Decent instability, modest shear Sunday

With yesterday's storms restricted to the southernmost areas of Michigan, our next chance at significant rainfall appears to be Sunday. With all of the troubles the models have been having with the forecast in our state this year, I'm going to try something different going forward: I'm going to discuss each model I have access to individually, briefly touching on the more significant factors that the models are contributing to the forecast, then summarize my opinion.

• NAM: Instability is forecast to increase throughout the day ahead of a cold front which would be approaching from the northwest. A weak cap would be in place and is forecast to erode over most of the state by 2PM EDT. This would support thunderstorm development in the afternoon. Very modest unidirectional bulk shear of 25-30kts would be present around this time, so this forecast does not advertise much of a severe threat. Still, some marginally severe weather (namely, hail) would be supported in the areas of highest instability. The NAM is predicting very high temperatures approaching 90ºF. It has the cold front begin to move through Michigan around 2PM, and thunderstorms throughout the afternoon.

• GFS: The extent of decent instability is forecast to be a bit more limited by the current GFS run, indicating greatest convective potential along and south of the US-10 corridor, and especially east of I-75. Capping is forecast to be a bit stronger in these areas as well. This could go either way - either the cap would erode enough so that thunderstorms, likely initiated elsewhere, would grow in intensity as they approached, or there'd be just enough left to choke off the fuel supply. More time and more runs are necessary to make that determination. Like the NAM, shear is forecast to be modest, in the 25-30kt range, but lapse rates would support a hail threat. Timing of the cold front appears to be similar to the NAM's timing.

• ECMWF: A bit of an outlier in that it has the cold front approach much earlier, and much less instability is forecast to develop over the state, although in a pattern similar to the GFS with the highest instability south of the US-10 corridor and to the east. Same as the rest in terms of shear values, though if this forecast's timing verifies it may not matter. Storms would likely fizzle out due to the cap not having enough time to burn off.

Summary: I'm going with the NAM/GFS on this one as the similar timing overrules the ECMWF solution. Rain and thunder will be likely Sunday afternoon, with a marginal severe threat, especially toward SE Michigan where instability should be a bit higher. The mitigating factor will be cap strength, and whether or not it will erode prior to the arrival of storms which should initiate farther north and west. I do not anticipate any widespread severe hazards at this time. NWS Detroit mentions severe thunderstorm possibilities in latest forecast discussion, though I'm still of the opinion that conditions will be marginal at best due to weak shear values. The SPC lists day 4 (Sunday) potential as too low for a 15% risk area, so they would appear to be in agreement. As such, my official outlook is "isolated severe" for areas along and south of US-10 and east of I-75 (limiting northward extent at this point to favor GFS solution on moisture), and a general thunder risk everywhere else.