WEATHER By Jack Williams A DERECHO has winds that are 57 mph or stronger and follows a path 240 miles long. DERECHOS GO MAINSTREAM A WEATHER PHENOMENON TO AVOID P ilots have been learning about—and worrying about—squall lines since the 1920s. Nevertheless, many still haven’t heard of the super squall line called a “derecho.” That surely changed in June 2012. A derecho killed 22 people as it spread destruction from the Midwest to the Mid-Atlantic on June 29 and 30. It left millions without power and left thousands to cope with damage caused by high winds and fallen trees. If nothing else it should give pilots a new respect for squall lines—lines that can be more than 300 miles long—of powerful thunderstorms. It also should reinforce the idea that a pilot can’t stay too updated on the weather. FORECASTING DERECHOS ISN’T EASY. Many pilots like to take a look at the National Weather Service 42 / FLIGHTTRAINING.AOPA.ORG Storm Prediction Center’s (SPC) first “Day 1 Convective Outlook” on the mornings of days they’ll be flying to see whether thunderstorms could be a problem along their route. When the SPC issued its first Day 1 Outlook at 2 a.m., chances of high winds in Maryland and Virginia were less than 5 percent. As the day went on, however, new observations and predictions fromhttp://FLIGHTTRAINING.AOPA.ORG