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Breaking Down Thursday's Severe Weather Threat

Our threat of severe weather is increasing for Thursday, specifically the afternoon and late night.  Almost all of our WDRB counties are included in a Slight Risk (level 2 out of 5) with our northeastern counties in the Marginal Risk (level 1 out of 5).  It's certainly possible the SPC could put out an Enhanced Risk (level 3 out of 5) Thursday morning in or near our area. 



Believe it or not, this severe weather threat is not solely caused by Alberto, the tropical system to recently move past us.  As Alberto leaves, that opens us up to the approaching cold front and the warm/moist air mass it will push ahead of it (ushered in by Alberto). 


Let's start by looking up through the atmosphere.  This low level flow shows a jet - an area of stronger wind - centered over us late Thursday.  As we will look at below, there are two waves of storm chances Thursday.  These large-scale factors are driving both. 


In the mid-levels of the atmosphere, we look at vorticity which is basically spin.  How these areas of "spin" move is most important in storm forcing.  You can clearly see the area of higher vorticity (red/orange) here Thursday night.  The fact that it's positioned right next to an area of weak or negative vorticity (dark blue) shows us we will have positive vorticity advection in our area late Thursday. That means the area of positive vorticiy is moving into the area of negative vorticity, creating movement and spin in the atmosphere. 


In the upper levels of the atmosphere, we can look at the jet stream.  At first glance you might think this map doesn't show you much, but look again.  The white lines showing wind direction seem to separate from each other over Kentuckiana.  That movement apart is called divergence, and divergence aloft leads to rising air in the atmosphere.  We need rising motion to support storm development and growth. 



The first round of storms starts in the early afternoon, and will be driven by daytime heat and humidity.  This first wave should enter our western counties around 2 PM, closer to Louisville around 3 PM, and our eastern counties by 4 PM. 


The AdvanceTrak image above shows that temperatures will be in the middle to upper 80s by this time of day, providing ample heating.  It also shows breaks in clouds.  Those small pockets of sun will increase our instability during the day. 


Full disclosure: these numbers are on the high end of possibility.  A safe bet for CAPE numbers will be 1500-2500 during peak heating.  The higher the number here, the more energy storms have available to them. Numbers in the 2000-3000 range are high for this area. 


Dewpoints will also be high- lower to middle 70s.  Through the morning you will notice the air feels humid.  Moisture is a key ingredient in storm development, and we definitely have that here. 



First, we need to recognize that whatever happens with this earlier round will impact the later round.  If the earlier storms are stronger, it would likely rob the later storms of some energy.  If the early round doesn't come together as planned, that leaves more energy for the evening storms. 


Dewpoints will still be high for our second round of storms.  CAPE numbers won't be as high because we lose the daytime heating, but we will still have ample moisture. 


This second round will be forced more by a weak front, so we will start to see more storms after sunset.  You can see in these images how this becomes more organized the longer it stays "alive." 


These will intensify rapidly "blowing up" as they move across Kentuckiana.  The trick here will be watching how they interact with each other.  Those interactions can cause either strengthening or weakening depending on what happens, so those will become "nowcast" situations. 


Models suggest this becomes a Mesoscale Convective System ("MCS").  That means it will be larger than individual thunderstorms; think like a cluster of strong cells all grouped together. 


This is truly an overnight event as we don't clear these clusters out until nearly daybreak! Make sure you have a way to receive weather alerts while you are asleep in case we need to share warnings with you. 



The main threats will be large hail and damaging wind gusts.  Hail could be larger than 1" diameter and wind gusts could be stronger than 60 mph.  The smaller threats are heavy rain (leading to flooding) and isolated tornadoes. Marc will continue to share updates with you on WDRB News tonight. 

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-Hannah Strong



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