Questions on High Precipitation Supercells?

[Jason Harris]

Although so many people here do their chasing out on the plains with the classic supercells since HP Supercells seem to be especially challenging and yet rather common for people in the Midwest and South and pose some of the greatest dangers of chasing, I thought a thread with that focus could be useful.
Also, since Shane Adams today mentioned in the Non-supercell winds threat the following, it seemed like a relevant transition to set up a HP Supercell thread:
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High-precipitation supercells (HPs) often produce tornadoes in the FFD region, but getting close enough to feel the effects is dangerous and not highly-sought after; the viewable window for HP FFD tornadoes is small to begin with, and as Gene mentioned in an earlier post, some of the largest hail you can find will occur within this region, making it a dangerous place to try and get close to.
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So, to start with, I'd like to ask four questions:

1. Any particular visual aspects for identifying a HP supercell? (over various distances and parts of the storm--from 30 miles away to right underneath)
Any pics to clarify points would be very much appreciated.
2. Tips for chasing one?
3. Radar distinctions? Kidney-bean shape yes?
4. Reliability and variability of where the tornado(s) will be?

 

[Jason Harris]

And I'm copying Gene Moore's post here about approaching a HP supercell from the "approaching a supercell" thread--just to try to consolidate, and it's obviously relevant:

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One more thing about staying ahead of the storm. In HP (heavy precip) supercells as mentioned earlier, you don't want to get behind and then there's flooding, not to mention the view of the wall cloud or tornado will get cut off. Problem is, just getting out in front of an HP may lead to other problems. That is, rain out of the anvil canopy, or as some call it the FFD (forward flank downdraft)....but to me it's the storm anvil and it's way up there. For that reason there is considerable light getting in under that part of the storm, so from a distance it looks good. That said, be careful to watch for long streamers coming out of the anvil downstream. It is likely rain, but in a strong supercell it may be (very) large hail. In fact some of the largest hail will be released in this area downstream from the main precip core. Some chasers like to be in giant hail, you may not at this point in your chase career. Additionally, even light rain from the anvil will sooner or later begin to spit out lightning. So, you have yet a few more reasons why most chasers dislike HP chasing. There is but a narrow corridor to the SE where the visibility is good and the lightning is minimal.

 

[Jason Harris]

By the way, I can delete this later, but I wanted to sneak in this pic of part of what seemed to be a HP supercell that produced the tornado first near Charlotte, MI and then a little one in Lansing, MI on 8-27-07. This storm has now passed to the East and I didn't get to see anything but a lot of rain and lightning, but looking East are there any interesting features here? Or could this be any garden-variety thunderstorm at this point?

 

[Shane Adams]

Any particular visual aspects for identifying a HP supercell? (over various distances and parts of the storm--from 30 miles away to right underneath)

I'm not great at pinpointing exact supercell characteristics by schematics, but generally HP storms will have, obviously, a large precipitation core, often-times flanked on the front (or inflow) side of the storm by a second area of precipitation. They tend to be more linear in appearance over time, as displayed by the typical 'kidney bean" shape on base reflectivity. It's basically a supercell that you cannot see very well beneath because of all the rain. The closer you get, the less you're gonna see, unless you find that window near the updraft that is always closing as the saturated RFD wraps around the circulation. Believe me, once you chase a few you'll know them when you see them because you will learn to hate them LOL.

Tips for chasing one?

My basic rule is keep to the northern end of HPs. I don't understand the whys and hows so much, but I have learned through experience that these "FFD" tornadoes are fairly common, and the best place to see them (from what I've observed) is to be towards the northern end of the storm. It's hard to explain the thought processes I go through during a chase, but when the storm gets that "HP" look to it, I naturally gravitate to the north. This happened to us on May 22, 2007 in Graham county Kansas. We were setting up on the storm in "classic" style when we realized it was becoming more HPish, so we busted north. An area of rotation developed northwest of us, well north of where we had been before we went into "HP mode". A tornado developed, and we had a great view of it during the entire lifecycle, a far better view than we would've had from the southern end of the storm (where we were initially sitting). It doesn't always work like that, but it's an example of how it can. You have a greater margin for error with classics and LPs, but HPs you need to be out in front of them. Once you commit to an upclose encounter, you pretty much get what you get and then the show's over. HPs will drown you with heavy rain and they tend to pick up speed and curve to the right the longer they live.

