PRELUDE

At 4:30 a.m. on June 29, I began my day with a quick look at the weather charts and prepared to make a forecast. It was to be yet another hot day across the region; you did not need a meteorologist to tell you that it would be hot because, you could feel already it before the sun ever rose. The morning air was thick, heavy and the winds offered no relief, they merely stirred the soupy air in the kettle of the Great Lakes. I knew that thunderstorms were expected that day, I had been eyeing a system on the charts for the past three days, and it was to arrive during the late afternoon across the region I forecasted for (Cleveland, Ohio; Erie, Pennsylvania; Fort Wayne, Indiana; and Toledo, Ohio). I remember vividly making the comment “You’re going to need a big [weather] event to push this heat back” on my website’s morning forecast update. Little was I prepared that the event would be a derecho – an event that squall-line lovers, like myself, adore.

Lucille was returning from Chicago, Illinois on her way back to Lima, Ohio, so I had to watch the weather of northern Indiana since I was expecting thunderstorms. I was located in Bowling Green, Ohio – a town which is no stranger to straight line winds from the squall lines that inhabit our world in the summer time. To our south was the city of Findlay, Ohio, where Kelly and her church were located near the downtown region, a city where their biggest weather event was the semi-decadal floods that occur along the banks of the Blanchard River. Across the Ohio River in the town of Williamstown, West Virginia, I was forecasting for Megan and her coworkers at a flower shop. Finally, there was Guy in Greensburg, Pennsylvania, a professor at the University of Pittsburgh enjoying another day off. As for me, it was going to be another fun day of thunderstorm forecasting, but for the others, they were to experience an event that occurs only once a year at most.

The morning had started out rather slowly; the only thunderstorms that were on the radar that I could catch was a dying system out over extreme southeastern Iowa. The temperature had soared into the lower 90s by the time noon arrived, and dew points were up, which caused the air to be so thick that you began sweating before ever closing your door. Around noon, I went for a quick walk outside, just to feel the heat – and I sure felt it – but I also experienced a rather interesting sound; throughout the town you could hear the sound of air conditions rumbling, it sounded almost like a colony of bumble bees was surrounding me. Returning from my walk, I went back to the radar, waiting and waiting for the storms to develop.

I returned to the radar and that was when the storms caught my attention. On the radar, it looked as though they were the typical afternoon thunderstorms that develop in the Chicago region; but when I checked out the damage reports, I noticed isolated pockets of hail and wind damage and knew right away it was going to be a derecho. At 2:00 p.m., the derecho was moving into Indiana and getting stronger, feeding off the destabilizing atmosphere produced by daytime heating. It nearly doubled in size outside of Indianapolis and tore through the state like a knife, people caught off guard by the 70 mph winds that knocked the commuters around.

In Ohio, this storm blasted through Lima, and Findlay, with estimated winds of 95 to 100 mph, destroying half of the trees in both cities and transforming the city into an impassable zone of destruction. Power lines were snapped from broken polls across the flat landscape of Ohio, houses took damage from the branches of trees slamming against the siding of the house. When the trees could no longer take the force, they toppled over, crushing through roofs. In Columbus, the boiling temperatures were made worse by a combination of storm damage and a loss of power.

West Virginia and southwest Pennsylvania were not spared either; those unlucky enough to be just slightly within the storm would see what a hurricane force wind will do. Motorists became trapped on interstates as trees and overturned trucks blocked the roadways. While there may not have been much warning for those in the mountains, the damage reports that flooded the local National Weather Service offices gave plenty of warning time for Washington, D.C. and Baltimore, MD who were next in line after Charleston, WV and Richmond, VA.

HOW A THUNDERSTORM CAN DEVELOP INTO A DESTRUCTIVE DERECHO AN ANALYSIS OF THE METEOROLOGICAL EVENTS THAT CAUSED THE 29 JUNE 2012 DERECHO.

