Tyler Hudson
EF0
I was just curious as to why back veering winds cause HP supercells? Seems all 2015 has been for the most part. And this seems to happen a lot. And how does it affect tornado potential from the storm?
HP Supercells w/ Tornadoes
- Thread starter Tyler Hudson
- Start date
I wouldn't say the veer-back wind pattern causes HP supercells. Supercell morphology is strongly controlled by two factors: anvil-relative flow and upstream seeding.
When storm-relative flow at the anvil level is weak, it leads to precipitation particles falling back down near the updraft, which will lead to an HP mode. With stronger flow, particles are carried farther from the updraft before falling down, thus helping to keep the updraft region relatively free of precipitation.
Even if anvil-relative flow is strong, however, you can still get HP storms if new updrafts/storms form upstream of the current one and spread precip into the updraft of the current one. That's what happened this past Saturday with the southernmost storm in Texas. It was initially a classic supercell (despite somewhat weak anvil-relative flow). But the flanking line appeared to blossom with new updrafts immediately behind the storm. The precip in those new updrafts were blown right into the updraft of the initially classic storm, thus causing it to go HP. It was interesting to watch that storm evolve over the course of a few hours, because you could see it try to go back to classic several times as each new updraft would dump its precip core into the updraft of the main storm, but then die out, leaving time for all the precip to fall out and clear the updraft.
The idea of seeding is one big reason why dryline storms are so desirable. It's pretty rare to get additional development behind the first storms. This past Saturday was an exception because of a general lack of capping.
Regarding the veer-back profile: I think it causes messy storms but not necessarily because of the factors above. Rather, the shear vectors get screwed up. When you have a nicely cyclonically curved hodograph with no kinks or backing in it, you get a gradually veering shear vector as well. This fits textbook supercell structure where you get a nice clean right moving split. When you have the veer-back pattern, the shear vector may initially rotate cyclonically, but then it abruptly shifts to anticyclonic, thus screwing up the dynamic vertical perturbation pressure structure in the storm. Things don't go along with the textbook anymore, and the splitting is messy, weak, or intermittent. You can still get tornadoes from such storms, however, because tornadoes are dependent much more on shear in the lowest 500-1000 m than deep shear.
When storm-relative flow at the anvil level is weak, it leads to precipitation particles falling back down near the updraft, which will lead to an HP mode. With stronger flow, particles are carried farther from the updraft before falling down, thus helping to keep the updraft region relatively free of precipitation.
Even if anvil-relative flow is strong, however, you can still get HP storms if new updrafts/storms form upstream of the current one and spread precip into the updraft of the current one. That's what happened this past Saturday with the southernmost storm in Texas. It was initially a classic supercell (despite somewhat weak anvil-relative flow). But the flanking line appeared to blossom with new updrafts immediately behind the storm. The precip in those new updrafts were blown right into the updraft of the initially classic storm, thus causing it to go HP. It was interesting to watch that storm evolve over the course of a few hours, because you could see it try to go back to classic several times as each new updraft would dump its precip core into the updraft of the main storm, but then die out, leaving time for all the precip to fall out and clear the updraft.
The idea of seeding is one big reason why dryline storms are so desirable. It's pretty rare to get additional development behind the first storms. This past Saturday was an exception because of a general lack of capping.
Regarding the veer-back profile: I think it causes messy storms but not necessarily because of the factors above. Rather, the shear vectors get screwed up. When you have a nicely cyclonically curved hodograph with no kinks or backing in it, you get a gradually veering shear vector as well. This fits textbook supercell structure where you get a nice clean right moving split. When you have the veer-back pattern, the shear vector may initially rotate cyclonically, but then it abruptly shifts to anticyclonic, thus screwing up the dynamic vertical perturbation pressure structure in the storm. Things don't go along with the textbook anymore, and the splitting is messy, weak, or intermittent. You can still get tornadoes from such storms, however, because tornadoes are dependent much more on shear in the lowest 500-1000 m than deep shear.
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Steve Miller TX
EF3
The only thing I would like to add to Jeff Duda's excellent explanation is an important consideration of storm-relative flow at all levels. I was on the Saturday storm that produced the tornado near Cisco and watched it from birth. The updraft region with violently rotating wall cloud was nearly stationary until the tornado really cranked up. This increased the storm-relative winds in the mid and upper levels. Up until that point, it was a classic as the precip was stretched out and dispersed well downstream. As best as I can determine with sounding data, the 300-250mb winds were pretty weak and my guesstimate is around 40-50 knots and 500mb winds 35-40 knots.
http://www.spc.noaa.gov/cgi-bin-spc...2=00&align=V&Levels=500&Levels=300&Levels=250
Once the storm went tornadic, it began to move about 25 mph if my memory serves correctly. That eastward motion significantly reduced the storm-relative winds significantly and thus the updraft was moving into it's own precip core...basically.
