METEOROLOGIST JEFF HABY
A derecho is a widespread severe wind event resulting from persistent and violent outflow from a
MCS (Mesoscale Convective System). The derecho environment includes dry mid-levels winds that
are ingested into a
squall line or a segment of a squall line. The forward motion of the storm along
with an intense downdraft produced by negative buoyancy via evaporational cooling brings down momentum from the middle
levels of the storm to the surface.
A severe wind is one with wind speeds
of 50 knots (58 miles per hour) at the surface. In a derecho these severe winds encompass a distance of at least
400 km (250 miles) either out ahead of or along a squall line MCS. The length of time the severe winds
last can be particularly damaging. While a
severe thunderstorm may produce severe convective wind gusts that
last for several
minutes at a point location, derecho wind can last 30 minutes or longer. Derechos can be tracked from radar
and severe weather
reports while they are occurring since severe weather reports will be given in sequence
as the derecho traverses along.
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