Severe Weather was Expected -- What Happened?
For the last couple of days, we had been advertising the potential for severe weather -- a squall line, with bowing structures, possible supercells within the line and maybe even ahead of the line -- last night and this morning.  When the line got to central Alabama, it ended up being nothing more than a couple of general thunderstorms.  What happened?  Here is our main culprit:
 

BMX Upper Air soudning 7 am April 18

BMX Upper Air soudning 7 pm April 18

 

To understand what occurred (or didn't) last night, you have to look at what the atmosphere did ahead of the cold front.   Above are the upper air soundings from Birmingham yesterday.  At 7 am, on the left, notice the warm air aloft that we highlighted with the black circle.  12 hours later, on the right, notice that the atmosphere tried to modify as the cold front crept closer to central Alabama, but was not able to erode away the warm air aloft.  This warm air aloft "capped" the atmosphere, which kept strong updrafts from being able to form, therefore thunderstorms are not able to develop.  Notice also, there was only a little bit of shear within the atmosphere at 7 pm (right).  The shear did increase as the line approached, but without the instability, storms just weren't able to get going.  Even more interesting, even had we realized enough instability, the LCLs (base of the clouds) were so high, we may not have been able to get tornadoes, anyway.

In contrast, the soundings below are from Jackson, MS.  At 7 am, on the left, they too had a nose of warm air aloft, but by 7 pm, (right) that warm air was able to be eroded, giving them the necessary instability for strong to severe thunderstorms to develop.  With ample shear in place, they ended up with just enough instability (CAPE) for a couple of storms in west-central Mississippi to spin up a couple of weak tornadoes, as welll as one to two inch hail.  Instability and shear values are highlighted in the green ovals in each of the 7 pm soundings.

 

JAN Upper Air Sounding from 7 am April 18, 2013  

JAN Upper Air soudning 7 pm April 18


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