National Weather Service United States Department of Commerce

Clash of the Boundaries - Weather Review - April 17, 2010


The image below is a loop of composite reflectivity from 915pm until 1100pm CDT over east central Alabama.  The focus with this weather review is the collision of two boundaries.  There are many different types of boundaries that can form -- a front (cold, warm or stationary), a sea breeze, an outflow boundary from a thunderstorm or even a boundary caused by a temperature differential.  The two boundaries on this day were the sea breeze moving from south to north and a weak cold front that was moving north to south.  Many times, a collision of this sort will produce at least a shower or maybe even a thunderstorm.   As you can see, this is an example where nothing occurred.  (Click to enlarge)




In the study of weather, meteorologists can not limit themselves to why something occurs; you have to try to answer why something does NOT occur, as well.  After digging through the upper air soundings for the morning of April 17 (below left) and that evening (below right), the evidence becomes apparent as to why there was no convection along this boundary collision.  First, looking at the Skew-T on the left from the morning, this would be the atmospheric conditions in between these boundaries.  Clearly, you can see an isothermal layer (layer of temperatures of the same value), which would act as a "cap".  Pilots in the area that morning reported the tops of the clouds at about 10K ft, and this is exactly where this isothermal layer is set up.  For the Skew-T on the right, which would be the atmospheric conditions along the front (northern boundary), you can plainly see a similar type of "cap".  Another component could be the depth of the actual cold front.  At approximately 6K ft in depth, the cold front was just not deep enough to reach the Level of Free Convection (LFC), which had this happened, would have aided in the developement of convection.  In the same vain, the sea breeze was only approximately 1700 ft in depth. (Click on the Skew-Ts to enlarge).


skew-t skew-t


For an example of when a boundary collision did produce precipitation, click here.  Watch for more boundary collision weather reviews as the NWS in Birmingham focuses on summertime convection initiation this summer.