A bow echo is a bow-shaped radar reflectivity signature within a convective line, produced when a focused rear-inflow surge pushes the middle of the line forward. It is often associated with a swath of damaging straight-line winds and brief tornadoes.
How a bow forms
The AMS Glossary describes a bow echo as a bow-shaped line of convective cells, typically 20 to 200 kilometers long, that persists for three to six hours and is often accompanied by swaths of damaging straight-line winds and small tornadoes. Structurally, a strong rear-inflow jet penetrates into the back of the line, and paired vortices sit at the northern and southern ends of the bowed segment.
Physically, a mature line generates a deep cold pool. Cool, dry mid-level air is drawn in from behind and accelerated forward through the middle of the line by the pressure and buoyancy contrast. That rear-inflow jet reaches the surface as intense downdraft outflow and pushes the middle of the line ahead of the ends.
The wind field and mesovortices
At the surface, the rear-inflow surge is felt as a burst of straight-line winds often exceeding severe thresholds and, in strong cases, reaching well above 80 mph. The strongest winds tend to be near the apex of the bow at its most forward push.
The northern bookend vortex often intensifies as the system matures and can develop into a mesoscale cyclonic circulation that gives the whole system a comma appearance. Mesovortices along the leading edge of the bow can produce brief, sometimes strong tornadoes with short warning lead times.
How forecasters read a bowing segment
On radar a maturing bow shows outward curvature in the reflectivity band, a rear-inflow notch of weaker reflectivity punching in from behind, and a velocity signature of accelerating flow at mid-levels. Bookend vortices show as cyclonic and anticyclonic couplets at either end of the bow.
When multiple bow echoes chain together within a single mesoscale convective system and produce continuous wind damage over hundreds of miles, the event is classified as a derecho. Bow-echo forecasting therefore blends storm-scale radar interpretation with mesoscale awareness of the environment supporting a long-lived line.
Where it misleads
Not every bow yields damaging surface winds. Low-topped, cool-season bows in weaker environments can look impressive on radar without matching the wind swath a warm-season bow would produce. The environment beneath the storm matters as much as the shape above.
The tornado threat inside a bow is easy to underestimate. Mesovortex tornadoes can form quickly within a wind swath that already justifies a severe thunderstorm warning, and they can occur before any classic supercellular signature appears. If you are in the threatened region, have a plan to take shelter if a warned segment approaches you.