7/16/07 FCST: MN / WI / IL / IN / MI / OH

[Scott Weberpal]

84 HR WRF paints a beautiful picture for tornadic supercells across far E. MN and all of southern WI. Strong NW H5 flow juxtaposed over a NW to SE oriented warm front, coupled with strong deep layer shear (forecast helicity values 300+), and 65+ Td's; should set the stage for a fantastic chase day. fast moving, compact shortwave forecast in the H7 and H85 flow reminds me of many typical upper-midwest tornado days. Could prove interesting come Monday...

 

[Derek Weston]

Oh, that is looking nice...

Nice sw'erly flow at 850... shear actually looks pretty good. (haven't been able to say that in a while)

From the early looks of it, S. Wisconsin would be the place to be.

Fingers crossed...

 

[Derek Weston]

Probably should include IA/Ill now as the predicted threat has shifted south.

I find it interesting that the SPC hasn't noted anything about supercell type storms as a possibility. Although this isn't looking to be a major setup, the directional wind-shear is interesting enough to make me think that it's a possibility if favorable conditions materialize...

But what do I know...

 

[Bill Schintler]

Chase target for Monday, July 16

9:48 PM CDT, 07/15/07

Chase target:
Delhi, IA (30 miles west of Dubuque).

Timing and storm mode:
Storms will fire between 2 and 3 PM CDT. Storm mode will be medium- to high-based multicellular storms which should remain below severe limits.

Synopsis:
A couple of H3 S/WV’s were noted in NWRLY flow. Divergence in association with a lead wave over ERN SD/NEB has been associated with a cluster of storms in CNTRL NEB. This storm complex will propagate to the S for several more hours into an increasing 20kt SRLY LLJ. A stronger upstream wave has been responsible for a variety of severe WX with ongoing storms in ERN ND this afternoon and evening. Model progs radar extrapolation, and Corfidi vectors take this complex on a SERLY track overnight though MN. In IA, ERLY SFC flow has commenced in the WRN half of the state, while dewpoints were 60-65F throughout the state. Moisture depth is shallow in most areas of the Upper-Midwest; however an axis of deeper moisture extended from OK into ERN NEB/WRN IA as indicated by 15C H85 dewpoints. A WRLY LLJ will transport this moisture towards ERN IA overnight. Upstream soundings (MPX, ABR, BIS) also indicated modest mid-level lapse rates.

Discussion:
Over the next several days, the ridge over the WRN CONUS will continue to build to the N, causing a blocky pattern to evolve. In the short term, one more day of active WX is on tap for the Upper-Midwest, and then a quiet pattern will resume for the first part of the week. The MCS presently approaching WRN MN will track SE through MN overnight and then into extreme NERN IA and WRN WI by 12Z. This in turn will push an E/W-oriented OFB S into US-20 in ERN IA by 18Z, which should provide a focus for storm development by 20Z. The strongest deep-layer shear will be located well to the north of the instability axis in ERN IA and NRN IL, however backed flow N of the OFB will locally enhance SRH’s in this area. Instability will be limited by aforementioned modest lapse rates, with MLCAPE’s AOB 1000J/kg.

- bill

 

[Andrew Pritchard]

I think there might still be a decent shot at some supercells across eastern Iowa and potentially even into northwestern Illinois. Storms should refire along the ofb/front laid down by previous storm, and with potentially extreme instability, the redevelopment could be quite vigorous. Atop that we'll be looking at effective shear values around 50 knots, which is more than enough for supercell structures. Top that with srh values aoa 200 m2/s2 and some boundary interactions and a couple brief tornadoes are possible. Now of course, with a setup like this even making a forecast the night before is stupid because it's all dependant on the current convection and boundary locations in the morning. We'll have a clearer picture then, but I sure wouldnt rule out a surprise or two where ever it is that boundary lays out in the evening.

 

[Dave Winslow]

South Dakota too please?

I have been watching a stationary front over southern MN/E. South Dakota move slowly north this morning. There are some elevated thundershowers over NE SD and outflow is moving southeast into Eastern SD (at Noon Aberdeen is reporting a strong N wind gusting to 27 mph). If this OFB can continue south and interacts with the stationary/warm front, we may get some discrete storms in a highly sheared environment.

I am concerned with T/Td spreads in this region, especiallys SW Minnesota because of moderate drought and probably very pitiful amounts of Evaportranspiration. If anyone chases this setup, there is much better soil moisture in SD comared to southern MN and NW Iowa so maybe the LCLs would be more managable.

Certainly something to watch though this afternoon along that front.

 

[Andrew Herron]

You can remove MI as we have lots of convective baloney around that's not going to give us much of an environment. The best we can hope for is a strong gust of wind.