Location: Ocean Springs, Mississippi
Date: 9 July 2014
High amounts of moisture available in the atmosphere combined with slow moving storms created a recipe for periods of heavy downpours near the Gulf Coast of Mississippi on 9 July. You can see the high-intensity rain shaft in this picture from Shawn, revealing an area of strong downward motion. This rain-cooled, downward air spreads outward away from the storm, lifting warm moist air out ahead of it. This can lead to new storm formation in nearby areas, which is a common scenario for flash flooding events.
These two radar images (separated 45 minutes apart) provide a view of these storms and the interaction the outflow they produce. Note that a line of storms form along the intersection of two boundaries (seen as fine lines of greens/blues on radar). These storms merge and intensify to produce an outflow boundary of their own, which can go on to produce new storms, and so on.