Radar distinctions? Kidney-bean shape yes?

Yeah, definitely. They tend to appear more smooth or round than the classic variety, and their hooks aren't always so obvious; sometimes the tornadic area in an HP will appear more like a subtle, curved indention than a classic "fish hook" or "number six" shape. If you're running radar and are near an HP, it would be best to be on velocity mode so you can "see" what you can't see with your eyes.

Reliability and variability of where the tornado(s) will be?

They can be in the classic area on the backside (as with classic supercells), or they can be on the front side (FFD area). They can be as predictable/unpredictable with HPs as they are with the other varieties of supercell storms.

It's important to note that supercell status/type is always in a state of flux (barring the occasional long-lived, "steady state" LP); what might be LP could become classic and what is classic could turn HP. Also, many times during a cyclic, classic supercell, the "cycling down" phase of the storm may cause it to briefly take on HP-like characteristics, as the RFD crashes in and wraps the precip core around the circulation. Don't be lulled into thinking the storm has turned to mush and the day is done...these storms can cycle back quickly and be producing another tornado near you or even behind you if you lose track of everything happening in the near-storm environment.

 

[Gene Moore]

It's important to note that supercell status/type is always in a state of flux (barring the occasional long-lived, "steady state" LP); what might be LP could become classic and what is classic could turn HP. Also, many times during a cyclic, classic supercell, the "cycling down" phase of the storm may cause it to briefly take on HP-like characteristics, as the RFD crashes in and wraps the precip core around the circulation.

Bouncing off Shane's good comments about the changing state of supercells: A classic supercell that's developing into an HP is difficult to see coming. Many watch the updraft area near the flanking line/ meso. This works as the mesocyclone occludes (wraps up) and rotates around the storm to the NE. There is another way storms become HP and you can catch it in advance by watching the FFD, or the precip wall east of the meso/ wall cloud region. For example, you're ESE of the main updraft watching a tornado. During this time it's difficult to look at anything else...after all it's why you made the trip. If possible keep a casual eye straight north. You should see a very well defined line across the sky. This is the updraft/downdraft or high shear region of the supercell. As a storm prepares to turn left this area begins to bulge out and slowly sag to the south. In some cases where you may be close to the mesocyclone it may sag south to your east. This puts you in the radar notch. Supercells don't like to be notched, and they will try to fill it in by rotating precip...rain/ hail into the clearing. So, if you're in there if can be fun, but your time is limited. Most often the storm will force you out of the pocket to the south as the FFD sags south. It's this sagging FFD where the big hail is generally located. This is the downstream fall out region from the storm's meso updraft. It's likely in this area you'll see a transition from no precip to large hail almost immediately as the leading edge gets close. Best to watch this region for a couple of reasons, first video taping in blowing rain and hail produces ugly footage, second you'll likely lose sight of the tornado just to your west (not good) and finally if there is not exit south you could get trapped in the worst of a no visibility core. If you watch the motion of the FFD it's likely you can stay a step ahead of the storm. Remember, your eyes are at least 6 minutes faster than the radar, probably better like 10 minutes.

 

[Shane Adams]

As a storm prepares to turn left this area begins to bulge out and slowly sag to the south. In some cases where you may be close to the mesocyclone it may sag south to your east. This puts you in the radar notch. Supercells don't like to be notched, and they will try to fill it in by rotating precip...rain/ hail into the clearing. So, if you're in there if can be fun, but your time is limited. Most often the storm will force you out of the pocket to the south as the FFD sags south. It's this sagging FFD where the big hail is generally located. This is the downstream fall out region from the storm's meso updraft. It's likely in this area you'll see a transition from no precip to large hail almost immediately as the leading edge gets close. Best to watch this region for a couple of reasons, first video taping in blowing rain and hail produces ugly footage, second you'll likely lose sight of the tornado just to your west (not good) and finally if there is not exit south you could get trapped in the worst of a no visibility core.

I can't be positive, but I've read that quote about six times in a row trying to visualize this in the real world. After thinking about it, this sounds very similar to what happened to several chasers on May 12, 2005. That day there was both a south and an east escape option; however downed power lines blocked the south route and a new circulation/tornado blocked the east route (even as we were getting pounded by hail, other chasers 1/4 to 1/2 mile east of us were getting absolutely clobbered with the biggest stones). But if memory serves, a lot that transpired that day after the South Plains tornado developed seemed similar to the scenario Gene described above.