Strong winds are created by an area of rain-cooled air thru (Evaporative Cooling) held up by the storm’s updraft. This is the same setup that usually creates hail, as well. Since cold/dry air is very dense or heavy, it will eventually fall to the surface at a high rate of speed. Once it hits the ground, the winds will spread out in all directions producing strong winds called straight line winds or downburst. Unlike tornados, winds from a downburst are directed outwards from the point where it hits land. Dry downbursts are associated with thunderstorms with small amounts of rain, while wet downbursts are created by thunderstorms with high amounts of rainfall.
Wet downbursts are usually the cause of torrential rains and flash flooding.Damaging winds can also be the result of that same cold pool of air increasing in size and elongating. Sometimes this could cause fast moving upper level winds to create an inflow of air known as the rear-inflow jet, behind the thunderstorm. This fast moving current aloft will hit the storm’s updraft, but instead of going through the updraft, it tilts downwards towards the surface, bringing very gusty winds with it. This will cause the storm’s updraft to tilt as well, allowing the thunderstorm to live longer and expand outward. This cold air accumulating beneath the storm strengthens the gust front, causing it to take on a bow like shape. The resulting acceleration and forward motion of the gust front subsequently forces warm & moist air upward creating more storms, and if this process repeats itself, a Derecho could result.
Derecho: The criteria for severe winds are 58 miles per hour or greater at the surface. In a Derecho, these winds can travel a distance of at least 250 miles (400 km) or greater – ahead & along a squall line. A Derecho’s wind can last up to 30 minutes or longer. Due to the length of time these winds are sustained, they can cause considerable damage over a large area.