Damaging winds in the northern USA

  • Radar showing the signature "bow echo" of a derecho. This one was over eastern Iowa and northwest Illinois on 29 June 1998. Image courtesy of NOAA

    How it might look from the ground: a shelf cloud ahead of an approaching derecho in Minnesota. Image courtesy of NOAA

    Storm reports from the Storm Prediction Center (SPC) Sunday night/early Monday. Image courtesy of NOAA/SPC

    ... and from the rest of Monday, clearly showing the eastward path of the derecho. Image courtesy of NOAA/SPC

  • Damaging winds in the northern USA
    13.07.2011 08:57


    Fierce winds in Nebraska and Iowa, USA, destroyed crops, ripped roofs from houses and felled trees and power lines on Monday - but the cause was not a tornado. It was a "derecho".

    A derecho, which is Spanish for "straight" or "direct", is characterized by a line of long-lasting winds (sustained winds of 58mph+) rather than the localized rotation of a tornado or short-lived gusts of a squall. It is not a common phenomenon but neither is it all that rare in the Corn Belt. Two or three might be expected per year across the USA, most often during June or July.

    It is caused by a wide area of vigorous convective activity (a Mesoscale Convective System, or MCS) producing severe thunderstorms and powerful downdraughts and outflows. Looking at precipitation via radar these can often be identified by the heavy rain and hail echoes appearing in a signature bow shape.

    To complicate matters further, a derecho may or may not incorporate a tornado. This does not seem to be the case with Monday's event but winds were strong enough for nearly one million people around Chicago to be left without power as it surge eastwards across northern Illinois, southern Wisconsin, northern Indiana, Michigan and Ohio.

    The storms became less powerful as they moved farther east across Pennsylvania to the Eastern Seaboard, finally exiting into the Atlantic via Washington DC. In all, the derecho affected 18 states to a greater or lesser degree.

    By: Stephen Davenport