The inversion prevents any nascent convective clouds from rising above its level, which is why this phenomenon is called a capping inversion. Warm, moist air is trapped below the inversion, where it stores up its convective energy.
The line forms when convergence causes this low-level warm air to burst through the inversion, then rise into the cold air of the trough aloft. Storm cells form, and when their precipitation falls through the mid-level dry air, evaporative cooling occurs. The downward rush of this cold air causes huge outflow boundaries, which in turn cause more convergence ahead of the line, and help sustain and perpetuate its life cycle. This is especially true when the mid-level dry air moves so fast that it overtakes the squall line's surface location.
This means that — unlike most slower-moving air mass thunderstorms — a squall line's updrafts and downdrafts occur in separate locations. That's what keeps their updrafts and downdrafts from canceling each other out, and keeps their energy cycle moving on a "conveyor belt" of vertical motions.
Like all thunderstorms, squall-line storms can produce severe icing and turbulence. Their tops can shoot past the tropopause, and Geostationary Operational Environmental Satellite (GOES) imagery will show these as distinct blobs of extra-white cloud, illuminated by the bright sun (visible imagery) or the cold temperatures (infrared imagery).
Fortunately, advancing squall lines often can be spotted in the clear air ahead of them. You'll see a wall-like cloud formation that contains, yes, a wall of water. The only safe action plan is to reverse course or land at the first practical opportunity.
Dry lines
Dry lines primarily live in west Texas, New Mexico, and Oklahoma. They generally run along a northeast-southwest orientation, and mark the boundary between air masses of two different moisture contents. For this reason, dry lines are sometimes called dew point fronts.
Unlike the situation across fronts, temperatures across a dry line may not vary by much, and pressure differences are negligible. There are wind shifts across dry lines, but these have more to do with diurnal heating cycles than with pressure gradients. That's why you can imagine dry lines as a sort of inland sea breeze: Gulf air blows far inland with the heat of the day, then the breeze subsides when the sun goes down.
East of the dry line, elevated dew points — in the 60-plus degree Fahrenheit range — reflect the influence of the damp air transported north from the Gulf of Mexico. West of the dry line, dew points below 30 degrees — sometimes way below, into the single digits — reflect the parched air over the Desert Southwest.
When a trough aloft — one of those "pools of cold air" that you hear television meteorologists frequently mention — orients itself just upwind of the east (moist) side of a dry line, the humid low-level air begins its launch. The trough's divergent wind motions help boost the already-destabilizing effect of the cold-air-over-warm condition, and presto — a dry-line thunderstorm is born. It can form in lines or clusters, and deserves a wide berth.
Sometimes a bulge develops in a dry line, and a low pressure center forms at the bulge's apex. When this happens, meteorologists worry about tornados.
Pilots traveling through Texas to the Desert Southwest ought to think hard about dry lines, but the phenomenon is all too often relegated to footnote status.
Mesoscale convective complexes (MCCs)
MCCs were discovered in the early 1980s, when satellite meteorology began to really mature as a science. These are large circular thunderstorm formations that originate just east of the Rocky Mountains, then move eastward. They affect the central plains states, and seem to plague the northern plains states (Nebraska, Iowa, and the Dakotas) with a higher frequency.
It's an MCC's size that gets your attention. One of these complexes can cover an entire state, and once in place the system can regenerate day after day, causing severe convective dangers like tornados, hail, intense lightning activity, and high surface winds. MCCs can also produce derechoes, which are families of widespread thunderstorm lines or clusters with strong downbursts and surface wind gusts up to 60 knots.
MCCs seem to form as the result of poorly understood interactions between groups of storms that pop up in different locations. As with squall lines and dry lines, lifting forces from winds circulating around troughs aloft help energize the storms — and in fact many MCCs form at the southernmost boundaries of squall lines associated with weakening cold fronts. Others form to the north and east of stationary fronts. MCC formation usually begins in the late afternoon, and peak storm activity occurs in the hours after local midnight. In the morning hours, storm activity subsides.
One reason for this nocturnal pattern has to do with the low-level (say, at the 5,000- to 10,000-foot-altitude range) jet stream's behavior over the plains states. Strong low pressure systems over the central United States are known for their low-level jet streams (LLJs), which shoot that necessary storm ingredient — Gulf air — northward through a low pressure system's warm sector. It's the LLJ's transport of Gulf air that helps feed and rejuvenate the storm systems at night.
LLJs occur at night because of a "decoupling" between surface winds and winds aloft. During the day, thermal mixing causes interaction between winds at the surface and aloft. This results in a slowing of the low-level winds aloft. When the sun goes down, surface winds drop off, but the LLJ — unhindered by mixing — picks up speed, keeps on cranking through the night, and serves as a heat engine in much the same way that solar heating drives the convective process in daytime thunderstorms.
Should you notice strong southerly winds while cruising at low altitudes over the plains, be on guard. You might be in that LLJ pipeline. You'll know, because wind speeds in an LLJ can reach 50 to 80 knots. Are you in cloud? Is it becoming more turbulent? Then start planning your escape. You may be about to enter an MCC. By the way, don't try to circumnavigate an MCC — they are too large, and most general aviation airplanes don't have the speed or range to tackle the job.
Every thunderstorm is unique, but squall lines, dry lines, and mesoscale convective complexes should make the most grizzled, weather-beaten, storm-tossed pilots sit up and take notice. They provide even more reason for the best advice of the season: Stay well clear of any convective clouds.
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