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November 11th-12th storm system


Thundersnow12

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Kinda goes without saying but it's too early to call on things like will there be enough instability etc as you allude to. Pattern recognition wise, if something as strong as some of these operational runs are showing occurs, I'd be surprised if it's not accompanied by a fairly robust severe threat. The non t-storm wind threat looks impressive from this vantage point. Overall think a cautious approach is best for a few days and look at the op runs as part of the ensemble envelope. Didn't get a chance to look at individuals on the 12z GEFS, but there had to have been a decent amount of weaker/less organized solutions based off the mean. Hopefully it remains something fun to track up to go time.

 

Well said and good advice.

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The CIPS is showing an outbreak

 

That's not how analog guidance works. It's merely showing setups that are somewhat similar, you can have a ton of other complications that mitigate or enhance things (in this case, 500 mb temps are almost certainly going to be a mitigating factor). The likelihood of an outbreak with this is pretty low as of now, despite how impressive the dynamical aspect of the system is, go figure in 2015.

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That's not how analog guidance works. It's merely showing setups that are somewhat similar, you can have a ton of other complications that mitigate or enhance things (in this case, 500 mb temps are almost certainly going to be a mitigating factor). The likelihood of an outbreak with this is pretty low as of now, despite how impressive the dynamical aspect of the system is, go figure in 2015.

 

I should have put outbreak in quotes

 

"outbreak"

 

I know people take too much faith in the analogs.

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Just for fun, pulled the MDW BUFKIT sounding at 9z (3am) Thursday from the 6z GFS. At that time, sustained west-southwest wind of 35 kt (40 mph), 58kt (67 mph) mean mixed layer gust using the momentum transfer tool and top of the mixed layer which is about 865 mb, 69 kt (80 mph). And that's the peak of the winds. 6z and 12z show 52kt/53kt mean mixed layer gusts and 64kt at the top of the mixed layer. Whether this comes close to verifying or not, the most impressive synoptic winds I've seen modeled in this area.

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Impressive wind shear for sure. Looks like great directional shear with the negatively tilted trough. Really don't need that much instability for a system like this is progged to be. The one thing that concerns me is moisture. Someone mentioned above that this system is likely to make up for what it's lacking in other ways and I fully believe that. Octobomb system in 2010 produced strong tornadoes in IL at 6 A.M. driven on forcing and shear alone I believe.

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My initial WAG on the severe threat is that it will be a good bit more impressive than the previous system if it's as amped as modeled...with the caveat that how much instability we ultimately see will determine whether it's just a nice little event or a higher end outbreak.

 

The wind fields along and ahead of the cold front on the GFS are extremely impressive...60-70+ knots at 850mb and 50-60 knots at 925mb, with strong height falls and PVA along the front Wednesday evening combining with very strong surface convergence along the front to provide for much stronger forcing along the front than what we had with this last event. Verbatim, even with the very meager instability shown, I'd have to expect at least a strongly forced squall line with a fair number of wind damage reports and probably at least a few tornadoes. What the weak instability does is limit momentum transfer some and also makes it harder to see any convection other than a forced line along the front, which would ultimately keep the tornado potential lower. It's unfortunate that this weekend's cold front will get almost down to the Gulf Coast.

 

The synoptic wind gusts in the CAA behind the surface low could be very impressive as has been mentioned...50+ knots at 925mb and 60+ knots at 850mb in the CAA behind the surface low with extremely strong pressure rises behind the surface low seems like a slam dunk high wind warning event for a good number of areas.

 

It's obviously still early enough for things to change (although I think the severe threat stays well to my wind and the strongest synoptic winds also stay to my northwest either way)...however the op GFS and Euro have been fairly steady in the idea of a deep system tracking into the Great Lakes for several runs now, and the strong temp gradient over the Plains that the low forms on combined with a 150 knot jet streak ejecting out of the southwest both argue for a fairly deep system. The shortwave moving out of the southwest won't be dinky and sheared out like the last system.

 

 

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Impressive wind shear for sure. Looks like great directional shear with the negatively tilted trough. Really don't need that much instability for a system like this is progged to be. The one thing that concerns me is moisture. Someone mentioned above that this system is likely to make up for what it's lacking in other ways and I fully believe that. Octobomb system in 2010 produced strong tornadoes in IL at 6 A.M. driven on forcing and shear alone I believe.

Yes I just learned this at a work shop that Atmospheric Science students had at Ohio State. All you need is Moisture, Lift, and Instability.  If you have the bare minimum of Instability but it's replaced with high wind shear and high helicity values it can more than make up for. An example of this is the Fort Worth Mayfest storm of 1995. There wasn't much instability, temps were only in the mid 60s, but Helicity values were in the 600-700 range and they received grapefruit sized hail.

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Yes I just learned this at a work shop that Atmospheric Science students had at Ohio State. All you need is Moisture, Lift, and Instability.  If you have the bare minimum of Instability but it's replaced with high wind shear and high helicity values it can more than make up for. An example of this is the Fort Worth Mayfest storm of 1995. There wasn't much instability, temps were only in the mid 60s, but Helicity values were in the 600-700 range and they received grapefruit sized hail.

