High Winds Warning– This is a warning issued for sustained surface winds greater than 40 mph/64 kph lasting more than an hour or winds over 58 mph/93 kph over land that are either predicted or occurring for an unspecified period of time.
High Winds Warning (Derechos)
Derechos are widespread, severe wind events resulting from persistent and violent a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms-a Mesoscale Convective System (MCS)-but smaller than hurricanes. These winds normally persist for several hours or more. The type that forms during the warm season over land has been noted across North America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.
The Derechos environment includes dry mid-level winds that are ingested into a squall line (a line of thunderstorms that have a common lifting mechanism) or just a segment of a squall line. The forward motion of the storm along with an intense downdraft (vertical movement of a very cool air mass ahead of the thunderstorm) produced by negative buoyancy via evaporational cooling. This brings down momentum from the middle levels of the storm to its surface.
A severe wind is one with wind speeds of 50 knots (58 miles per hour) at the surface. However, 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. 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 winds 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.