Storm Mode

[JamesCaruso]

In the thread on Hook Slicing, Rob H. wrote the following in reference to El Reno 5/31/13:

"SR anvil winds of 45kt and 1.7 pwats suggested that the El Reno environment was likely to be more on the HP side of things. "

This got me thinking that after a long time chasing, I really still don't have a good handle on what storm mode to expect. I would have thought that upper level winds of 45kt were sufficient for classic supercells, but I am speculating that the threshold probably changes based on the pwats, i.e. the higher the pwats the stronger the upper winds need to be to avoid HP mode?? I imagine it is more about the mix and balance of different parameters, as opposed to absolute thresholds required for each individual parameter? Guidance from those in the know will be appreciated!

Jim

 

[Shane Adams]

I imagine it is more about the mix and balance of different parameters, as opposed to absolute thresholds required for each individual parameter?

Pretty much that right there. I have always simplified things. I don't really look at anvil level flow speeds so much as those speeds relative to mid level flow. If I've got decent mid level flow but my anvil level winds are barely stronger, or even the same, I know to expect some degree of self-seeding from storms. Even if you've got 50kts at h5, h25 winds of the same speed are not going to vent properly for that degree of mid level flow. Regardless of each level's actual speed, you want a "stair step" of increasing speeds the further up you go, with the possible exception being a super-strong LLJ that might be as much or even stronger than your h7-on up flow.

Also, if you're looking at a storm you're trying to approach on radar, the tell-tale sign of great anvil level flow is a green "smear" type display, indicative of powerful upper flow that's adequately blowing the top of the storm downstream from the updraft. HPish type storms will tend to be more "blobby" on radar, as their weak upper flow keeps the updraft/downdraft interface more closely attached, thus keeping the precip "blow off" very limited or nil.

 

[Rob H]

I usually look at three things: precipitable water, storm-relative anvil winds, and instability.

The Anvil Level SR (Storm Relative) winds and SR winds from 9-11-km are meant to discriminate supercell type. In general, upper-level SR winds less than 40 knots correspond to "high precipitation" supercells, 40-60 knots SR winds denote "classic" supercells, while SR winds greater than 60 knots correspond to "low precipitation" supercells.

It's important to note the "storm relative" part of that. 60kt 500s with storm motion of 60kt in the same direction doesn't really help you - precip goes up, then falls right in the path of the updraft.

0.50 inches or less = very low moisture content
0.50 to 1.25 inches = low moisture content
1.25 to 1.75 inches = moderate moisture content
1.75 to 2.00 inches = high moisture content
2.00 inches or above = very high moisture content

[...]Severe Storms: Tropospheric moisture determines whether a supercell storm will be HP (High Precipitation), Classic, or LP (Low Precipitation). HP supercells often have PW of 1.75 inches or above and LP supercells often have a PW value of less than 1 inch.

You're absolutely right, like many things with forecasting, the mix is what's important. Also, from personal experience chasing in 75° dewpoints in the upper midwest, it seems that you need at least 3000 MLCAPE to keep updrafts in HP supercells from being outflowy. 3500-4000 MLCAPE seems to be the sweet spot for tornadoes in SD/ND/MN in these extremely moist summer environments.

Here's a good paper that ties a lot of previous research together: http://journals.ametsoc.org/doi/pdf/10.1175/1520-0493(1998)126<2406:VISMPI>2.0.CO;2

I try to get an idea of what to expect, and then I look at the things Shane mentioned to see how things are actually panning out.

 

[JamesCaruso]

Thanks Shane and Rob.

Shane, as a follow-up to your points on relative wind speeds at different levels: oftentimes the 700mb flow will be relatively weak, even in otherwise pretty good setups. What are your thoughts on how much this influences storm mode? Are we still in good shape as long as 500mb and 250mb are progressively stronger?

Jim

 

[Paul Knightley]

Weak mid-level winds can also be an indicator of storms on the HP side. Experience from a number of sources probably suggests that storms are more often towards the HP end of the spectrum, but then again, it depends on the area you're in somewhat.

 

[Warren Faidley]

This link might help:

http://www.stormeyes.org/tornado/stmtype.htm

W.

 

[Rob H]

It should be noted that some of the values on that page are derived from the environment, and are not necessarily factors having to do with precipitation efficiency. LCLs for example, tend to be lower with HP storms because they're usually in lazy summer zonal environments with an extremely moist atmosphere.

To answer an earlier question, while looking back to see if anything new has surfaced since Rasmussen/Straka (there hasn't really), I also came across the conclusion in their research that anvil level winds have a much better correlation with storm mode than mid-level winds. I'm guessing this is because the most efficient way to vent a storm is at the top of the updraft.

 

[Jeff Duda]

Thanks Shane and Rob.

Shane, as a follow-up to your points on relative wind speeds at different levels: oftentimes the 700mb flow will be relatively weak, even in otherwise pretty good setups. What are your thoughts on how much this influences storm mode? Are we still in good shape as long as 500mb and 250mb are progressively stronger?

Jim

Haven't seen official research on this, but in my experience, a weakness in the flow between 850 and 500 mb usually makes things messy. You can still get supercells, but they tend to be more transient or can become disorganized quickly without giving much warning. Usually pretty frustrating to chase storms in that environment because individual storms can persist for a long time without ever really giving a show.

ADD: Best example I can think of right now is April 17, 2013 in SW OK. There were many supercells that day, but they fluctuated between HP and classic and had a real tough time producing tornadoes despite looking pretty good visually and at times having low-level mesos.

 

[Shane Adams]

oftentimes the 700mb flow will be relatively weak, even in otherwise pretty good setups. What are your thoughts on how much this influences storm mode? Are we still in good shape as long as 500mb and 250mb are progressively stronger?

Sorry it took so long to to reply, I just haven't looked back in for a few days. Anyways

Not always. As another poster has mentioned, weak areas in lower level flow, even in otherwise decent supercell wind fields, can create situations where storm mode is adversely influenced. If the overall directional shear is there AND strong instability is realized, weak flow at h7 is less of an issue. In situations where the directional flow is there but the instability is "okay" it can become more of a problem. However, if I could have a decent sup setup but be forced to pick one level that would have to be lacking, I'd settle for weaker flow at h7. Obviously you need the decent h5/h25 flow to be "stepped" for venting to create sustainable updrafts, and you want a decent LLJ (h85) to help create rotation near the ground (especially if you're chasing tornadoes as opposed to just storms). If the LLJ/h5 flow is strong enough (i.e., the LLJ speed almost matches h5 speed), it can help compensate for weaker h7 flow in some cases.

In summary, I basically use instability as the measure of how much the atmosphere can get away with on days with good directional flow but marginal speeds. Strong/extreme CAPE can greatly ad to vertical acceleration in moderate/weak directional flow fields, compensating for the lack of horizontal speed, which when stretched by an intense, extreme instability-driven updraft, can create mesocylones/tornado cyclones the same way a textbook directional/speed shear type of wind field does.

 

[JamesCaruso]

Thanks all, very helpful as always!