Derecho, a Spanish word meaning “straight”, is a squall line which produces excessive wind damage across a wide region. In every situation, the derecho is a complex of thunderstorms whose conglomeration produces a swath of wind damage for at least 250 miles (400 km) and producing winds that exceed 58 mph. Derechos almost always surpass the threshold needed for it to be categorized as a derecho; most feature winds upwards of 75 mph – category 1 hurricane force – and generally traverse at least 400 miles before calling it quits. On 4-5 July of 1999, the Boundary Waters – Canadian Derecho travelled nearly 1300 miles (2100 km) and produced winds around 90 mph (144 km/h). Some, but not all, occasionally produce tornadoes, but they are almost always weak tornadoes, generally around F/EF-0 or F/EF-1. These storms often travel at high rates of speed, sometimes close to 70 mph (113 km/h), and generally do not display hindrance when crossing the Great Lakes or Appalachian Mountains. Such was the derecho that slammed Delaware, Illinois, Indiana, Kentucky, Maryland, New Jersey, North Carolina, Ohio, Pennsylvania, Virginia, West Virginia, and Washington, D.C on the 29th of June, 2012.

Figure 1: Confirmed Damage reports received by local National Weather Service offices. Blue squares = wind damage from winds over 58 mph, yellow squares = wind damage from winds over 75 mph. Green = hail, and the lone red square is a tornado/funnel cloud report. By Timmy Albertson

THE LONE RIDGE RIDER

Derechos hold a nickname – a rather catchy one, if you will – that describes it both in origin and in life: Ridge Rider. While it may sound like the character from a 1950s John Wayne western film, the lone Ridge Rider combines its birthplace and life into one name. The origin of a derecho is almost always found in the same spot of a particular system, though the system may not always be in the same location, the relevant position of the ridge rider to the weather system will be nearly the same.

►►Heat Waves – The Birthplace

Like all systems in meteorology, the derecho has separate categories depending on its characteristics: Serial (single- or multi-bow), progressive, low dew point, and hybrid. Of these four categories, the most intense derechos fall into the progressive derecho category. Why is the most intense derecho the progressive, and why does that matter? It matters because the progressive derecho can be found in the location of a specific weather event that we call a heat wave.

Observations have shown, and statistics has done more Figure 2: Annotated 700 mb pressure height map from 29 June than just confirm, that there is a direct relationship 2012. between the summer heat waves in the central United States and a derecho. Not all heat waves will be accompanied by a derecho, but most derechos will be preceded by a heat wave. The conditions in meteorology that are needed for a derecho to develop are found to be the best within large scale heat waves. The type of heat waves that meteorologists are referring to are the same ones that can be found in the Southeast, Northeast, Mid-Atlantic and Great Lakes of the United States and southeastern Canada.

Thus, the derecho typically develops within an upper level ridge, therefore explaining the first half of the nickname ridge rider.

►►The Upper Levels – Jet Stream

These heat waves occur as the result of an upper level ridge of high pressure that moves into the southern and central part of the United States, and remains stationary, whilst an upper level low pressure and its associated trough dig into the Rocky Mountains. This causes the upper level wind pattern to develop a shape that is similar to a sideways S shape, which places the middle bend through the plains and the final hump over the Great Lakes. This feature in the upper level winds is known as the jet stream, which is semi-consistent stream of relatively fast moving air between two large air masses.

Figure 3: An idealized depiction of the mid-troposphere to support a derecho. Solid black lines are pressure heights. Often, the jet stream will dictate the direction and travel of a weather system, while at the same time, is altered by the weather at the surface at which it dictates. The jet stream, as it were, can thus determine the path that a band of thunderstorms will travel, or perhaps the next surface low pressure (pending everything remains constant). Since the derecho forms along the ridge of an upper level high, it will essentially ride the jet stream southeastward towards the next trough of low pressure.

