On the East Coast, before June 29, 2012, few people (meteorologists excluded) had ever heard of a derecho.  The sheets of rain and crushing winds that sweltering evening changed everything.

As a refresher, a derecho is a rapidly moving storm complex featuring damaging winds along at least a 240 mile path. No more than three hours can pass between winds of 58 mph or greater and there must be three high-end damage reports separated by no more than a moderate distance.

We’d seen a derecho as recently as 2008. But this one was different: a historic event impacting 11 states (plus our little non state) along its 700 mile path.

Here are 10 graphics that tell the story of the event…

Bring heat and humidity to a boil, then stir

With storms underway east of Chicago during the early afternoon, an unusually steamy air mass continued to bake out ahead of the developing line. High humidity and the summer sun combined with a capping inversion (a layer of warm air aloft that prevents “mess” storms from developing by cutting off rising updrafts) to create a volatile situation characterized by extreme instability ahead of the developing complex of storms.

Rocket fuel.

In Washington that day, the high reached 104 degrees. A record for the date and the month of June, and among our hottest temperatures ever recorded. When the derecho was knocking on the door, the temperature was 91 and the dew point was a suffocating 75. At 10 p.m. at night!!

The nation’s hottest summer day in 2012

Almost the entire region (mainly with the exception of the Gulf Coast and Appalachians) to the south of a cold front from Pennsylvania back to the Rockies was 100 degrees or hotter that day. Hundreds of records fell in an extreme heat wave that was just getting started.

According to NCEP reanalysis data provided by Dr. Ryan Maue of Weatherbell (see image), June 29 was likely the hottest day of the year for the lower 48.

Holy sounding Batman!

At least twice a day, teams release balloons from nearly 100 locations across the country, and many more around the world, to get atmospheric reading from the surface to about 100,000 feet in altitude. The charts of their data findings are typically referred to as soundings. The information gathered at the 8 p.m. June 29 launch from near Dulles Airport made it abundantly clear this was no ordinary night.

Even at such a late hour, surface CAPE (convective available potential energy) remained above 5,000 — a practically unheard of level locally, and somewhat unusual even out in the Plains. A remnant but weakening cap of warm air (partially shown as negative CINH in the chart) above the surface kept storms from forming out ahead of the advancing line, containing the energy for the main event – like the subtle prong on a mouse trap holding back the bow until pressure is applied and it snaps, violently. Other notables include a surface lifted index of -14 (negative numbers indicate instability, -14 is VERY unstable).

From a cluster to a “land hurricane”

Most derechos form from seemingly innocuous (though often locally severe) clusters of storms. Only with time do these features sometimes mature into a mesoscale convective system or, in the more extreme cases, a derecho.

The satellite imagery loop provided by NASA shows this process very well. The earliest of reports came during the morning in Iowa. From there, the storms wandered along before becoming hyper-energized heading into Indiana. Then, an expanding and self-perpetuating “cold pool” of air associated with the storm outflow reacted explosively within the available moisture and heat out ahead of it.

Mesmerizing regional radar loop

In a recent post comparing last year’s event to the series of storms a few weeks ago, Kathryn Prociv noted an average forward speed of 65 mph for the 2012 derecho. The somewhat hypnotic regional radar loop covering the derecho path from near Chicago through the Mid-Atlantic highlights it’s incredible motion quite well.

Pretty much wherever you see reds along the main path, significant and often widespread wind damage was noted. Don’t be fooled by any “color loss” in West Virginia, as that was one of the hardest hit spots overall.

A closer look at the Mid-Atlantic

Ken Pryor (NOAA/NESDIS) created an animation as part of his fantastic derecho synopsis which meshes radar and satellite imagery to show how massive and widespread the mature storm system was upon its approach and passage through the D.C. area.

The anvil clouds from the derecho eventually spread out to cover an area about twice its actual north to south size. This was, in part, due to the vigorous convection expelling moisture non-stop well beyond where the rain shield was.

Intense winds as seen by D.C. area radar

A majority of local severe weather events have a handful of zones within a squall line that actually contain severe weather, and they tend to be relatively short lived at true “severe” criteria of 58 mph or greater. Often, it’s just a radar scan or two that we see “pings” of this higher wind activity.

The wind velocity radar loop above shows damaging winds along the whole segment of the line as it passes the D.C. area.

If the initial blast of 60-80 mph winds wasn’t enough, periodic gusts powerful enough to cause problems (~50 mph+) continued behind the main destructive burst for up to 15-20 minutes along with formidable sustained winds. In some cases, violent downbursts, following the initial blasts of wind, caused additional areas of major to extreme damage.

Big list of big winds

The June 29, 2012 derecho truly carved a destructive path through multiple locations.

Typically (both in the D.C. area and even more storm-prone areas in the Ohio Valley) severe weather events result in isolated to scattered damage. In this case, damaging winds were the rule rather than the exception: you were lucky if you came through the derecho unscathed.

Reports on the low-end of severe wind criteria (58 mph or higher) were seemingly fewer than those on the high-end. All three local airports recorded gusts to near 70 mph or higher. In addition to the certified wind measurements in the chart above, the National Weather Service assessment notes that a station near Cambridge, Ohio measured a gust to 106 mph.

Millions without power, 1 million+ in Virginia alone

Approximately 4.2 million customers were left in the dark in the immediate aftermath of the derecho, according to the Department of Energy. Outages per percentage of total customers from the event were the highest on record for the D.C./Md./Va./W.Va. region.

It was not too unusual for it to take up to 7 to 10 days for unlucky ones to get their power back. Making matters worse was the excruciatingly hot conditions in the days following. D.C.’s coolest high temperature from June 30 through July 8 was 95, with half a dozen days near or past 100 in that stretch.

Who knew?

Last year, I discussed the fact that the derecho was poorly forecast ahead of time. Many others came to similar conclusions.

In some ways, the forecast snippet above, included in Baltimore Gas and Electric’s after-action report, is among the most astounding. Wind damage on par with a hurricane and a few hours notice or less? Still kind of hard to believe!

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