Sunday's Shear Profile

[Jared Orr]

I've been chasing for a while, so I should know this...But what is the behavior of a supercell near a warm front when there is strong south wind aloft and SE or ESE surface winds? I would imagine that the supercell would move NNE until it delves into cooler air...then it would become weakened...cells would probably congeal with the southern cells being the most potent....but what about tornadic potential?

This extreme closed upper low/negative tilt setup on Sunday has me wondering where to target, because I'd like to stay north if possible (closer to home).

 

[Warren Faidley]

If you are going to chase north you must also consider the CAPE and where the best instability ends up at max heating. The DL location will also be a factor. The ICT hodograph (ATM) looks insane for Sunday. That could change. Storm movement (direction and speed) can also be calculated from upper air data.

W.

 

[Skip Talbot]

North moving cells have a tendency to quickly overrun the warm front, at which point they often become elevated as they run into the cool, stable air north of the front. Helicity ramps up right on the warm front and north of it, however, so your storm will often try to spin up as it encounters the warm front. It's a balancing act between how long the storm can maintain its updraft and surface based inflow while taking advantage of the increased helicity. Visually you could see the storm's base lower, a wall cloud or low level rotation develop, and then striations and laminar inflow bands as the storm starts to pull in more stable air, at which point it could still be a prolific hail producer but the tornado chances will be diminishing.

If the storm stays in this favorable window near the warm front long enough, you may see a tornado before it weakens or goes elevated. The ideal scenarios, however, are when the warm front is lifting north as well, or the cell encounters the warm front at and angle closer to parallel than perpendicular. These situations keep the cell in that favorable zone for a much longer time, and can allow for long lived or multiple tornadoes and cyclical supercells.

Storms often turn right and try to latch on to or ride a boundary that they encounter as well. This is also an ideal situation as well, as it keeps your storm in that favorable zone for a long time. A storm approaching a warm front at an angle closer to parallel may be more likely to do this. You may see an initiating cell with an almost N track turn hard right and move E along the warm front. This usually happens where you've got a veering wind profile with SW flow aloft though, and the cell is steered to the right as it matures. S flow aloft may keep that cell moving N even as it hits the warm front.

So try to gauge your storm motion when looking at those wind fields and the motion of the warm front as well. Maybe right near the surface low, storm motion will be to the north and storms will very quickly cross the warm front, but perhaps the storm motion becomes more northeasterly to the east and storms might stay within the warm sector longer, allowing the to mature and take advantage of the helicity on the warm front, or even root to the boundary and become long lived, cyclical supercells.

I'm not sure what you mean by congealing with southern cells. Sometimes you can get north south lines of training cells that congeal into a solid mess when you've got southerly flow that parallels the initiating boundary. Other times you get a nice ring of discrete cells if that boundary bows out to the east like the GFS was showing for Sunday last night. However, you can still get a line of discrete north moving tornadic supercells paralleling a north south boundary that doesn't congeal if you have the right shear profiles in the warm sector. 3/28/07 is a good example.

 

[Jared Orr]

Thanks for the detailed reply skip!

North moving cells have a tendency to quickly overrun the warm front, at which point they often become elevated as they run into the cool, stable air north of the front. Helicity ramps up right on the warm front and north of it, however, so your storm will often try to spin up as it encounters the warm front. It's a balancing act between how long the storm can maintain its updraft and surface based inflow while taking advantage of the increased helicity. Visually you could see the storm's base lower, a wall cloud or low level rotation develop, and then striations and laminar inflow bands as the storm starts to pull in more stable air, at which point it could still be a prolific hail producer but the tornado chances will be diminishing.

Okay, That's what I was thinking. I knew that helicity was enhanced along the front, but I never thought of the striations and laminar flow bands as being indicative of stable air being pulled in.

The ideal scenarios, however, are when the warm front is lifting north as well, or the cell encounters the warm front at and angle closer to parallel than perpendicular.

In my question, I wasn't taking into account the movement of the front. Thanks for pointing that out.

I'm not sure what you mean by congealing with southern cells. Sometimes you can get north south lines of training cells that congeal into a solid mess when you've got southerly flow that parallels the initiating boundary.

I guess I was assuming that with isentropic lift and screaming upper winds, forcing would be strong enough to keep a steady stream of cells forming in the same location, one after another (assuming a stationary front). In my mental radar simulator, I see cells forming and weakening to the north right as new cells are reaching maturity to the south due to their access to more unstable air. Maybe congeal isn't the right word. (I can just see southern cells stealing the energy from cells that are finally starting to enjoy that warm front helicity.

However, you can still get a line of discrete north moving tornadic supercells paralleling a north south boundary that doesn't congeal if you have the right shear profiles in the warm sector. 3/28/07 is a good example.

Thanks for the example. I'll look up the parameters on that day.