Shane Young
EF1
Looks like, to me, that the key to the event for the southeast was the location of the jet streak, particularly around 500 mb. Plus some good strong diffluence aloft over the southeast.
And the effects both had on the surface low progression.
To see what I'm looking at, pull up the Mesoanalysis archives (http://w1.spc.woc.noaa.gov/exper/ma_archive/). I looked at 300 mb, 500 mb, and 700-400 mb vorticity advection.
I am surprised, looking back, at how massive the diffluence was in the region. Surprised this wasn't mentioned more... I definitely overlooked it earlier.
But the big thing that jumped at me is the how the vorticity advection aloft was shifting east until about 19 or 20Z, then quickly began to broaden and move away NE.
Up until that time, it's situated basically over the surface low. If you watch the sfc plot (I was watching RAP during the afternoon, but it shows up easier in the Mesoanalysis archive), the low was dawdling ENE through Missouri. As soon as the upper dynamics shift away north around 20Z, the surface low begins to quickly eject away NE-NNE as well.
And this is why the low-level winds in Mississippi and Alabama really veered around this time..... leading to helicity decrease..... and the storms stopped rotating as much, lost their discreteness, and it all petered out from there, particularly from south to north.
NOTES:
- It really doesn't look like the helicity changes show up very well in the Mesoanalysis, but to me, Mesoanalysis never really seems to capture how the variations in low-level flow modify helicity very well (for example, boundaries (where there is a strong helicity gradient) don't stand out typically).
- As I understand it, the winds veer out in low-levels as the surface low moves further away because of the decrease in isallobaric flow.
[Isallobaric wind is a wind pointing from where pressure is rising the most to where pressure is falling the most [in a reference frame with respect to the moving low] (i.e. towards the center of a deepening low). As the pressure fall move further away, the isallobaric flow diminishes..... flow becomes more isobaric (as it usually is nearer the surface), and you end up with flow more SWerly for a front oriented SW-NE.]
- Of course the helicity evolution didn't mean much without quite healthy thermodynamics. The moist axis set up nice today and the patchy breaks in the clouds was a very rare treat in the southeast, where we don't need much with such low LCL's.
As to why the vorticity advection did shift north around 20Z? I think that's where the 500 mb map comes in key (upper levels help clarify too). The 500 mb jet max is coming through the base of the trough up until around 20Z, and that means greatest vorticity advection. As it starts moving up the right side, the PVA decreases AND moves away. I think you can see some good evidence of the jet streak coming around on the water vapor imagery.
I've watched quite a few of these southeast events now, and found it very difficult usually to pick out the big event from the bust event... especially when it came to finding discrete supercells. (I didn't chase today, but did base........ they had every possible thing go wrong, so we won't talk about that anymore!
).
Maybe others are already watching out for these things, and I just now finally caught on more to the specific importance of the midlevel jet... but I definitely think this is something to pay attention to in the future... especially here in the southeast, where winds commonly veer early, and it has seemed to me that the very lowest-level flow is most critical to holding the helicity needed to get the rare discrete event (the position of the surface low is so crucial down here).
Anyways, that's what I was thinking about today. Please offer your comments if you have any
Shane (of the non-Adams variety)
And the effects both had on the surface low progression.
To see what I'm looking at, pull up the Mesoanalysis archives (http://w1.spc.woc.noaa.gov/exper/ma_archive/). I looked at 300 mb, 500 mb, and 700-400 mb vorticity advection.
I am surprised, looking back, at how massive the diffluence was in the region. Surprised this wasn't mentioned more... I definitely overlooked it earlier.
But the big thing that jumped at me is the how the vorticity advection aloft was shifting east until about 19 or 20Z, then quickly began to broaden and move away NE.
Up until that time, it's situated basically over the surface low. If you watch the sfc plot (I was watching RAP during the afternoon, but it shows up easier in the Mesoanalysis archive), the low was dawdling ENE through Missouri. As soon as the upper dynamics shift away north around 20Z, the surface low begins to quickly eject away NE-NNE as well.
And this is why the low-level winds in Mississippi and Alabama really veered around this time..... leading to helicity decrease..... and the storms stopped rotating as much, lost their discreteness, and it all petered out from there, particularly from south to north.
NOTES:
- It really doesn't look like the helicity changes show up very well in the Mesoanalysis, but to me, Mesoanalysis never really seems to capture how the variations in low-level flow modify helicity very well (for example, boundaries (where there is a strong helicity gradient) don't stand out typically).
- As I understand it, the winds veer out in low-levels as the surface low moves further away because of the decrease in isallobaric flow.
[Isallobaric wind is a wind pointing from where pressure is rising the most to where pressure is falling the most [in a reference frame with respect to the moving low] (i.e. towards the center of a deepening low). As the pressure fall move further away, the isallobaric flow diminishes..... flow becomes more isobaric (as it usually is nearer the surface), and you end up with flow more SWerly for a front oriented SW-NE.]
- Of course the helicity evolution didn't mean much without quite healthy thermodynamics. The moist axis set up nice today and the patchy breaks in the clouds was a very rare treat in the southeast, where we don't need much with such low LCL's.
As to why the vorticity advection did shift north around 20Z? I think that's where the 500 mb map comes in key (upper levels help clarify too). The 500 mb jet max is coming through the base of the trough up until around 20Z, and that means greatest vorticity advection. As it starts moving up the right side, the PVA decreases AND moves away. I think you can see some good evidence of the jet streak coming around on the water vapor imagery.
I've watched quite a few of these southeast events now, and found it very difficult usually to pick out the big event from the bust event... especially when it came to finding discrete supercells. (I didn't chase today, but did base........ they had every possible thing go wrong, so we won't talk about that anymore!
).Maybe others are already watching out for these things, and I just now finally caught on more to the specific importance of the midlevel jet... but I definitely think this is something to pay attention to in the future... especially here in the southeast, where winds commonly veer early, and it has seemed to me that the very lowest-level flow is most critical to holding the helicity needed to get the rare discrete event (the position of the surface low is so crucial down here).
Anyways, that's what I was thinking about today. Please offer your comments if you have any
Shane (of the non-Adams variety
)
