Another hot and muggy day is expected across the area today with temperatures in the low-to-mid 90s with dew points in the mid-to-upper 70s, creating heat indices in the 100° to 110° range. This will create a very unstable atmosphere over much of the upper midwest and will be key in enhanced storm development later today. The impulse that is expected to let storms fire is currently sitting over the Rockies and should move into the Northern Plains by mid-to-late afternoon. A warm front will also be draped at a southeastern angle with an associated low pressure center that will also serve as the stage for storm development. As the impulse moves through and the cap begins to break, storms should quickly begin to break out across the eastern Dakotas, western Minnesota, and northwestern Iowa, and turn severe quickly. Instability mentioned earlier will create a very dangerous environment for supercellular thunderstorms with very large hail to baseball sized, damaging winds, and an isolated tornado all possible initially.
Discrete supercells should continue for a short time in the evening before things begin to line up into a Mesoscale Convective System. Concern is growing for the potential of widespread damaging winds with the line that develops and heads to the southeast, possibly as a derecho. How long this line lasts is up in the air. There is enough instability to continue these storms across much of northern Iowa and continued movement into Wisconsin and Illinois. Due to the enhanced damaging wind threat, the Storm Prediction Center has upgraded much of northern Iowa and Minnesota to a moderate risk of severe weather. Significant damaging winds to hurricane force is a big concern as things begin to evolve as a line and move southeastward.
It’s important to note that there are some uncertainties with this. We are currently not 100% on how far south this could go and how far south the severity will go. In all likelihood, those north of Highway 20 should have the most concern about this risk. Those to the south of Highway 20 should still keep a close eye on things. It really will all depend on the end location of the warm front and where that will setup also. There are a lot of different scenarios at play with the warm front and what could happen there, so it’s important that you check back in case there are future changes.
Another risk of severe weather exists for Monday, however the majority of this risk should be to the east of our forecast area where a moderate risk of severe weather has been posted. We will not detail this too much as there is a lot of unpredictability in regards to what the storms from today will do and how this will impact the forecast. Current expectations have the MCS moving through from today continuing into the Ohio Valley and leaving behind several outflow boundaries. As daytime heating begins to cook in, isolated strong-to-severe storms will be possible along these boundaries with a risk for large hail, damaging winds, and an isolated tornado. There is also a chance for additional storms later in the night, but this will all depend with how well the atmosphere recovers.
Stay tuned to the Iowa Weather Network for the latest information!