Severe weather is going to be a factor in some part of the central Midwest Saturday afternoon. As I write this the storm Prediction Center has an enhanced risk in place over southeast Iowa and much of northern Missouri and Illinois.
As with every severe weather set-up, there are factors that will determine the strength and location of the greatest threat. One of the issues that will complicate Saturday's outcome is the outflow from Friday nights storms. Outflow is cool air aloft that is brought to the surface by downdrafts in the thunderstorms. Sometimes a major cold pool forms from what's known as a mesoscale convective complex (MCS).
The MCS is essentially a group of individual storms that merges into a large cluster, sometimes covering the length of an entire state. The cool pool that's generated from the complex can have the same effect as a cold front pushing instability south. In this satellite image centered on western Iowa you can see what the MCS looks like from 22,000 miles in space. The red indicates the high storm tops that can reach 55-60,000 feet high tapping the cold air at the level of the atmosphere.
Here's and example of what the radar might look like in a mature MCS. The storms in the case are covering much of northern Illinois sending rain cooled air southward in their wake.
In Saturday's set-up models are having some difficulty determining where Friday nights outflow boundary sets up. That's critical as it determines the northward extent of the instability that will be available for storms to tap.
The latest trends have been for most models to drop the boundary further south....somewhere into southern Iowa or even into extreme northern Missouri. That implies the greatest threat of severe weather over the SE 1/4 of Iowa into northeast Missouri and WC Illinois. The solution that looks most plausible to me is what the NAM and NAM NEST are showing. A surface low enters SE Iowa late in the day preceded by a warm front that runs from it through the Quad Cities. Near that warm front the flow will be backed and there is at least a minor tornado threat. You can see where the storms are shown firing below.
Virtual soundings on both the NAM and NAM NEST show PDS soundings (particularly dangerous). That is cause for concern but it's far from reality. We won't know the real potential until the set-up is in play around mid-day. Right now SPC thinks its more of a wind threat and I defer to the experts on that. Here is what a PDS sounding looks like near near Washington, Iowa on the NAM.
After the initial cells go up they will eventually grow upscale and may form a squall line that races across Illinois and Missouri towards the Ohio Valley Saturday evening. This has the potential to be a big wind producer (perhaps a derecho) as it sweeps southeast.
For now that's the latest and greatest very late on a Friday night. We'll see where things stand Saturday morning when we can assess where the outflow establishes the firing zone. Bottom line, if you are in SE Iowa, WC Illinois, or NE Missouri the situation is worth keeping an eye on. Roll weather...TS