Thus, we now understand the meaning of the nickname ridge rider. Though we may have taken a cool nickname and dissected the reasoning behind the choice of wording, I have yet to tell you how these storms developed in the first place.

MID-LEVEL CONDITIONS

The mid-level of the troposphere has always been an important location for the development and growth of thunderstorms in general. Most thunderstorms develop in the lower boundaries of the boundary layer, but others develop just a little bit higher in altitude. We refer to these kinds of thunderstorms as elevated thunderstorms. An elevated thunderstorm is crucial in developing a thunderstorm into a derecho, and in order to get an elevated thunderstorm, we need a little bit of help from the Rocky Mountains and western Plains.

►► (mT or cT) What’s it Gonna Be?

Paul Revere and the Raiders said it best: “I can still recall when you told me I was all…” but they were not singing about the maritime tropical air mass. In our case however, let us pretend that they were. So, as they asked so well in their first hit, the question of “What’s it gonna be?”

It is important to note that an upper level high pressure does not translate to a surface high pressure directly below it; rather, the surface high pressure will be to the east under the western edge of the upper level trough preceding the upper level high. This becomes an important component during the thunderstorms initial development along the surface frontal boundaries. The high pressure at the surface is an area of low level divergence which disperses in a clockwise rotation around the center of the high pressure. This rotation of air provides cooler air to the east of the center, but it introduces warmer air to the other side of the center. In the case of our heat wave, the high pressure center is off to the west, ushering in relatively moist and sultry air that is best described as a maritime tropical (mT) air mass.

At the surface level, this humid mass of air is what produces the conditions we know as the heat wave – for the most part, anyway. The mT air mass is also important for the development of the derecho, but it is accompanied by yet another mass of air, continental tropical (cT). As it turns out, the cT is a mass of air which has all the heat of an mT, but much less of the moisture, thereby making it almost desert like, but only in extreme cases.

►►The Elevated Mixed Layer

The Storm Prediction Center best describes the Elevated Mixed Layer as “a layer of mid-tropospheric air that originates over the arid, elevated terrain.” While the surface high pressure is ushering in mT air from the south and southwest, the mid-level high pressure is bringing in air from the Rocky Mountains, air which is hot and dry, better known as cT.

During the summer time, the cT air mass in the mid-level atmosphere does not make it much further to the east than the Great Plains and Canadian Prairies. When a heat wave is in place, which means that the high pressure system is not moving, suddenly the mid-level component of the high pressure begins to continuously pull this air eastward with its circulation. The result is that this dry air in the mid altitudes has now migrated way past the Great Plains and Prairies and extended itself into the Midwest and Great Lakes regions. Despite being dry air, the cT air provides a significant purpose for the development of a derecho.

Since dry air cools faster with height than moist air; even though the air may be warm at the surface, it cools much faster with increasing height (mid-levels) than it does should it have been moist. Therefore, the injection of cT air into the middle level of the atmosphere is highly welcomed, especially north of the stationary front. But what about this level of dry air makes thunderstorm develop this anticipated, should it prevent the development of thunderstorms? Actually, while it may at first, on a hot day along the surface and the air cooling rapidly with height, this actually allows for strong updrafts to occur, which are necessary for the development and sustainability of a thunderstorm, especially a derecho.

THE NIGHT BEFORE

Ah, thank you very much Paul McCartney for writing the song The Night Before, it makes it much easier to draw a parallel between a song on the Beatles’ Help! album and the final process to developing a derecho. In the end, what Timmy written weather guide would be complete without a reference to his favorite band?

►►The Death of a Mesoscale Convective System

The final ingredient to get a derecho to develop is that we need an ignition source. Though it is not always the case, in the event of June 29, 2012, this was the ignition source. Back in eastern Iowa, the folks around Davenport were watching as a type of storm complex, known as a mesoscale convective system (MCS), was dissipating. The death of an MCS is often new breeding ground for storms to develop, and if the conditions – as mentioned prior – are favorable for derecho development, than a dying MCS is all you need.

