POSSIBLE DERECHO OR CONVECTIVE CLUSTER SUNDAY NIGHT INTO TUESDAY MORNING

The atmospheric pattern is becoming increasingly favorable for a corridor of strong west-to-east flow to develop this weekend. At the same time, a strengthening low-level jet is expected to emerge by Sunday night, providing an abundant supply of warm, moist, and unstable air. This combination creates the ingredients needed for the development of a large convective cluster—potentially even a derecho—that could track from the Midwest through the Great Lakes and into the Appalachians between Sunday night and Tuesday morning.

The strong upper-level jet stream will provide the steering flow necessary for these organized thunderstorm complexes to maintain their intensity and travel hundreds of miles across the eastern United States. If a mesoscale convective system (MCS) or derecho develops, widespread damaging winds, torrential rainfall, and frequent lightning would be the primary hazards, with the potential for impacts extending well downstream from where the storms initially form.

EXPLAIN WHAT IS A DERECHO

A derecho is a widespread, long-lived windstorm associated with a fast-moving line or cluster of severe thunderstorms. Unlike a tornado, which produces localized rotating winds, a derecho causes damaging straight-line winds over a very large area.

What makes a storm a derecho?

For meteorologists to classify a storm as a derecho, it generally must:

• Produce a continuous swath of damaging wind reports extending at least 400 miles (about 650 km).

• Generate numerous severe wind gusts of 58 mph (50 knots) or greater.

• Include several significant wind gusts of 75 mph or higher.

• Maintain an organized thunderstorm complex for many hours while moving rapidly.

How does a derecho form?

A derecho develops when several atmospheric ingredients come together:

• Hot, humid, unstable air provides the fuel for explosive thunderstorm development.

• A strong low-level jet transports warm, moist air into the storms, helping them intensify and persist.

• Strong winds aloft (jet stream) steer the storms and allow them to organize into a long-lived mesoscale convective system (MCS).

• As thunderstorms mature, rain-cooled air rushes to the surface, creating powerful straight-line winds. When these downdrafts continually regenerate new storms along the leading edge, the system can maintain itself for hundreds of miles.

Why are derechos dangerous?

The greatest threat is widespread damaging winds, which can:

• Snap or uproot thousands of trees.

• Damage roofs, homes, barns, and commercial buildings.

• Knock down power lines, causing widespread and prolonged outages.

• Create hazardous travel conditions due to falling trees and debris.

• Produce wind damage similar to that of a weak hurricane over a large region.

While damaging winds are the primary hazard, derechos can also produce:

• Large hail

• Torrential rainfall

• Frequent cloud-to-ground lightning

• Isolated tornadoes embedded within the line of storms

How fast do derechos move?

Derechos typically move at 40 to 70 mph, although some have traveled even faster. Because of their speed and size, they can affect multiple states in a matter of hours.

Derecho vs. Tornado

Derecho

Tornado

Straight-line winds

Rotating winds

Hundreds of miles long

Usually a few miles long

Damage spread over a broad area

Damage concentrated in a narrow path

Produced by a large thunderstorm complex

Produced by individual rotating thunderstorms

Why this weekend's pattern is worth watching

The forecast pattern you're describing includes several ingredients that can support an organized convective system:

• A strengthening low-level jet to transport warm, humid air.

• A strong jet stream providing the wind shear and steering flow needed for long-lived thunderstorms.

• A corridor of instability extending from the Midwest into the Great Lakes and Appalachians.

If thunderstorms organize into a mature mesoscale convective system, they could evolve into a derecho if they remain intense and produce a long, continuous swath of damaging winds. While it's too early to determine whether a derecho will develop, the atmospheric setup suggests the potential for a widespread damaging wind event should the storms organize as expected.