Downbursts, gust fronts and derechos

Air coming down

A downburstA strong downdraft that hits the surface and blasts outward, producing damaging straight-line winds. It is the general term for what a microburst and a macroburst are size classes of. is a downdraftAir descending through and out of a storm, driven by the weight of the precipitation and by rain evaporating into drier air below. It is what produces the gust front and the damaging winds. that hits the ground and spreads out radially. That is all it is, and it produces winds that match a respectable tornado with none of the warning signs people are watching for. Fujita split them by size:

Microburst

Affects an area 4 km, about 2.5 miles, or less across. Small, intense, and over in minutes.

Macroburst

Larger than 4 km. Same mechanism, wider footprint, usually less extreme at any one point.

The split matters to a spotter mostly as a reminder of scale: a microburstA downburst less than 4 km across, usually lasting 5 to 10 minutes. Small, brief and capable of winds over 100 mph, which is a bad combination for anything trying to detect one. can flatten a neighbourhood and leave the next one untouched, so "it was fine where I was" is not evidence about the next street.

Wet and dry

Dry microbursts fall through deep dry air, cool by evaporation, and arrive with dust and little rain. They belong to the High Plains. Wet microbursts, buried in heavy rain, dominate the humid Southeast, and Atkins and Wakimoto's study of them over the southeastern United States is the anchor for this region: in a pure wet-microburst environment the cloud base sits near 850 mb.

What outflow looks like on the way

The leading edge of outflow is a gust frontThe leading edge of cool air rushing out of a storm, ahead of the rain. It is what makes the wind pick up sharply just before a storm arrives., and the cloud that marks it is a shelf cloudA low, wedge-shaped arcus cloud on the leading edge of a storm, attached to the cloud base above it. It is an outflow feature, it means damaging wind rather than rotation, and it is the feature most often misreported as a wall cloud., with the ragged turbulence on its underside sometimes called a whale's mouthThe turbulent, sagging underside of the cloud deck behind a gust front, seen looking back up after the shelf cloud has passed over. By the time you can see it the wind has already arrived.. Module 3 lesson 6 is the full treatment; the point here is the same one that module made: a shelf cloud is outflow, so it is a wind warning to you, not a rotation report.

A rain footA horizontal bulge near the ground at the bottom of a precipitation shaft, where the downdraft has hit the surface and spread outward. It is a visual sign of a wet microburst., where the bottom of a rain shaftThe visible column of rain falling between the cloud base and the ground. At a distance one can be mistaken for a wide tornado. splays outward sideways, and a dust foot doing the same thing over dry ground, both say a downdraft is reaching the surface. Those are the cues you get, and you get them late.

Derechos

When a fast-moving line of storms produces damaging wind along a very long swath, it earns its own name. SPCStorm Prediction CenterThe national office in Norman, Oklahoma that issues severe weather outlooks, mesoscale discussions and watches. It does not issue warnings. sets five conditions, and all of them have to hold.

1. A wind damage swath at least 250 miles, about 400 km, long.
2. Wind gusts of at least 58 mph (93 km/h) along its length.
3. Several well-separated gusts of 75 mph (121 km/h) or greater.
4. It originates from an MCSMesoscale Convective SystemA collection of thunderstorms behaving as one system rather than as separate storms. Clusters, squall lines and bow echoes are all examples. moving faster than the mean wind, so it is primarily driven by its own cold poolThe dome of rain-cooled air a storm leaves sitting on the ground under and around itself. Denser than its surroundings, so it spreads out, and what it spreads under decides whether the storm lives or dies..
5. The system shows the radar signatures that go with that: bow echoes and rear-inflow jets.

The cool-season serial derechoA long-lived, fast-moving windstorm produced by a line of thunderstorms, leaving damage in a swath rather than at scattered points. One of only two things that can put an area under a HIGH risk., a line-echo wave pattern rather than one long bow, is characteristic of the southern Mississippi valley and the humid Southeast, which makes it the version most likely to arrive here.