The night before the June 29, 2012, there was in fact an MCS making its way through Iowa. During the early morning hours of June 29, this MCS was on its way out. Later that day though, while the MCS may have been gone, its lingering effects were not. In fact, this left behind the ingredients for thunderstorm initiation during the early afternoon in neighboring Illinois.

►►The Bow Echo

At around noon in Illinois, a thunderstorm erupted and began to intensify, this storm quickly became what is referred to as a bow echo. A bow echo is a meteorology term referring to a thunderstorm that takes on the shape of a bow (like a bow- and-arrow) on the radar screen. A bow echo evolves from a cluster of thunderstorms, but sometimes it may develop on its own, but in either case one criterion is almost always present. The winds at the lowest levels of the atmosphere (say surface to about 8,000 feet) are blowing in the same direction.

So now, we have a bow echo on the radar, in an atmosphere which is conducive for the development of a derecho. So what happens next?

Figure 4: A bow echo in Iowa, as seen as base LET’S ROCK! reflectivity radar, on 5 May 1996. Courtesy of Iowa State University. The immortal words of Ed O’Neill’s character Al Bundy from the television show Married…With Children echo through the minds of nearly every forecaster. Our bow echo is now moving through Illinois and continues to intensify. This is because the bow echo happens to be in a location that not only favors thunderstorm develop, but also favors a long track derecho. The unidirectional winds will allow for the storm to pick up some speed, which will help to prevent the storm from strangling itself with rain cooled air or, as they are known as officially, downdrafts.

►►Downdrafts…

The updrafts lift air upwards into a thunderstorm, and since we know from elementary school that whatever Figure 6: Diagram of the ideal microburst. comes up must also come down, there is the updraft’s twin: the downdraft. A downdraft is the result of the rain fall, but it is not the physical rain that brings the downdraft, rather it is the air it drags down with it.

Cold air tends to be much denser than warm air – especially when the warmer air is moist. As the updraft lifts the warm air into the thunderstorm, the altitude at which these updrafts will lift the air to (remember the condition of the atmosphere: it is cooling rapidly with height) will result in it becoming cooled, this also where rain develops. As the rain falls, so too will the cooler air as they are both connected in their respective physical processes. Thus results in the premise that the more rain that falls, the more that colder air is being pulled down towards the surface.

All thunderstorms have a downdraft: storms have an updraft, so they also have a downdraft. Generally, the downdrafts in thunderstorm are short lived and often race out ahead of the storm. When the downdraft hits the surface, it spreads out and causes wind damage – depending on the intensity of the downdraft, the winds will vary from 30 mph to 100 mph. In the event that the winds are strong, but short lived, the event is referred to as a microburst (micro- because occurs in such a small Figure 5: Simplified diagram of the flow of air in and out of the surface area). Jokingly, one can think of a derecho as a front of a thunderstorm. series of seemingly never ending microbursts spread out over hundreds of miles. These downdrafts, upon hitting the surface will race out in nearly every direction – pushing both out of and further into the storm, thus we refer to it now as an outflow or in the broader sense, an outflow boundary. Outflow is just the downdraft (rain cooled air) that has hit the surface or near-surface and is racing out of the thunderstorm in all directions. Retrospectively, the air coming into the thunderstorm, that ultimately becomes the updraft, is known as inflow, or in a broader sense, inflow boundary.

►►Lightning Speed!

The outflow boundary may rush out so far ahead of the storm that it will actually slam into the inflow which the storm is feeding upon. When this occurs, the thunderstorm will often dissipate, but given the right circumstances, new storms can and sometimes will develop near the convergence zone between the outflow and inflow ahead of the storm. The cutting off of the inflow typically occurs in a storm of slow speed, and typical storms move at a speed between 40 and 50 mph. Derechos, on the other hand, do not move at that speed – rather they move at a lightning 60 to 70 mph, and in some cases, up to 80 mph.

