Cut-off Lows vs. Open Troughs

[Brian Press]

Hello all!

Along with most of you, I have been pouring over the model data religiously trying to figure out when to make the trip out there. Coming from California, tickets are not too bad, and I have the flexibility (owning my own business) to come out when it gets good. Last year, I feel like I kept waiting and waiting for a good setup and never made it out and missed all the high plains stuff, and this year I told myself, just go when there is moisture and a front! So, I am heading out next week (Long intro to my question..)

So, my question is, why does it seem like the cut-off lows tend to not be the best for chasing? Surely they suck up plenty of juice from the GOM, but as I have been observing over their years, and seems like the storms tend to line out quickly and have a have a strong cold front (like today)? Not the classic dry line? Just wanted to get some insight on these cut-offs. Thank you!!
Brian

 

[Marshall Stoner]

I'm not sure what you mean by cut-off. The problem today was a lack of adequate deep layer shear for sustained supercells.

 

[Brian Press]

Thanks Marshall, I was referring to a Cut off low pressure system, like what came through on Tuesday. Any other insight would be great!

 

[Paul Knightley]

Every system is different - but what you really need for a 'good' chase set-up is a reasonable SW'erly upper flow over the Plains, in order to bring an EML across to give some (but not too much!) capping. This will prevent numerous storms forming.

Cut-off lows tend to 'wander' slowly across the area - at first, they will provide such a SW'erly flow aloft, but as they get closer the flow may weaken, or back more to the south - or they pull in sub-tropical moisture and create areas of mid-upper cloud, all of which will affect the ability for discrete supercell development. Additionally, they will tend to slowly fill and weaken - the shear will reduce. Also, as they weaken the low-level flow may become weaker, with less decent moisture streaming in - and plentiful convection will reduce low-level theta-e via overturning, also making prospects poorer.

The ideal set-up for a chase period is a long wave trough to the west with shortwaves quickly moving NE over the Plains - or a succession of troughs moving in on a strong jet. This create multi-day chase episodes with a couple of down days in between systems. The dryline will be active - which is less common as a slow-moving upper low moves into the Plains.

 

[Jeff House]

Paul covers it very well.

Note he's talking about upper level winds backing to the south, instead of the preferred SW. Straight south upstairs is messy.

This is not to be confused with 850 mb and below, where more backing is better. Basically a chaser wants gradual turning with height, going from southeast low levels (at least SSE) to southwest upstairs (WSW better).

The favorable hodograph is a smooth sickle shape. Also called veering with height (good), not to be confused with veered off at the surface (bad).

Trouble with cut-off or closed lows, the hodograph can look like a plate of spaghetti instead.

 

[Joshua Horenstein]

Great discussion; could you folks add about negative vs neutral vs positive tilting; I know classically we want negative but why, and then I seem to hear complaints of Veer Back Veer with these, or am I conflating two issues.

 

[Dustin Knight]

A number of factors are at play in regard to negative vs positive tilt vs neutral troughs so I'm not putting them in any order in terms of priority or anything like that. A big factor is the temperature advection regime associated with each. With a "textbook" negatively-tilted trough you'll have a surface-low somewhere near the inflection between the trough axis and downstream ridge axis. Because of this you often see colder air in the upper troposphere being transported atop a warmer, more moist boundary-layer which results in positive buoyancy. That's not to say you can't get this with a positively tilted trough as we've probably all chased those setups too, but generally speaking a negative-tilt trough favors that process.
You mentioned the wind profiles and this is also a factor. Basically flow aloft is geostrophic while the flow in the boundary-layer is impacted by friction, this ends up producing a wind profile comprised of wind veering from S/SE in the low level to more westerly aloft, which often supports the development and organization of severe storms.
Just to add to this, in simple terms, there are also effects that the vorticity advection associated with each type of trough will have as well as the processes related to speed maxima embedded within the flow aloft but I don't want to go overboard with all of that here. Basically though, you can get divergence aloft which will lead to pressure falls at the lower level, which leads to a deepening low assuming mass continues to be removed faster than it's being replaced, and this can produce the strong surface-lows you like to see with deep moisture return and strong, backed low-level flow. Put simply, negative-tilt troughs are pretty good at doing that and in an orientation that often supports severe weather. Positive and neutral troughs can and do result in severe storms, it's just a matter of negative troughs being a bit more consistently effective at assembling severe storm ingredients. I would never write off a setup just because I don't see that classic, deep trough though, there are plenty of ways for the atmosphere to put together a favorable temporal and spatial overlap of ingredients supportive of severe storms.

I tried hard to hit the main points without getting too complex or going into our favorite equations (QG, Omega, Height-Tendency) and all that. I know this is the advanced forum though so if you would better understand this if I did go into the more technical stuff I don't mind to. I didn't get into lapse rate tendency stuff or any of that either so anyone can absolutely elaborate on what I said if they'd like. I just wanted to hit the basics to answer the question first.

 

[Jeff Duda]

Great discussion; could you folks add about negative vs neutral vs positive tilting; I know classically we want negative but why, and then I seem to hear complaints of Veer Back Veer with these, or am I conflating two issues.

Really the only reason negatively tilted waves tend to result in better severe performance is because the lift associated with them tends to sit right over the surface forcing mechanism (in the Plains, the dryline) rather than lagging way behind the fronts, which is more common with positively tilted troughs. Thus negatively tilted troughs are more commonly associated with a superposition of favorable upper-level dynamics and surface based forcing compared to positive troughs.

I still long to chase open waves. They're so uncommon, but when they occur the surface pressure pattern over the Plains can look so awesome (basically perfectly circular isobars and you can get damn near due east winds in the warm sector, thus making for incredible shear profiles).

 

[Joshua Horenstein]

Thanks guys, that's really helpful!

 

[Marshall Stoner]

Okay. I understand. I think it's fairly common to see a closed 500mb isobar as systems cross the rockies in spring and summer. Not all such systems are bad though as they can still produce good shear profiles on their southeastern fringe as they emerge onto the plains. The problem is lows that reach a peak depth early then begin to fill, as they become vertically stacked in the process. In this case the shear becomes unidirectional in theory because there's no more warm air adviction. However, immediately east of the rockies there's almost always some veering with height below 3 km just because of the terrain. When a low closes off you typically lose most of the veering above 3 km which hurts storm-relative inflow of horizontal vorticity (think of the horizontal rotating tube created by the 0-3km shear) by decreasing rightward storm movement. Sometimes you can still get a good setup on the periphery of a trough that has a closed 500mb isobar though, because there's a more subtle open wave flowing around the cutoff, increasing the veering aloft in it's vicinity.

 

[Brian Press]

Thanks guys! This is great!! Very helpful info @Dustin Knight! Really appreciate the effort! All of the things you talked about makes all the sense. @Jeff Duda, I have seen many times those neg. tilt troughs and how they form those super tight crazy surface lows. I have always know those are the best set ups, but like you mentioned, we do not see that too often. Thanks again!