6/25/10 FCST: SD,NE,IA,MN

[Derek Weston]

Interesting, though tough to nail down setup for tomorrow.

Focusing on S MN into N IA and nearby areas specifically. (given that this is the most favorably sheered region)

Latest NAM projects healthy CAPE of 2,000 J/kg to 3000 J/kg + through out the western 2/3 of S MN/N IA.

Cap looks to be mostly eroded by 0z with managable 700temps.

Nice 500mb jet streak across S central MN will give us plenty of deep layer shear. 850 flow is decent enough by 0z (if slightly veered SW) through out this region. Good enough hodographs for supercells and tornadoes.

The main issue with this setup had been getting our upper level support juxtaposed over the better lower level shear. Latest runs have come around to a more favorable solution where this is concerned.

Complicating factor here is possible ongoing convection from earlier in the day near our target area. Also not a huge fan of the paltry surface winds we're likely to be dealing with. That said... turning with height and CAPE are present.

 

[Jeff Duda]

IF the cap can be broken (with strong forcing from a cold front, there's a good chance that'll happen), then it looks like there will be sufficient shear for supercells and tornadoes. The caveat, however, is that the forcing is a cold front...which is likely to promote linear storm modes. BUT, I see that shear vectors are going to be WNW to NWly, which would make them have a good component perpendicular to the boundary, which could promote discrete storms.

(digression: "I can clearly not choose the wine in front of me". "But I can clearly not choose the wine in front of you".)

This has the looks of something like a NW flow setup, but maybe not as tilted.

 

[Derek Weston]

IF the cap can be broken (with strong forcing from a cold front, there's a good chance that'll happen), then it looks like there will be sufficient shear for supercells and tornadoes. The caveat, however, is that the forcing is a cold front...which is likely to promote linear storm modes. BUT, I see that shear vectors are going to be WNW to NWly, which would make them have a good component perpendicular to the boundary, which could promote discrete storms.

(digression: "I can clearly not choose the wine in front of me". "But I can clearly not choose the wine in front of you".)

This has the looks of something like a NW flow setup, but maybe not as tilted.

right, that's not ideal. . . though the CF is more of a windshift than anything from what I see, not as worried about it being to strong or aggressive. Believe we got some tornadic sups off the CF during the last big Minnesota outbreak. Think it could work.

Of course, I've been missing chases lately due to work... so, my enthusiasm might be a little higher here, please realize

 

[Mike Hollingshead]

The sw MN area has me somewhat interested. Like perhaps this is where the western most storm will be on the edge of the cap and deviate itself right.

http://beta.wxcaster.com/cgi-bin/parse_skewt_trace.cgi?model=META&fcsthr=030&STATIONID=kmml

Interesting forecast sounding up there. Very far from a fan of these types of setups but at least it might produce a beasty mean storm to chase and have fun with. For whatever reason I foresee something happening like this chase in the same area... http://www.extremeinstability.com/05-6-20.htm Probably cause it was a big cape day that required heavy deviation in that same area. Guessing it too was a similar mid-level flow skirting the area deal. It seems setups like this don't end up with as much flow as the models said the day before. Like the flow trends north more or boundary south more. Anyway I figure it is probably wort a chase on the west edge hoping for something to do the isolated thing.

 

[Rob H]

As of right now, my target is Worthington, MN @21z. The warm 700mb temps in IA concern me a little, although the NAM has been trending towards a more favorable scenario. I'm eager to see what kind of temps the cap breaks with today, if at all. The slightly backed winds also are less than desirable with a NW flow setup, but as Jeff pointed out, the ridge is flattened out into more of a W flow from what was forecast earlier.

There are a whole host of favorable parameters in SW MN, plus at 21z it's in the left exit of the 850mb jet and right entrance of the 500mb with some very small impulses coming through. We need decent lift to get past the cap, but it's easy to have too much lift in this type of setup and go MCS. I'm not seeing the cold front that others are mentioning - I do however see a warm front hanging out in central MN that the NAM is showing precip breaking out on.

I wouldn't be surprised by an early cap bust in the target area, followed by evening MCS on the drive home - but it's my last chase with a friend from Germany so it will still be a good one.

 

[jamesmseitz]

With the 700mb temp seemingly cool enough and the exiting jet will create the lift needed I believe. This still however might lean to a evening to nighttime event the shear values are good enough and the relitive inflow from the SE makes the southern storms a great play. The cap just might be our friend instead of foe in this set up.

By the way..... this front that is being discussed..... It is aweful weak if not Non exsistent..... Rob is right.... The lift will come from the 2 jets mixing it up

 

[Jeff Duda]

By the way..... this front that is being discussed..... It is aweful weak if not Non exsistent..... Rob is right.... The lift will come from the 2 jets mixing it up

There are a whole host of favorable parameters in SW MN, plus at 21z it's in the left exit of the 850mb jet and right entrance of the 500mb with some very small impulses coming through. We need decent lift to get past the cap, but it's easy to have too much lift in this type of setup and go MCS. I'm not seeing the cold front that others are mentioning - I do however see a warm front hanging out in central MN that the NAM is showing precip breaking out on.

