8/18/05: FCST: Upper Miss. Valley / Great Lakes

Well, it appears that tomorrows events will be soley dependant on how well the showers will clear out in the morning. Along with the fact their could be some clouds hanging around/WAA being a potential limiting factor for some of the threat area... Which primarily includes eastern IA, northern/central IL, southern WI and possibley even western lower MI. Deep-layer shear appears to be pretty strong across eastern IA/northern IL/southern WI, with >45kts in the warm sector. In additon, low-level shear appears to be quite strong in northern IL/southern WI (>350 M2/S2). NAM forecasts >3500J/KG of sbCAPE across the area forementioned... So, as for supercell / tornado prospects - it all depends on cloud cover/lingering morning showers. We have extreme instability, good moisture pooling, strong wind fields, etc., we shall see...

At any rate, the day appears pretty favorable... We just need to get rid of the cloud cover, so we can get insolation/instability. I expect the genesis region to be in eastern IA/western IL in the mid-afternoon, with the main convective mode being supercells (...with tornadoes, especially along any OFBs), possibley transitioning into a line, traveling across the rest of the lakes before weakening by late night THUR.
The 00z NAM leads me to believe that with any insolation, the supercell and attendant tornado threat across E. IA/N. IL appears good. Unfortunately, my confidence in this occuring is limited. However, I DO like the orientation of deviant storm motion on forecast soundings with the forecast position of the WF. Tomorrow could prove fruitful if insolation can occur along or near the WF. It's time for Illinois to light up.

I currently like the area around the Quad Cities, however, I wouldn't mind initiation further east away from the big river.
Nice outflow boundry showing up on satellite in NE Iowa. I'll be heading to DVN shortly to play along that boundry and better lapse rates in IA. Davenport sounding showed good turning with height this morning but no cap. Hoping things can build up a little before firing and turning to grunge.
Jerry Funfsinn
I'm not convinced that SPC's assessment of this afternoon's potential will pan out in IL. Convection has continued to fire of the OFB for some time now and destabilization may be a problem. I may hold out and play S. WI which has cleared nicely. The 15z RUC also moves the surface low across S. WI into SE WI by 03z. I am also a little discouraged by the veered low-level flow across N. IL indicated by the RUC. The only area where low-level backing remains by 21-00z is S. WI. Subsidence in the wake of the morning convection should enhance clearing through most of the day and allow more than ample destabilization.
I am also concerned about the veering winds, but at this hour they are still backed north of dvn. The station mxq in eastern Iowa had a 7 degree jump in dewpoint in the last hour and a half. Its up to 77, now thats some juice. I'm playing the wait and see game right now.
Jerry Funfsinn
Debris from morning convection (including a severe bow echo) has cleared nicely in south-central WI providing for decent insolation.

The main concern I have centers on whether the light subsidence in the wake of that convection/weakening low level convergence/ will inhibit initiation over E. IA even if the cap weakens. We've seen this happen a few times before during this season and ruin a few good setups.

Given initiation, the chance for genuine tornadic supercells across S. WI looks at least as good as any I've seen this year-esp. given the continued backing winds Scott mentioned

Post-event edit: How prophetic.
The 15z RUC leads me to believe all subsidence will be gone from S. WI by 00z. However, it's a model and I'm a little discouraged by the veering flow in the last 30 minutes in SW WI. I'm going to sit here in Janesville until it's more clear whether the OFB in IL or SW WI will be the game today.