 The traditional criteria that distinguish a derecho from a severe thunderstorm are sustained winds of 58 miles per hour (93 km / h) during the storm as opposed to gusts, high or rapidly increasing forward speed, and geographic extent (typically 250 nautical miles ( 460 km; 290 mi) in length. ) In addition, they have a distinctive appearance on radar (bow echo); several unique features, such as the rear inflow notch and bookend vortex, and usually manifest two or more downbursts. Although these storms most commonly occur in North America, derechos occur elsewhere in the world. Outside North America they may be called by different names. For example, in Bangladesh and adjacent portions of India, a type of storm known as a "Nor 'wester" may be a progressive derecho.
