DANGEROUS DOWNDRAFTS !!!
    KEEPING TRACK OF THE WIND BEFORE IT STRIKES

When rain falls into dry unstable air aloft, much of it evaporates. This process causes the air aloft to become cooler and more dense relative to the surrounding atmosphere.  Under certain conditions the cool air aloft descends rapidly to the ground where it spreads violently outward, resulting in strong damaging downdrafts, which are sometimes referred to as downbursts or microbursts.  Damaging downdrafts may have wind speeds of up to 100 mph and can therefore seriously damage homes and buildings. These severe weather phenomena have also caused several major airline crashes with huge loss of life.

Detecting damaging downdrafts is difficult and challenging, especially here in the southwest. While these weather elements are often associated with thunderstorms which produce moderate to heavy rain, in some circumstances destructive dry microbursts will occur  where there is almost no thunder and little or no rain reaching the ground. Since radar will usually not detect a dry microburst, they can strike with little or no warning. 

                                                       
                                                  Visual Characteristics                                 
Downdrafts produced by thunderstorms with moderate to heavy rainfall are often associated with a low hanging and rapidly moving shelf cloud (left). Damaging winds are frequently along or just ahead of the shelf cloud making it an important visual clue that destructive winds are in the vicinity.  In the case of dry microbursts, there may be little or no thunder and rainfall. However the strong winds may produce a wall or cloud of dust (right), a sure sign strong and possibly damaging winds are occurring. 
      
                     Radar Signatures

A common radar signature associated with damaging thunderstorm downdrafts is the bow echo (left). The bow echo is often associated with strong straight-line winds, brief but very heavy rainfall and frequent cloud to ground lightning. Strongest winds are almost always at the apex of the bow; thus if weather radar shows a bow echo approaching your area, seek shelter in a strong building. Remember, damaging downbursts can destroy mobile homes, can blow over motor vehicles and boats, and can even remove the roofs off stronger buildings.


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