The flow aloft tomorrow will be from the Northwest while at the low levels, the flow will be from the southwest. That will lead to increasing shear and the potential for supercells to develop that can support tornadoes and large hail.
Why do we always show the lifted indexes? Because Storms needs enough liftto rise to 45,000 feet + for them to be of concern. Tomorrow evening, lifted indexes at 7pm (shown) will be at about -4, meaning that at 500 mb, a parcel of air will be 4 degrees warmer than its environment (meaning that it will keep rising).
The dangers Wednesday are not associated with the cold front (we will have to worry about that Thursday) but the warm front. Warm fronts are less likely to spark squal lines, but carry a greater risk for tornadic activity. We can show this on a Energy Helicity Index (EHI) Plot (below). These plots measure the risk a given supercell has at becoming tornadic--- and tomorrow the risks are relatively high. While this does not mean we are going to have a tornadic outbreak tomorrow evening, people should check local radars, watches, and warnings before heading out (or just check out the blog!).
EHI values at 7pm tomorrow are expected to be at about 2 in the
city, and 3+ in the western subburbs--- meaning that any supercells
that develop run the risk of spawning a tornado
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