Derechos are most common in the Midwestern United States, but are still fairly rare. You might see a derecho about once a year there. They can occasionally be found all the way up into the Northeast. The best thing to do in the event of a derecho is to go someplace safe and protected—high winds and falling trees can be quite dangerous! Weather satellites can view derechos from space and help communities predict when they might form.
The group of thunderstorms produced heavy rainfall and strong winds estimated to be 70 to 80 miles per hour that downed trees and power lines, along with frequent lightning. More than 1 million homes and businesses lost power due to the storms.
Credit: NOAA. Slide the bar to see the areas where power was lost as a result of the June derecho storm. The Short Answer:.
Derechos are fast-moving bands of thunderstorms with destructive winds. The photo was taken on the evening of July 10, in Hampshire, Illinois. Credit: Brittney Misialek. Derecho development is necessarily tied to the formation of bow echoes. A bow echo usually arises from a cluster of thunderstorms, but also may evolve from a single strong storm. Bow echoes most frequently occur when atmospheric winds are relatively strong and unidirectional i.
As the rain-cooled downdraft of a thunderstorm reaches the earth's surface, it spreads horizontally, most rapidly in the direction of the mean atmospheric flow. As the cool, dense air spreads outward, it forces the lighter, warm and moist air surrounding the storm up along the leading edge of the outflow, or gust front see figure below, with mean flow assumed to be from left to right.
Gust fronts often are marked by a band of ominous, low clouds known as "arcus. The rain produced by the newer storms reinforces the cold pool, strengthening the inflow of air from the back side of the developing storm complex and encouraging the downward transport of higher momentum winds from aloft. These processes can enable the system to attain a nearly steady-state condition. This increases the longevity and strength of the entire system and is what allows the storm to travel such a large area over a short amount of time.
At this point, the convective system typically exhibits a pronounced bow shape on radar see figure below. As the thunderstorms continue to increase in coverage, even more rain-cooled air reinforces the cold pool pictured below. The line of storms continue to accelerate either as one large bow echo or multiple smaller bow echoes within an overall line. At this point, widespread and persistent wind damage has been occurring for a prolonged period of time.
Below is an example of damaging wind reports left behind by the June 29, derecho that swept from the upper Midwest to the central Atlantic coast in a matter of hours. Almost 1, reports of damaging winds were received with numerous reports of wind gusts at hurricane force or greater 74 MPH.
Powerful winds likely extended into the Atlantic Ocean. Below is an image depicting favorable weather patterns for derecho formation. A progressive derecho needs a very unstable hot and moist environment with relatively strong winds aloft. Therefore, the most favorable area for this type of storm to develop is on the periphery of a strong area of high pressure aloft where the greatest overlap of hot and dry conditions and stronger wind aloft occurs.
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