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
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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 th the 29 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.