Why is the derecho moving so fast? The winds in the lower troposphere are unidirectional and this aids the thunderstorm immensely in travelling. These winds push the bow echo along, and also begin to spread the bow echo out. The outflow of the thunderstorm around it helps to expand the size of the storm as well, but the derecho has another advantage to it. Since it travels so fast, it often takes a few hours before the outflow can start cutting off the inflow.

THERE YOU HAVE IT!

►►The Derecho

Intense falling rain producing downdrafts that hit the surface and spread out into outflows, the air hits the surface and promptly races out in all directions – the outflow. This outflow is hurried along by the high speed of the derecho. The high speed of the derecho is the result of the unidirectional lower level winds, which accelerate it forward and help to expand it. With the storm moving at a high rate of speed, it will travel hundreds of miles before the outflow cuts off the inflow and slowly begins to collapse the storm.

The heat is amply supplied by from the surface due to the heat Figure 7: This image shows where and how the derecho appeared on the base reflectivity radar over the course of 6 hours. wave, and this air is jettisoned upwards, in what is known as an updraft. The updrafts bring air upwards into the cool – dry air – and it condenses, resulting in the development of a thunderstorm. Since the temperature difference can often be fairly impressive, the precipitation will generally be heavy. In the end; heavier the rain, the heavier the downdraft – and the heavier the downdraft, the stronger the outflow.

The real damage begins the moment the downdraft slams into the surface.

Congratulations! You have now accompanied a derecho from its birth through the point where it slams into towns and cities. I hope that you have learned from this! That’s your weather!

-Timmy Albertson

Figure 8: Because of the conditions needed to create a derecho, we see that the Ohio River Valley and Mississippi River Valley favor derecho the most. Note, the Ozarks in Arkansas receive the most. This may be in part to the convergence of supercell thunderstorms following a tornado outbreak in Oklahoma.

Figure 9: Taken just outside of Portage, OH; this is the tail end of the section of the derecho that developed roughly 15 minutes before slamming into Findlay, OH on 29 June 2012. Because this branch of the derecho developed so rapidly, the city of Findlay, OH and neighboring communities had short warning and little information.

Figure 10: The calm before the storm, this photo was taken in south Lima, OH moments before the derecho hit on 29 June 2012.

Figure 11: Though it looks like a scene from a Florida hurricane, this is Lima, OH during the 29 June 2012 derecho. Winds caused damage that resulted in power loss for two days for much of the city.

Figure 12: The Derecho of 29 June 2012 produced severe wind damage in Williamstown, WV. Power in much of Washington County, OH and Wood County, WV was lost for roughly one week. The subsequent days saw temperatures soar to above 100 °F.

Figure 13: The residential streets of Findlay, OH were closed off for two days as downed trees clogged the roads. Other streets were able to move the trees out of the way after the first day, but traffic remained blocked for utility crews.

Figure 14: The same home as the one in the prior picture.

Figure 15: Walking along the sidewalks in Findlay, OH after the derecho was a difficult task. Downed tree limbs and downed power lines made the walk difficult, and clean up slow.

Figure 16: The 29 June 2012 Derecho tore through Findlay, OH, destroying roughly 50% of the city's trees. The trees along Main St. took power lines down with them as they collapsed. Findlay was without electricity for three days.

Figure 17: A massive tree collapsed in the estimated 100 mph winds that tore through Findlay, OH from the 29 June 2012 derecho.

Figure 18: Derechos and bow echoes do not typically produce tornadoes, but when they do, the tornadoes are generally short lived and very weak. The town of Milton Center, OH received an EF-0 tornado from a massive bow echo on 12 June 2013. The estimated winds from the bow echo were roughly 85 to 90 mph.

Figure 19: The sidewalks of northwest Bowling Green, OH after the massive bow echo of 12 June 2013. The winds on the northwestern edge of town were estimated to be roughly 70 mph, the rest of the town escaped with minimal damage.