I've seen many instance of weak mid and upper level winds but a stationary of even strongly deviant moving/propagating storm enjoy favorable storm-relative winds and go nuts while other storms around it are mushy HPs. My favorite instance was years ago in the Texas PH near Pampa. Mid level winds were only 10-15 knots from the west! But, with strong instability, an OFB and dryline, the storm moved S and even SSW against that mid level flow and ended up with minimal storm-relative flow and produced tornadoes as well as one incredible merry-go-round mesocyclone. It was an HP, but definitely one helluva storm.
Another example was the Lake Whitney tornado a few years ago...and quite an impressive tornado/waterspout it was. Another case of strong/extreme CAPE, boundary, and strongly deviant moving/propagating storm with weak mid and upper level winds. I remember this one because I missed the incredible tornado by 10 minutes. Ugh.
Another more extreme and rare example was the Jarrell, TX event in 1997. Extreme CAPE, a nice boundary, but only ~15-20 knots mid level westerlies. Because that storm moved/propagated SSW and even SW, the storm-relative winds were ideal. Here is an excellent case study from a name you might recognize from some SPC products.
http://www.spc.noaa.gov/publications/corfidi/jarrell.htm
Like Jeff mentioned above, the continued storm development behind the Cisco beast had a great deal to do with it being HP because of the seeding which I also contend reduces updraft velocities. This has happened ALOT in 2015. I agree that was the main reason. I hung back to the SW and watched this occur repeatedly as I hoped something would anchor back along the dryline and fresh OFB. It tried, but couldn't quite do it. With the storm-relative winds also coming into play, it ended up being quite the HP bowling ball so to speak. Another factor to weigh is precipitable water values in the atmosphere which I am learning to appreciate more in 2015. They have been rather high this year resulting in heavier precip-producing cells.
That's my $0.02 anyway.
http://www.spc.noaa.gov/cgi-bin-spc...2=00&align=V&Levels=500&Levels=300&Levels=250
Once the storm went tornadic, it began to move about 25 mph if my memory serves correctly. That eastward motion significantly reduced the storm-relative winds significantly and thus the updraft was moving into it's own precip core...basically.
I've seen many instance of weak mid and upper level winds but a stationary of even strongly deviant moving/propagating storm enjoy favorable storm-relative winds and go nuts while other storms around it are mushy HPs. My favorite instance was years ago in the Texas PH near Pampa. Mid level winds were only 10-15 knots from the west! But, with strong instability, an OFB and dryline, the storm moved S and even SSW against that mid level flow and ended up with minimal storm-relative flow and produced tornadoes as well as one incredible merry-go-round mesocyclone. It was an HP, but definitely one helluva storm.
Another example was the Lake Whitney tornado a few years ago...and quite an impressive tornado/waterspout it was. Another case of strong/extreme CAPE, boundary, and strongly deviant moving/propagating storm with weak mid and upper level winds. I remember this one because I missed the incredible tornado by 10 minutes. Ugh.
Another more extreme and rare example was the Jarrell, TX event in 1997. Extreme CAPE, a nice boundary, but only ~15-20 knots mid level westerlies. Because that storm moved/propagated SSW and even SW, the storm-relative winds were ideal. Here is an excellent case study from a name you might recognize from some SPC products.
http://www.spc.noaa.gov/publications/corfidi/jarrell.htm
Like Jeff mentioned above, the continued storm development behind the Cisco beast had a great deal to do with it being HP because of the seeding which I also contend reduces updraft velocities. This has happened ALOT in 2015. I agree that was the main reason. I hung back to the SW and watched this occur repeatedly as I hoped something would anchor back along the dryline and fresh OFB. It tried, but couldn't quite do it. With the storm-relative winds also coming into play, it ended up being quite the HP bowling ball so to speak. Another factor to weigh is precipitable water values in the atmosphere which I am learning to appreciate more in 2015.
They have been rather high this year resulting in heavier precip-producing cells.That's my $0.02 anyway.
Royce Sheibal
EF3
Jeff is a beast, nailed it. A couple of other factors to consider if a storm will be HP or not: Precipitable Water Value: IF PW's are close to or above 1.5, even with good venting you're likely to get an HP. There is just too much water, too much RFD for a classic cell. The other factor is location. Nebraska is infamous for being HP-heavy and still producing tornadoes (often rain wrapped). Last year we chased the mother's day tornado from Beaver Creek all the way to Omaha, and of the hundred chasers out there I think maybe two of us got decent photos (not me). The rest of us were starting at the HP wall of doom rain wrapping a 1.5 mile wide tornado. This sort of scenario happens quite often around here, most likely due to Nebraska's proximity to moisture convergence long the warm front which really can cause PW's to get crazy. As for shear, veer back scenarios often are coincident with poor upper level flow due to non-ideal positioning of the surface and upper lows, which may be why you see HP's often associated with them.
Tyler Hudson
EF0
Thank you all for the replies! A lot of it is over my head but I'm here to learn, so I will do my research!!
Paul Knightley
EF5
Tyler - your reply is something which is a joy to read! Learning is a great part of chasing and all of us can learn more. But it's great to hear that someone who is fairly new to it all is prepared to put in the leg work. I sometimes think it seems that those who are somewhat newer to this all don't put in the groundwork - I'm sure that's not the case overall but it sometimes can seem that way! Keep up the good work!
