A microburst can develop from a passing thunderstorm. An updraft of air is pulled into the storm, while on the other side a downdraft of air is occurring.
A downdraft may develop within the storm's cell and rain and wind is pushed down to the surface. As the forced air hits the ground, it is forced outwardly and spread along the surface.
The wind speeds within the downdraft could be in excess of 150 MPH, therefore called a microburst.
These microbursts can initiate quickly during a thunderstorm.
The critical problem is when these storms and their microbursts are occuring, planes that flying through or within the vicinity could be adversely affected by it's intense downward wind speeds.