06/27/09 FCST:MN, IA, IL, WI, MO

[Mark Sefried]

Interesting little setup possible on Saturday for southern WI, northeatsern IA and northern IL. NAM has a nice negatively tilted low centerd over Lake Superior at 0z Sun with a cold front extending southward from there. Nice SE surface winds are forecasted with 500 mb winds out of the west around 40-45 knots. Should be plenty of shear for the storms to rotate especially over norheast IA, far sw WI and Northern IL . The nothern 2/3 of WI may be socked in with precip for most of the day. Far southwestern WI looks great with LI's near -6 and sbcape of 2500 j/kg, and enough helicity to get things going. I will not be able to chase because of prior cominitments, so something big will happen The only down side is that sw WI is not the best terrain wise, however since this possible event is several days out maybe the system will slow down and the best action will be over northern Iowa.

 

[Andy Wehrle]

I just started looking over this setup in more detail today. NAM is progging dewpoints of 75 or higher into southwestern and far south-central WI along with good turning from 850 up to 500 MB. Speeds could be a little faster but still not too shabby. There appears to be a 500 MB jet streak that is lagging off to the west by 00Z Sunday. If that could get here a little faster...

I'm sure those forecasted dewpoints are overdone, but they should still be into the 70s and with temps in the mid to upper 80s LCLs should be pretty good.

GFS does not look quite as good, especially the screwy surface pattern, but still presents the possibility for some severe weather.

Mark is right about the terrain in SW WI- I have chased there, and there are almost no roads that lead directly from somewhere to somewhere. The ones that exist are state or county highways full of sharp curves and lined with steep bluffs that make it impossible to see the sky other than straight above/ahead of you.

Further east in south-central Wisconsin, the terrain is marginally better, still plenty of hills and trees but there is at least an abundance of straight, well-marked, well-paved roads (provided the one you need is not closed for construction... ).

 

[Skip Talbot]

I have been watching this setup for a few days and its nice to see the GFS and WRF in fairly good agreement. One of my biggest concerns with this setup is the storm mode. With the forcing on the cold front and veered surface winds across much of the warm sector, we'll probably be dealing with a big line. As Mark pointed out, however, the winds do back more towards the northern portion of the warm sector. If the 12z WRF is correct, I'd be targeting Mason City, IA @ 18z-21z Saturday. Directional shear is actually better earlier on in the day. The WRF has a stout morning cap, which is pretty much gone by 18z. Shear and instability are more than sufficient for supercells at this hour, and with the strong forcing we should see initiation shortly afterwards. You'll want to be on the storms at initiation too before we have a transition into a linear mode.

 

[Matthew Piechota]

Looks to me this is going to be a day where there could be one or two nice sups early and then it transitions into a big squall line all up and down the CF. good shear so if we can get an isolated storm early, it could be good. we'll see.

 

[David Draun]

Lots of heat and humidity with this one. I really don't think the dewpoints are overdone with the model forecasting of this system. Already the evapotranspiration of the cornfields has created 80s dewpoints in Iowa this year. The IL cornfields are a bit younger due to later planting so they may not have the strong ET like to the west in Iowa, but this moisture should advect some. I really don't see much low-level shear farther south of the surface low. Pretty unidirectional all the way up the column. Looks like a squall line along the cold front for sure, and if the wind damage is pretty intense, it might be a nice serial derecho event.

 

[Joel Wright]

Well, the big question for Saturday is whether the veering low-level winds lead to linear mode, and if so can anything fire out ahead of the CF.

Even though GFS/NAM veer surface winds shortly before the frontal passage, well out ahead of the CF winds stay largely backed over southern WI and most of IL. So anything that can fire out ahead of the CF would have 0-3km SRH >150m2/s2 to play with. Another plus are the H5 winds nearing 50kts coming out of Iowa. Also, GFS forecasts CAPE nearing 4,000j/kg later Saturday. This time of year with that much instability and associated mid-level winds it's certainly nothing to sneeze at.

One thing I'm hoping for (wishcasting) is a weak surface low riding the cold front further south of the main surface low that could keep surface winds backed closer to the front. Something like that may not be resolved until the day of the event though.

Best upper-level winds will remain well to the north, but upper-level divergence looks to be quite good. I'd like to see a better LLJ, but at least it's not highly veered LOL.

 

[Mark Sefried]

Today's model runs do not look as promising as last nights. Looks like a linear event now except early on. Best chance of anything may be over eastern IA if storms develop on any outflow boundaries. I don't have much time to look at things in more detail as I am at work. Going out to dinner Sat night at one of the best steak joints in Central IL and I would rather have a big sirloin than chase Saturday's setup. Things may still change though for those of you that are free to chase so it may be worth keeping eye on.

 

[Andrew Pritchard]

Low level wind fields, while there is some turning present are really unimpressive. I'm sick to death of getting to my target area with barely a light breeze in the air this year. The only time that's going to get you anywhere is a day with a very pronounced outflow boundary. If this ends up being the case, you might get an isolated tornado threat along that boundary. Otherwise, this looks like a good severe weather day, but a pretty non-tornado event. I'm supposed to attend a family reunion in the Chicago area Saturday evening, but could pull some strings to chase but it doesn't seem that will be necessary 36 hours out.

 

[Joel Wright]

Don't see too much change with models over the last 24hrs. Still looks like a pretty good setup for severe storms, but supercell mode looks to be brief at best before quickly becoming more multicell/linear.

Looks like storms are going to explode along and ahead of a pretty decent cool front east of I-35 in Iowa by mid to late afternoon. There definitely will be plenty of instability, but low-level wind fields look pretty pathetic. Mid-level winds are pretty nice, but upper winds are a bit weak. There is nice turning with height, but the lack of velocity (especially at the lower levels) is a bit disconcerting. With a fairly potent cool front pushing in I think the convection is gonna have a tendency to go linear quickly. Like I said last night our only hope will be either a weak surface low developing further south along the synoptic boundary, or a leftover outflow boundary from morning convection. Both of those can't be ruled out.

Still up in the air whether I'll be chasing tomorrow or not. Since June is about over I'm a bit tempted to give it a run even with the above concerns. Will wait till tomorrow morning or early afternoon to see how things are looking and if a chase is worth it.