The set up for severe weather Wednesday afternoon is quite ominous. The following forecast is solely based on our Futuretrack model. Other models depict slight shifts north and south. So, while we can pinpoint exact locations of possible severe weather using this model, keep in mind it is just one model solution! We’ll have to forecast as we go through the day on Wednesday to get the exact particulars and locations effected.
Having said that, this is the forecast model I am going with right now. Before I get into the nuts and bolts, please know that a lot depends on the amount of instability available. (In my previous blog post, you can see how distinct the cut-off of instability is. This map shows how we go from a completely charged atmosphere to nothing is. Unfortunately, it’s an all-or-nothing scenario.)
First, let’s look at the surface wind directions. It’s obvious here we’ll have an east-to-west front lying right over Northern Illinois with plenty of convergence along the front (which promotes ascent). Areas north of the front will be slightly more stable, but areas south of the front will yield plenty of instability! Concerning to us is the area where there is a southeastly wind. This usually takes place along and immediately south of the frontal boundary. Clinton, IA, Sterling-Rock Falls, Dixon, DeKalb, Mendota, and the southwest suburbs of Chicago all lie in this sweet spot. (Again, keep in mind, this is one model and this boundary may set up a few miles north or south of there…causing this sweet spot to shift some.)
But we have to think of the atmosphere in three-dimensions. Simply looking at the surface wind gives us no clues as to the potential severity of the storms Wednesday afternoon. In this graphic, I superimposed the wind at 3,000 feet. They are noted as colored lines. The brighter (reds and pinks) indicate stronger wind aloft. Note, that is in Central Illinois. There, we can see quite a bit of “speed shear” which is a form of wind shear where wind increases with height. This set up will bring about the possibility of significant straight-line wind potential for Central Illinois…and Indiana later in the day. Closer to home, the wind over Northern Illinois is much lighter at 3,000 feet, but is moving perpendicular (or at a right angle) to the southeasterly wind. This maximizes the amount of wind shear over North Central Illinois. With that 90° change in wind direction: from southeast at the surface to southwest at 3K feet, I believe this is the area at highest risk for isolated tornadoes on Wednesday afternoon. You can see where the low pressure center is in Central Iowa around 6pm with the warm front extending eastward through Northwest Illinois and then toward the southern end of Lake Michigan. The storm motion tomorrow will be from west to east with a slight movement toward the southeast later in the day. The highest threat for severe weather will exist between 3-8pm.
Be alert for changing weather and have a way to get warning information, whether that’s from the WREX Weather App (Search on the App/Google Play stores) or a NOAA Weather Radio.
Meteorologist Joe Astolfi will be updating the blog first thing Wednesday morning. -Eric
Posted under severe weather
This post was written by Eric Sorensen on June 11, 2013