There was a terrible storm that hit part our area back on June 29th. This powerful storm left thousands of people without power in the Washington D.C Metro area. Meteorologists kept saying that is was a Derecho. A "Derecho" is a Spanish word for Straight and this is regarding the type of winds that this type of storm brings.
It is a widespread, long-lived, straight-line windstorm that is associated with a fast-moving band of severe thunderstorms. Generally, derechos are convection-induced and take on a bow echo form of squall line, forming in an area of wind divergence in the upper levels of the troposphere, within a region of low-level warm air advection and rich low-level moisture. They travel quickly in the direction of movement of their associated storms, similar to an outflow boundary (gust front), except that the wind is sustained and increases in strength behind the front, generally exceeding hurricane-force. A warm-weather phenomenon, derechos occur mostly in summer, especially during June and July in the Northern Hemisphere, within areas of moderately strong instability and moderately strong vertical wind shear. They may occur at any time of the year and occur as frequently at night as during the daylight hours.
Definition by and more detailed information at; http://en.wikipedia.org/wiki/Derecho
As of today, the National Weather Service and The Storm Prediction Center, is monitoring a possible repeat of this storm that may come near us tomorrow to the north of Philadelphia (7-26-12)
Here is a video of that last event.