Bow Echoes

Derechos typically are associated with bands of showers or thunderstorms that assume a curved or bowed shape. The term "bow echo" is based on how bands of rain showers or thunderstorms "bow out" when strong winds, associated with the storms, reach the surface and spread horizontally. A derecho comes from a long-lived bow echo or a series of bow echoes.

Bow echoes usually arise from a cluster of storms, but also may begin from just a single supercell thunderstorm. As the rain cooled downdraft reaches the earth's surface, it spreads horizontally. This downdraft marks the dissipation stage of that particular thunderstorm cell.

The cooler, more dense air hugs the surface as it spreads, forcing lighter warm-moist air up into the atmosphere. The boundary between the rain-cooled air and warm-moist air is called the gust front.

Air forced up by the gust front begins the next new thunderstorm cell formation. As this new cell matures, the rain it produces reinforces the "pool" of rain-cooled air allowing the gust front to maintain its strength.

As the cold pool increases in size, it induces an inflow of air (orange arrow at right) on the trailing side of the thunderstorm complex. This causes the updraft to tilt toward the trailing edge.

Tilting the updraft allows the cumulonimbus cloud to expand further, increasing the aerial coverage of rain which, in turn, further adds to the cold pool of air under the thunderstorm and thereby strengthens the gust front causing it to bow out.

The bowing of the gust front forces more warm moist air up, creating new thunderstorm cells and the process repeats.

The additional rain reinforces the cold pool and strengthens the rear inflow of air, with the thunderstorm complex reaching a semi-steady state condition. At this point there is a pronounced bow in the storm(s) as seen by Doppler radar and an area of moderate to occasionally heavy rain near the center of the cold pool well behind the gust front.

As long as there are new thunderstorm cells forming on the gust front as it advances, replacing the older dissipating cells, the cold pool and rear inflow air will continue. Also, all along the leading edge of the bow echo, thunderstorms may be producing downbursts and microbursts.

About one half (46%) of bow echoes begin as unorganized thunderstorms; 30% form from squall lines, and about one-quarter (24%) from supercell thunderstorms.

If the bow echo (or series of bow echoes) progresses more than 250 miles (400 kilometers) with widespread wind gusts 58 mph (93 km/h) or greater, then the bow echo is classified as a derecho.