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Parts of the Ohio Valley and the Mid-Atlantic are reeling from the destructive and deadly line of storms that roared through the region on Friday afternoon and evening. The storms stretched 750 miles from Chicago to the Atlantic Ocean, causing at least 900 reports of wind damage as well as 2 fatalities and at least 5 injuries.

The storm reports map is astonishing. Blue dots represent a report of wind damage or gusts over 60 MPH. Black squares represent measured winds over hurricane force (74+ MPH).


Note: The above map is from 12PM EDT. The number will skyrocket as the full extent of the damage is realized and reported.

Jump the squiggle...

In this area, an event called a progressive derecho occurred. A progressive derecho is a powerful line of fast-moving thunderstorms that causes a very long swath of wind damage. The damage from Friday's derecho stretched over 750 miles from Chicago to the Atlantic Coast.

Derechos are a common type of severe weather during the summer, and they occur most frequently during intense heat waves like the one most of the central and east is experiencing this weekend.

This is a well documented, relatively well understood phenomenon that's happened for as long as there's been weather.


A derecho hitting the DC area on June 4, 2008.

A derecho is simply a bow echo, or a line of storms with damaging winds. The difference between a bow echo and a derecho is that a derecho lasts for hundreds of miles. It's essentially a bow echo on steroids.

There are two types of derechos -- serial and progressive. A serial derecho is probably what most everyone is familiar with -- a powerful line of thunderstorms forms along a cold front and marches across the eastern third of the country.

A progressive derecho is what happened today. They form in relatively quiet conditions when there's a heat wave resulting from a region of high pressure over the southern US. Extremely intense heat waves like the one we're having are breeding grounds for this type of a derecho, and especially the most powerful ones.

Here's the general setup of the atmosphere during a heat wave like this. A ridge of high pressure sets up over the eastern part of the US. Weather south of the ridge is usually clear, calm, and incredibly hot. Weather north of the ridge is usually pretty warm, moist, and unstable.

The ridge itself is usually where a stationary front sets up between the incredibly hot, calm weather to the south, and the more unstable weather to the north. Thunderstorms can't form deep in the ridge because subsidence (sinking air) overpowers and precludes thunderstorm development. Right at the northern edge of the ridge, along the stationary front, is where the deep pool of moisture and explosive instability are able to break through the sinking air and allow thunderstorms to develop.


The scabby looking things are supposed to be storms. Don't judge me.

As these storms rapidly develop due to the ample atmospheric fuel available to them, they're able to organize due to deep sheer (winds speeding up and changing direction with height) throughout the atmosphere.

This organized line of thunderstorms is able to develop its own jet of air called a rear inflow jet. This rear inflow jet moves from the back of the storm to the front, and rapidly descends towards the ground when it hits the front of the storms. This creates an extremely strong wall of wind along the leading edge of the storms. The rear inflow jet serves to strengthen the storms, and the storms strengthen the rear inflow jet. It's a feedback cycle that makes the derecho stronger and stronger.

This wall of wind is what makes a derecho, well, a derecho. The winds frequently reach 60 MPH, and can exceed 100 MPH at times. The persistence of these strong winds, mixed with the fast-moving, long-lived storms, can cause swaths of damage approaching 1000 miles at times.

Derechos have a kill switch built into their structure. If this wall of wind gets too far ahead of the line of thunderstorms, it'll cut off the feed of unstable air that's keeping the storms alive and thriving. When that flow gets cut off, the storms die. When the storms die, the winds die down too. The whole system will slowly wind down to nothing but rain, and even that'll stop after a few hours.

Derechos are a common, but very dangerous, type of summertime severe weather event. It's nature's way of balancing out the massive amount of energy built up in the atmosphere when a heat wave occurs.

If you're in the path of the storms tomorrow, be prepared to take immediate precautions to protect life and property. These storms can move at highway speeds.

If your power goes out, please remember to check on your aging and/or sick neighbors. The elderly and folks who are sick are extremely susceptible to illness or death from heat related complications.

Also, don't forget to keep an eye on any candles you light, and don't use gas powered generators or barbecues indoors.

You can keep track of the severe weather using the following links:

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National Weather Service Main Page
National Weather Service -- Wilmington OH
National Weather Service -- Charleston, WV
National Weather Service -- Pittsburgh PA
National Weather Service -- State College PA
National Weather Service -- Baltimore/Washington DC
National Weather Service -- Blacksburg, VA
National Weather Service -- Wakefield, VA
National Weather Service -- Philadelphia, PA
National Weather Service -- New York City NY

Storm Prediction Center Main Page
Storm Prediction Center -- Current Severe Weather Watches
Storm Prediction Center -- Convective (Severe Weather) Outlooks
Storm Prediction Center -- Mesoscale Discussions
Storm Prediction Center -- Storm Reports
Storm Prediction Center -- Mesoscale Analysis Pages

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