Dry Thunderstorms

Not all thunderstorms in Colorado produce measurable rainfall as all it takes to be considered a thunderstorm is the presence of lightning and thunder.  There are many days along the Front Range of Colorado where there is limited moisture and yet enough instability to produce high based thunderstorms.  What is meant by “high based” is the bottom of the cloud has formed well above the surface, usually 8,000 feet or more above ground level.  The greater the distance from cloud bottom to the ground the greater the evaporation of the rainfall before it reaches the surface and in many cases there is not any rainfall left at all.  This time of year from late June into mid or late July is one of the most common times of year for “dry thunderstorms”.  This is mainly due to the changing of seasons as the cool of spring has left and the heat of summer is upon us yet the monsoonal moisture has not arrived from the desert southwest quite yet.  Once deeper moisture arrives later this month the threat of dry thunderstorms will lower through August until we transition into fall.  Then as monsoon moisture departs the state dry thunderstorms will be likely once again by early to mid September.

Some of you have heard us as at Skyview Weather talk about temperature and dew point spreads.  What we are referring to is the air temperature and dew point at the surface which is a good indicator how much evaporation will occur as well as how strong the potential wind gusts will be.  At the base of the cloud temperature and dew point are the same, hence condensation occurs and clouds form.  While at the surface there may be upwards of a 50-60 degree temperature and dew point spread.  For example if it is 90 degrees outside and a current dew point of 30 degrees there is a spread of 60 degrees.  This magnitude of 60 can be used to predict potential wind speeds or gusts from thunderstorms; in this case a 60 mph wind gust is highly likely.  These wind gusts are caused by the evaporation of rainfall aloft cooling the air around it.  This cooler air rushes to the surface and spreads out from a central point.  These winds can travel quite a long distance slowly weakening the further away from the central point.  These winds as they spread out can be known as outflow boundaries.  Outflow boundaries can then push against the foothills or slam against another outflow boundary from another storm and can be a triggering mechanism for new thunderstorm development.  Many times these outflow boundary driven storms are “popcorn” type thunderstorms as they often strengthen and weaken rapidly.  Many times outflow boundaries and their associated winds can be surprising to some.  They can be that gust of wind out of nowhere while having that outdoor bbq, playing golf, fly-fishing, setting up that big outdoor tent, etc.