straight line wind/storm?

[county]

On Tuesday morning here in Memphis, TN we had a storm that included what has been described as “straight line wind” at some pretty serious speeds.

Anyway Over 300,000 were initialy wihtout power - still over 200,000 waiting (mine came on last nite)

So what’s the deal with this “straight line wind”?

[jackelope]

I’m pretty sure they say “straight-line” to differentiate it from a tornado with winds at that speed.

Wish my power were on. I’m at Ground Zero in Midtown.

[Richard_Pearse]

Got the computer running on pedal power jackelope?

[Polycarp]

I think that most people tend to think of damaging-level winds as associated with cyclonic disturbances – tornadoes, of course, hurricanes, and low-pressure systems creating severe thunderstorms and such.

But there’s a class of such storms associated with fast-moving fronts without the rotary motion of cyclones, called derechos, after the Spanish for “straight.” Microbursts are one form of derecho.

Jeff Haby, a meteorologist, gives this capsule description:

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.

(That’s the full definition from his weather site.)

There are a number of interesting discussions of derecho events online – most of them in .PDF or PowerPoint format, which the computer I’m working on either doesn’t support or is extremely slow loading. Google “derecho +wind” for a list of them.