Weather Explainers

Classic Ingredients For A Severe Weather Outbreak

At a Glance

Certain ingredients combine to produce swarms of severe thunderstorms and tornadoes.
These outbreaks can spawn dozens of tornadoes and hundreds of wind damage and hail reports.
Wind shear is a critical factor in the development and longevity of supercells.
Outbreaks occur when a powerful jet stream punches ahead of a cold front over warm and humid air.

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Severe weather outbreaks with hundreds of reports of wind damage, tornadoes and hail over multiple states are typically triggered by an extreme combination of atmospheric ingredients that set them apart from ordinary stormy days.

While there have been various proposals to define an outbreak, it's typically easy to spot one by looking at the density and number of reports of tornadoes, wind damage and hail after it happened.

In general, when at least several ingredients for thunderstorms are extreme in magnitude, numerous severe storms flare up. It's at this time of the year when these ingredients become explosive most often.

These ingredients can be most easily remembered using the acronym “SLIM” - Shear, Lift, Instability and Moisture.

Wind Shear: Thunderstorms have rising and sinking currents of air called updrafts and downdrafts. Downdrafts usually are laden with the thunderstorm's rain.

An ordinary mid-summer thunderstorm along the U.S. Gulf Coast, for example, will develop but then rain itself out as its downdraft chokes off its updraft.

However, in a severe weather outbreak, the difference in wind speed and/or direction from closer to the ground to jet-stream level can be massive.

This wind shear allows a thunderstorm to tilt so that its downdraft doesn't choke off its updraft, allowing it to last longer than the ordinary thunderstorm described above.

Wind shear may also produce a rotating supercell's updraft, which helps maintain the supercell's longevity.

It does this by first producing a broad area of slowly rotating winds around a horizontal axis. You can demonstrate this by placing a pen in the palm of your hand, then placing your other hand over the pen and sliding one or both hands in opposite directions. The pen rotates.

Air rotates horizontally based on the difference between wind speeds aloft and near the surface.

Heat and humidity then push that horizontal tube of rotating air into the vertical and stretch it, causing the rotation to increase.

If this change in winds and instability is extreme in the lowest few thousand feet above the ground, as might occur near the warm front or low-pressure center, tornadoes – some strong – could develop within supercells.

Schematic of a supercell, showing the (forward flank) downdraft displaced downstream, allowing the rotating updraft to persist.
(NOAA/NSSL)

While most strong tornadoes are spawned from supercell thunderstorms, some severe weather outbreaks aren't really about supercells.

For example, if a vigorous jet stream and cold front sweep quickly into warm and humid air without enough low-level shear for tornadic supercells, a long-lived squall line of severe thunderstorms may produce widespread straight-line thunderstorm wind damage over hundreds of miles and multiple states. This large-scale convective windstorm is known as a derecho.

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Lift: We often first need some sort of triggers, those atmospheric features that force a large number of thunderstorms to occur.

Often the parent of such a trigger in the United States comes in the form of a sharp southward plunge of the jet stream, pivoting eastward out of the West.

This U-shaped jet stream plunge – known to meteorologists as a trough – forces a low-pressure system to intensify to the east of it, drawing northward increasingly warm and humid air.

To contrast this soupy air, the air in the U-shaped jet stream trough is very cold, and winds aloft also transport dry air from the western U.S. or northern Mexico.

These contrasts, known as frontal boundaries, force this potentially unstable air to rise, resulting in an explosion of severe thunderstorms.

Instability: Thunderstorms blossom as long as air is rising, or in an unstable environment.

The amount of instability is determined by how much colder the air is above a warm near-surface layer.

The greater this difference, the more vigorous the thunderstorms, with air currents soaring tens of thousands of feet into the air.

There's a simple analogy you probably see almost every day in your kitchen.

When you put water on the stove and turn on the burner, nothing happens immediately. The water (and pot) are both slowly warming, relative to the air above. The instability is rising.

Once the water begins boiling, you notice steam rising. That's the process of convection – rising warm, moist air – that eventually condenses into clouds and forms thunderstorms.