I'm not sure if the dynamics that apply to upper level jets also apply to mid and low-level jets. Thus I'm not sure if crossing jet maxes at 850 and 500 mb will necessarily provide the lift needed. I would agree that the boundary/front does appear to be pretty weak, and thus forcing may not be as strong as I once thought.

 

[Dean Baron]

FWIW, June 17th was a dryline, not a cold front.

Parameters look decent today for supercells, the only really concern is the cap. WRF has H7 temps cooling to below 12C throughout the evening across southwest MN which for the average front would be enough to get storms going. However, considering the front today is rather weak, I wonder if the forecasted H7 temps are still too warm. Guess we'll just have to wait and see. Target is Tracy, MN and adjusting from there.

 

[Tim Marquis]

. I would agree that the boundary/front does appear to be pretty weak, and thus forcing may not be as strong as I once thought.

This morning's convection was more robust then what some of the models were forecasting. This convection will leave a boundary behind and could provide enough forcing to overcome the cap. Looking at the 12z MSP sounding, you are going to need to get up to 90 to break it. A pretty robust cap. The true question is where this boundary is going to set up in the afternoon. However, another concern is the storm mode. The high res 4km NSSL wrf run has the cap breaking over eastern minnesota by 21z (although, the models have had a poor handle on when the cap will erode) going linear by 23z, a relatively small window of opportunity. This model had somewhat a handle on this morning precip ( had it a little further north and not as robust) which gives me some higher confidence in how its going to handle the afternoon. I agree with spc pushing the moderate risk further west as this airmass will be relatively undisturbed from this mornings convection.

 

[Rob H]

I'm not sure if the dynamics that apply to upper level jets also apply to mid and low-level jets. Thus I'm not sure if crossing jet maxes at 850 and 500 mb will necessarily provide the lift needed. I would agree that the boundary/front does appear to be pretty weak, and thus forcing may not be as strong as I once thought.

I had assumed that the same balance between pressure and Coriolis would apply at different heights, although to a lesser extent - but I've never seen anything specifically saying so, I don't know the extent, and I probably took one piece of knowledge and over-applied it. LLJs can give some extra lift that storms might need to get going, although I'm starting to think that would be more due to moisture/warm air advection and not the dynamics normally assigned to the UA jet stream.

The WRF-NMM shows what looks to be a lone cell just north of Worthington, MN so I'm probably sticking to my previous target until I start seeing what the RUC is doing and get to look at the 18z soundings.

edit: Out of curiosity, what processes are causing the decent, localized 700mb vertical velocities in west-central MN on the 12z RUC run for 19z on?

 

[Edward Ballou]

Minnesota has been my bane. I've always been late to the show - the front speeds to the east. My advice would be to show up early as to be on time, if you're on time, you'll be late.

Now to the forecast - KFSD is calling main threat along Hwy 14 from KBKX to KMML. RUC shows 2m temps approaching 100 at 21z. While that might not be accurate, the tight circle definitely fits and Tds in the low to mid 70s should provide enough moisture - advection from Marietta to Marshall. The various models seem to be agreeing on this area which may shift slightly to the north.

Things seem to start up north and this was the case the other day (Wadena, MN). Hwy 75 should be an ample start for N/S travel and the intersection (Hw 68) at Canby should provide for a SE route if things start firing southward.

 

[Dean Baron]

Ay, that precip needs to dry up and quick. We have a pretty soild cloud deck here in the Twin Cities keeping temps cool, with an area of precip also out towards the SD/MN doing the same thing. Visible satellite shows decently clear skies across southwest MN but cloud debris off to the west. Really hoping that stuff dries up before it gets too far east.

 

[Tim Marquis]

edit: Out of curiosity, what processes are causing the decent, localized 700mb vertical velocities in west-central MN on the 12z RUC run for 19z on?

My best guess is that there is some subsidence behind that precip line. Its like a boulder in a stream, ahead of it water rushes upwards to go over it. Behind it, water rushes back towards the boulder causing a sinking motion to appear.

 

[Jeff Duda]

My best guess is that there is some subsidence behind that precip line. Its like a boulder in a stream, ahead of it water rushes upwards to go over it. Behind it, water rushes back towards the boulder causing a sinking motion to appear.

Can a 13km grid spacing model resolve such a detail from what appears to be a cellular storm?

 

[Tim Marquis]

Can a 13km grid spacing model resolve such a detail from what appears to be a cellular storm?

I wouldn't call it cellular, its more of a complex. For a single cell your absolutely right, you wouldn't get that kind of resolution on a 13km grid spaced model. For a small complex, perhaps.