The first is that instability over the region looks like it's going to be the real thing. Latest NAM4KM runs and even GFS runs are predicting dewpoints approaching 70°F. The amount of fuel that thunderstorms should have tomorrow cannot be understated - we're talking a full 50 gallon tank here. The models have painted pockets of 2500j/kg+ most-unstable CAPE over the area. CAPE (convective available potential energy) in excess of 1000j/kg is often associated with strong thunderstorms. 2000j/kg of CAPE is sufficient for severe weather even if wind fields aren't very strong. When you start approaching 2500-3000j/kg, large hail becomes a serious threat.
The other factor that has shifted more in favor of severe weather is timing. Yesterday, I mentioned that it looked as though the cold front would move through before peak heating, and that storms were looking like they'd skip Michigan and form over Ontario instead. Based on latest NAM 850mb temperature advection, this is no longer the case - the cold front should be to our west during peak heating hours, meaning storm initiation is likely right in our back yard. Storms are often the most intense right as and right after they form, so this is a significant development.
A wild card is presence of remnants, which I also talked some about last night. The Storm Prediction Center is basing its initial slight risk on a gradual slow down of the cold front; they're predicting that the cold front itself will not be what makes thunderstorms tomorrow in eastern lower Michigan, rather, outflow boundaries from the previous day's storms. This is the rain cooled air which is often pushed far out ahead of thunderstorms. Depending on how unstable the environment is, these boundaries can act as miniature cold fronts, igniting new thunderstorms as they pass across areas. These storms can fire up whether the previous line has dissipated or not; if it hasn't, the line often reintensifies. On the extreme end, this is how progressive derechoes work. Lines of intense storms produce ever intensifying outflow boundaries, and by the time the system really gets going, you wind up with a chain reaction of storm development, dissipation, and redevelopment that is almost seamless, rolling across wide areas of country. Now, we're not looking at anything NEARLY that extreme tomorrow, but the SPC is of the opinion that remnant convection will produce severe thunderstorms over the area by 2 to 3PM. Now, you may remember in June of last year how we had two rounds of severe storms crossing the area. This is basically what happened. Round 1 was the reintensification of previous day severe weather, and round 2 was caused by the actual cold front. Bearing that in mind, if "round 1" happens tomorrow, I don't think that temperatures/dewpoints will be able to rebound enough for a second round.
The real danger will come if the outflow boundaries/previous day convection either dissipates or slows down enough to overlap with the general cold front. If that happens, all the convergence will come in one fell swoop, and it'll happen at peak heating time. Some good news: wind profiles look to be a little more back veering than they were in previous model runs, so the tornado threat may be quite low. We'll have to wait for surface observations to come in tomorrow to know for sure. Right now, I'd say the primary threat is large, possibly significantly large hail. Damaging winds caused by wet microbursts would also be of concern.
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