Ehh if you have weak instability and such strong shear you need some pretty strong forcing to get organized convection, as the strong shear will rip apart updrafts that form from the meager instability...I think this was discussed some in the thread for the last system. Although yes at the end of the day we have seen some decent events with little CAPE when forcing has been strong enough. I'm not sure if you're from near Ohio State, but the Halloween 2013 event from that area is a good local example...even had a couple of tornadoes with that in central Ohio too...although I think the best with this one is mainly west of Ohio if it pans out like the recent GFS and Euro runs show.

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Ehh if you have weak instability and such strong shear you need some pretty strong forcing to get organized convection, as the strong shear will rip apart updrafts that form from the meager instability...I think this was discussed some in the thread for the last system. Although yes at the end of the day we have seen some decent events with little CAPE when forcing has been strong enough. I'm not sure if you're from near Ohio State, but the Halloween 2013 event from that area is a good local example...even had a couple of tornadoes with that in central Ohio too...although I think the best with this one is mainly west of Ohio if it pans out like the recent GFS and Euro runs show.

No, I'm from Texas, we get spared a lot there because when the cap breaks there's usually too much lift and moisture and all we get is rain showers that prevent the sun from coming out. So yeah I get what you mean, the shear can be tricky sometimes.

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Impressive wind shear for sure. Looks like great directional shear with the negatively tilted trough. Really don't need that much instability for a system like this is progged to be. The one thing that concerns me is moisture. Someone mentioned above that this system is likely to make up for what it's lacking in other ways and I fully believe that. Octobomb system in 2010 produced strong tornadoes in IL at 6 A.M. driven on forcing and shear alone I believe.

I wasn't interested in weather at the time so I don't know the specifics of it, such as CAPE... but I was under the impression there was respectable CAPE ahead of the line, early on the 26th, based on ILN's 12z sounding. It showed 1500 MLCAPE... but based on the temp profile and the ascending parcel temp, that doesn't sound right

 

85z1xXf.gif

 

 

Rich Thompson did that SPC tornado forecasting workshop early this spring... I remember he talked about this specific case when talking about convective mode. 

 

He showed some archived mesoanalysis from the event... looks like we had 250 j/kg at best by 15z on the 26th (when the front was about to approach IN/OH border). The only thing that surprises me about that event is that we had a 15% hatched tornado threat with that small CAPE. I know they were expecting discrete supercells to pop ahead of the line... but I wouldn't think that environment would be conducive to tornadic supercells.

 

Direct link to the talk:

https://youtu.be/GboE6X35T9Y?t=2059

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I wasn't interested in weather at the time so I don't know the specifics of it, such as CAPE... but I was under the impression there was respectable CAPE ahead of the line, early on the 26th, based on ILN's 12z sounding. It showed 1500 MLCAPE... but based on the temp profile and the ascending parcel temp, that doesn't sound right

85z1xXf.gif

Rich Thompson did that SPC tornado forecasting workshop early this spring... I remember he talked about this specific case when talking about convective mode.

He showed some archived mesoanalysis from the event... looks like we had 250 j/kg at best by 15z on the 26th (when the front was about to approach IN/OH border). The only thing that surprises me about that event is that we had a 15% hatched tornado threat with that small CAPE. I know they were expecting discrete supercells to pop ahead of the line... but I wouldn't think that environment would be conducive to tornadic supercells.

Direct link to the talk:

https://youtu.be/GboE6X35T9Y?t=2059

Honestly I didn't remember there being that much instability for that event....at least up in the Chicago area early on. I'll have to pull up some more soundings from that date. There could have been, so I'm not discounting it.

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Honestly I didn't remember there being that much instability for that event....at least up in the Chicago area early on. I'll have to pull up some more soundings from that date. There could have been, so I'm not discounting it.

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Yeah I dunno how you get 1500 MLCAPE with dew points near 60 and lapse rates that weak. 

 

GFS is coming in with a 976mb near-bomb... only 2mb short of being a bomb by definitioniC7FhF8.gif

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I'm not sure instability is as much as a limiting factor as many are making it out to be here. Halloween of 2013 hardly reached 100 J/kg along the line yet nearly 350 winds reports still came in. As of right now, this system is more dynamic, deeper, and the GFS already is indicating at least 100 J/kg across the northern potions of the squall line. And that's the GFS...I wouldn't be surprised if the NAM gets us up to 250-500 J/kg which would be legit moderate risk territory. Even then, the system is still 5 whole days out.sbcp_20131101_0300.gifgfsCGP_con_sbcape_108.gif

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I wasn't interested in weather at the time so I don't know the specifics of it, such as CAPE... but I was under the impression there was respectable CAPE ahead of the line, early on the 26th, based on ILN's 12z sounding. It showed 1500 MLCAPE... but based on the temp profile and the ascending parcel temp, that doesn't sound right

 

85z1xXf.gif

 

 

Rich Thompson did that SPC tornado forecasting workshop early this spring... I remember he talked about this specific case when talking about convective mode. 

 

He showed some archived mesoanalysis from the event... looks like we had 250 j/kg at best by 15z on the 26th (when the front was about to approach IN/OH border). The only thing that surprises me about that event is that we had a 15% hatched tornado threat with that small CAPE. I know they were expecting discrete supercells to pop ahead of the line... but I wouldn't think that environment would be conducive to tornadic supercells.

 

Direct link to the talk:

https://youtu.be/GboE6X35T9Y?t=2059

 

 

 

I remember the models progging a zone of like 500-750 J/kg MLCAPE back then.  That MLCAPE of 1500 on that ILN sounding is surprising.

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