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April 12 Severe Event


joshwx2003
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145 PM CDT SUN APR 12 2020  
  
AREAS AFFECTED...NORTHERN AND CENTRAL ALABAMA AND FAR SOUTHERN  
MIDDLE TENNESSEE.  
  
CONCERNING...SEVERE POTENTIAL...TORNADO WATCH LIKELY   
  
VALID 121845Z - 122015Z  
  
PROBABILITY OF WATCH ISSUANCE...95 PERCENT  
  
SUMMARY...THE SEVERE WEATHER THREAT CURRENTLY OVER MISSISSIPPI WILL  
SHIFT EAST INTO ALABAMA LATER THIS AFTERNOON. A DOWNSTREAM TORNADO  
WATCH WILL BE NEEDED SOON.  
  
DISCUSSION...A MCS MOVED ACROSS NORTHERN LOUISIANA THIS MORNING AND  
PRODUCED NUMEROUS TDS SIGNATURES. THIS SAME CLUSTER OF STORMS HAS  
PRODUCED A TDS MORE RECENTLY (~1830Z) IN WEST-CENTRAL MISSISSIPPI.  
THIS SAME LINE OF STORMS IS EXPECTED TO MOVE EAST NORTHEASTWARD INTO  
NORTHERN ALABAMA LATER THIS AFTERNOON AND INTO THE EVENING. THE  
AIRMASS ACROSS NORTHERN ALABAMA HAS DESTABILIZED ACROSS THE LAST  
SEVERAL HOURS AND CONTINUED MOISTENING/WARMING IS EXPECTED AS THE  
WARM FRONT LIFTS NORTH. THEREFORE, THIS ONGOING MCS IS EXPECTED TO  
MAINTAIN ITS CURRENT INTENSITY AND LIKELY STRENGTHEN FURTHER AS IT  
MOVES INTO ALABAMA.   
  
A 997MB MESO-LOW HAS FORMED IN SOUTHEAST AR/WESTERN MS. THIS HAS  
ACTED TO LOCALLY ENHANCE THE BACKING OF SURFACE WINDS AHEAD OF THESE  
STORMS. THIS HAS LED TO VERY LARGE 0-1 SRH VALUES NEAR 700 M2/S2 PER  
GWX VWP. DAMAGING WINDS AND QLCS TORNADOES WILL CONTINUE TO BE A  
THREAT WITH THIS MCS AS STORMS MOVE EASTWARD. HOWEVER, THERE IS A  
CONDITIONAL THREAT FOR SURFACE BASED SUPERCELLS TO DEVELOP SOUTH OF  
THIS ACTIVITY IN THE WARM SECTOR. IF THIS OCCURS, A SIGNIFICANT,  
THREAT WOULD BECOME MORE LIKELY.  
  
A TORNADO WATCH WILL BE NEEDED SOON EAST OF THE CURRENT WATCH IN  
MISSISSIPPI.  

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Honestly I doubt we’re gonna get any discrete supercells... Sure XYZ parameters are off the charts but good god LLVL LRs are so bad. 5-6C/km across southern Mississippi, I just don’t think that’s going to cut the mustard... and with super saturated profiles, I don’t think that’s going to be changing.

Everything is so messy too. 

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4 minutes ago, jojo762 said:

Honestly I doubt we’re gonna get any discrete supercells... Sure XYZ parameters are off the charts but good god LLVL LRs are so bad. 5-6C/km across southern Mississippi, I just don’t think that’s going to cut the mustard... and with super saturated profiles, I don’t think that’s going to be changing.

Everything is so messy too. 

Yup...agreed definitely could limit this threat.

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5 minutes ago, Indystorm said:

I think Quincy somewhere back in this thread said something to the effect that you don't need low level lapse rates to be great if other parameters are off the charts.

You don’t need low level lapse rates typically, but if there’s a cap storms won’t get organized if they even initiate. In this case we need them to overcome the cap that’s evident in the 18z JAN sounding.

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5 minutes ago, Indystorm said:

I think Quincy somewhere back in this thread said something to the effect that you don't need low level lapse rates to be great if other parameters are off the charts.

He has been touting that for days, but obviously they are limiting things here.

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3 minutes ago, jpeters3 said:

He has been touting that for days, but obviously they are limiting things here.

 

4 minutes ago, JasonOH said:

You don’t need low level lapse rates typically, but if there’s a cap storms won’t get organized if they even initiate. In this case we need them to overcome the cap that’s evident in the 18z JAN sounding.

Thank you guys for explaining how the LL lapse rates contribute to breaking the cap when one is present.

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6 minutes ago, Indystorm said:

 

Thank you guys for explaining how the LL lapse rates contribute to breaking the cap when one is present.

It's really an issue of low-level buoyancy.  If you can get enough low-level buoyancy, that aids (or at least doesn't act as a deterrent) in the vertical stretching of near surface vorticity, then you are golden.  Likewise, weaker low-level laps rates equates to less low level acceleration and makes it harder to stretch near surface vorticity.

So there may not necessarily be a CAP (there could be no CIN), but it's still hard to punch through the layer with very weak buoyancy.

It is possible for low-level dynamics to overcome this issue, but we don't really have a good understanding at this point of which conditions do, and which do not permit this "compensation." 


For those of you who are more technically inclined, it's not actually the low level buoyancy that does the near surface stretching.  It's actually the buoyancy-driven low pressure that sits at the updraft bottom, and the associated upward accelerations below the updraft base and the center of this low pressure.  When low-level buoyancy is stronger, this low pressure feature is also stronger.  Note that this effect is separate from the low-level dynamic accelerations driven by rotationally driven low dynamic pressure.

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23 minutes ago, jojo762 said:

Honestly I doubt we’re gonna get any discrete supercells... Sure XYZ parameters are off the charts but good god LLVL LRs are so bad. 5-6C/km across southern Mississippi, I just don’t think that’s going to cut the mustard... and with super saturated profiles, I don’t think that’s going to be changing.

Everything is so messy too. 

My exact thoughts and was my fear with this setup. Models never showed good low level lapse rates or 0-3km cape. Low level instability is crucial in tornadogenesis. I do think there will be plenty of qlcs tornadoes but discrete supercells with long track tornadoes is looking less and less likely each hour. However, qlcs tornadoes are still nothing to ignore and can be strong 

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1 hour ago, Quincy said:

Last check, low-level lapse rates and 0-3km instability are still fairly marginal. I think the most intense discrete action won’t be until 21-00z.

If you want supercell tornadoes, steeper low-level lapse rates are preferred. If you have a QLCS moving into an area with marginal lapse rates, like we’ve had today (around 5-5.5 C/km), but substantial low-level shear, that’s different. 

If you dig back, I cited that 6-6.5 C/km lapse rates will get it done. If you look at mesoanalysis right now, that’s confined to southern MS/LA. They’re barely around 5 C/km along the warm front, which is where SPC has been highlighting an “enhanced” sig tor potential.

As the warm front lifts, you should see lapse rates improve a bit, mainly down near I-20, not up near the TN border. 

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2 minutes ago, SluggerWx said:

Didn't expect to see a complete break in the clouds in extreme SW MS in just a couple frames on GOES-16, but there it is. Looks like overshooting tops on visible surround this little clear blob. Wonder if the CAP eroded.

mini patches of clear now surround some of those cells, an hour ago stratus did which is a clue of elevated over an inversion..

so maybe the inversion is eroding some

now the question might be is there too many of them

 

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