As it stands now, it appears likely the upper Midwest will see a threat of severe weather every day this weekend. Today’s threat will be well west of eastern Iowa. Saturday’s threat appears to be primarily for western portions of the state. The main threat now for eastern Iowa is Sunday and possibly Monday as well.
Let’s begin at first with a frontal boundary. Although I call it that it’s no different than the often talked about cold front or warm front. The more generic terminology (frontal boundary) is easier to use in this case because it will likely play the role of both warm front and cold front all at the same time.
How? well it’s simple. A front is a boundary between a warm air mass and a cold air mass. Generally they are unidirectional. Whatever air mass they bring with them determines whether they are a cold or warm front.
In this case, a cold front will push south across Iowa tonight and tomorrow. Then it will stall out, turning it into a stationary front. At some point on Saturday and/or Sunday, the front will push to the north, forcing the cold air mass to retreat and bring a warm air mass back into play. This motion turns it into a warm front.
If that all makes sense, good. But for the sake of sanity, I will simply refer to it this weekend as a frontal boundary.
Tonight, severe weather should remain well west of Iowa. But as the frontal boundary sags into the area, overnight thunderstorms lingering into Saturday morning are possible.
Turning to Saturday afternoon/evening, thunderstorms are expected to develop in Iowa along the front boundary. Determining a most at risk area is difficult because there is little confidence in pinpointing the location of the front. It’s also expected a strong cap will prevent the development of severe storms for much of this period. If storms can overcome the cap at a decent time on Saturday, all modes of severe weather are possible, especially in central and western Iowa.
Sunday things get really interesting. The Storm Prediction Center has a simple slight risk in place for the entire state. Again, capping will be an issue, but that could mean good things for a chase in eastern Iowa as it could delay development until late afternoon over the area. Uncertainty here remains as it’s unclear whether the cap can be broken and where the frontal boundary sets up.
Although slight risks are generic and minimal, I anticipate as Saturday and Sunday draw closer, increased confidence in frontal placement could lead to a better “bulls eye” being painted on the map. Monday also presents some possibilities as well. If the storm system stalls out at all, eastern Iowa could see a low pressure pass directly over the area on Monday afternoon.
The chase team will be ready for all eventualities this weekend and will continue to monitor the situation throughout the day today and tomorrow.