Thursday actually looks to be quite the unstable afternoon. Mixed-layer CAPE is expected to be on the order of 1200j/kg in most areas of lower Michigan according to the latest model data. Temperatures could easily rise into the upper 70s with dewpoints reaching as far as the upper 60s. Any time the dewpoints cross 60°, severe weather needs to enter the conversation - when they cross 65°, it's often much more significantly unstable air.
Despite what appears to be moderate to strong instability, the models are continuing to predict that wind fields will be weak in terms of speed shear. Storms would likely be moving slowly, which takes away some of the horsepower they might otherwise have. With the wind speeds aloft being as weak as they are, it may be difficult for thunderstorms to sustain updrafts for a very long time; they could form, but then collapse on themselves and dissipate quickly. On the other hand, if any do get going, there's definitely enough instability present for large hail, and with large hail/torrential downpours usually comes the risk of wet microbursts/damaging wind. And here's the kicker - though wind profiles look to be on the weak side, they are forward veering, meaning what updrafts do form could also easily rotate. The image below is a forecast sounding for the Thumb Area from the latest NAM run.
As you can see, it clearly indicates a tornado threat, and there are actually two weak tornadic supercells associated with similarly observed soundings (one from October of 2000 and another from June of 2003).
A couple of things are tempering my excitement for this one. The first is that while these conditions look good for severe weather, the timing of the cold front may not quite coincide with the day's peak heating. If this is the case, storms may not even initialize over lower Michigan as convective inhibition wouldn't erode enough before the cold front moved through. The last couple of NAM4KM runs depict this scenario - storms would ignite over Ontario rather than lower Michigan. I've come to trust the NAM4KM implicitly over the last couple of years as it tends to get simulated thunderstorms correct more often than not. However, should this model start to indicate storms forming more further west, my concern will increase.
The other factor is early day/leftover convection. Remnants from the previous day's storms (which look to explode over Missouri, southern Iowa, and western Illinois) are likely to move through at least a portion of the area prior to dissipating. We've seen this in the past, and when it happens it can put the big kabash on instability, not only preventing daytime heating by blocking out sunlight, but serving to increase temperatures at - and thus stabilize - the mixed layer, where all the action occurs. If a cloudy, rainy Thursday morning is in our future, I don't think there's going to be enough unstable air present to overcome inhibition (unlike June 22nd of last year). However, if the remnants do dissipate quickly and we have a warm sunny day, I'll definitely be a bit more concerned. If that were to happen, the cold front may not even need peak heating to fire storms.
As always, I'll be closely watching the situation unfold. If the NAM4KM starts to show signs of trouble, I'll be upgrading my outlook for sure.
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