2015-08-22 EVENT: MN/ND/SD/NE/IA

[Mike Marz]

A fairly potent trough for August standards is forecast to move through the northern plains this Saturday and it should bring decent severe wx chances with it. The models all seem to agree that a 60 to possibly 80 kt 500mb jet streak should crash through the warm sector satruday afternoon/evening. One concern that I have is that the strongest shear may lag behind the cold frong and not make it into the warm sector in time. The 850s look to be screaming out of the south between 40-50 kts all afternoon and the models that I have looked at have consistenly been forecasting mid 60s to around 70 degree dew points ahead of the cold front which should allow for 2000-3000 j/kg of cape to form at peak heating. The cap looks quite strong until about 21z but it shouldn't be an issue in terms of storms initiating along the front; hopefully it actually helps in the first few hours of initiation to keep things a little more discrete. I think that if we can get a few storms to stay discrete before this entire thing lines out and blasts east, there is a decent tornado threat. This setup reminds me a bit of July 11th, 2008 which produced a nice tornado near Willmar, MN. My target as of now would be near Morris, MN. Any other thoughts on this day?

 

[Jeff Duda]

Based solely on the shape of the 500-mb trough, this event is a little disappointing with the displacement between the shear and instability. The parent trough is neutral to slightly negatively tilted during this portion of the event. However, it is also occluding as the event is getting started, and I think when you couple that fact with the positive tilt the trough has when it is really starting to dig back across BC/AL, you get the nature of this event. The result is you get veering winds well ahead of the height minimum since the cold front becomes so well-established.

From what I've seen so far, the best combination of shear and instability may be across W IA into E/SE NE and extreme NE KS and NW MO. Nearly 5000 SBCAPE looks to be likely, but still enough forcing and not too much capping to completely inhibit convection. More veered mid-level flow also looks to enhance shear a bit in this region. I may have to make a very unusual late-August trip up to the KS-NE border.

 

[JustinPhillips]

Setup kinda looks like it may go MCS fairly quickly across Minnesota, could be robust though. Planning to chase since its late August and time is running down here in Wis/Minn. Tornado potential looks decent if the supercell window lasts awhile, dont think it will though, pretty strong deep layer forcing usually translates to quick upscale growth. Does look like a fun day though if your not setting real high expectations.

 

[Mike Marz]

Based on the latest model runs I am thinking of targeting somewhere in southwest or south central Minnesota along I90 and arriving early in the afternoon. This area is a bullseye for the 12z NAM's 0-1 EHI and sig tor parameters so it is not really going out on a limb to target it. I love the terrain in this area, as well as the roads, so it is pretty much a no brainer. The stronger 500s should be able to clip this area between 4-6pm and the low level winds look alright. I feel like this will be a classic cold front situation where the entire front erupts starting north of I94 and continutes to develop SW through the evening. Probably going to need any supercells that are able to form to produce within an hour or two of initiation before everything lines out...I am not overy enthusiastic about the tornado chances tomorrow, but there is definitely some potential. Should be a fun local chase either way!

 

[JustinPhillips]

Yeah mostly chasing tomorrow because it looks like a good structure day/cool shelf day. Think the tornado window is too short to get really amped about the tor potential but you never know. That strong deep layer forcing screams quick upscale growth, but again you never know and there probly isnt many more chances here in Minn/Wis this year.

 

[Ben Holcomb]

Having been practicing my geekery all week in Seattle, I haven't been paying attention at all. @Jeff Duda brought up to me the severe chances tomorrow, and I dove in and took a look.

The trough looks pretty strong for any time of year. Looking at water vapor this afternoon, the trough is crashing into the pacific northwest and already showing a negative tilt. 12Z soundings from today show a pretty stout EML across the central plains today. I would say the storms in NW KS have shown some pretty strong pulse activity supporting this, although they don't have the shear to keep them balanced, and they are spitting out cold air and choking themselves off. Tomorrow is a different story in SE NE with that potent trough cutting across the plains.

I will admit, I still am not super sold on the storm slinky part of SHARPpy (I don't understand it/have a lot of experience with it) but that is my main concern tomorrow, with the slinky barely coming off storm motion. That would lead me to believe linear modes even in Nebraska where forcing is a little less.

I've also learned that cold air always wins, and that is definitely a concern with the cold front racing ahead of the best support. WIth that said, it's still late August and hopefully cold air doesn't win as fast, as that could open up a pretty good window for tornadic storms.

So my gamble is whether or not I head to Dallas to see Jim Norton (comedian) perform or take a hope and a prayer to NE to see a very slight chance.

Will be reviewing 00Z data to make a final decision.

 

[Andy Berrington]

My overall thought tomorrow is that there is going to be some sort of window maybe around 21z where you'll get discrete/semi-discrete storms firing off the pre-frontal wind shift and within a strong low level shear environment. This window should be pretty short and upscale growth should take place rather quickly given increasing ascent approaching from the west and less than ideal upper level winds (some backing going on and in other areas they are quite weak), but this initial period may be fairly productive tornado wise assuming a few dominant storms can establish themselves. Low level shear profiles tomorrow are definitely in the upper percentile as far as summer is concerned with a near 50 kt low level jet punching into IA and MN. You just don't see that every year.

 

[Royce Sheibal]

I really don't know what to think about today. My chasing driving isn't available, and my wife would kill me if I took my car. Extreme NE Neb is looking really good, nice moisture convergence ahead of the front, good Theta-E advection. Models are all over the place, but radar VAD's show decent shear in the lowest 3KM, temps are near 85, and DP's are near 75 in some areas due to evap. from that 5-10 inch rain we had a few days back. GFS is way too slow today so it's wrong. HRRR isn't even remotely close, which is odd. NAM 4kM shows a line of mega-deth gorilla hailers trying to form from SW MN down into E Neb by 6pm. Probably the most likely scenario. For tornadoes today I really like central MN, cuz of dynamics, or extreme NE NEB where you might get storm motions close enough the curve of the front that you won't get cold pooled if you are on the tail end charley.

 

[David Williams]

Like Royce said, I noticed the HRRR was breaking precip out in west KS and TX panhandle and nothing in MN, IA, and NE. Bad sign. Looking at the 4K NAM an hour or two ago, it had a line of storms starting in nw IA and by the looks of things on radar right now, it hit the nail on the head. Cells should be exploding all down the line.