All morning I mentioned there was a lot of uncertainty with the forecast for not only Monday, but also for Sunday night. The atmospheric models I've been looking at through this morning and early afternoon have been very inconsistent. Not only have they been inconsistent, but they were wrong...when it came to what was happening this morning. When models can't even properly predict the first few hours of the forecast - they tend to go even more downhill as you move forward in time. This means we can't really trust what model guidance is showing...which makes things just that much more difficult.
This being said...there is a system to our northwest, and the weather pattern we are in right now is a bit unsettled. We already have storms SW of the STL metro right now, and we have storm chances scattered over the next few days. This system doesn't look like its going to move through until overnight Tuesday/early Wednesday morning.
For Monday, there will be clouds across the region, most likely from storms to our West. There will also be heat and humidity - which typically allows for cumulus clouds (and oftentimes even spot showers/storms) to develop in the afternoon.
Models have not taken the temperature decrease from the eclipse into account - and we really don't know how the eclipse will affect the weather tomorrow. We aren't even sure how much of a drop in temps we will see - that will also depend on how much cloud cover is overhead. Cloudy skies mean the decrease will not be as drastic. In areas where skies are clear - the temp decrease would be greater.
I would hypothesize that since we will have cooler temps during the eclipse, especially from 1-2pm, than what the models "think" we will have... is that that will delay the "heat of the afternoon pop up storms".
The way it looks right now...in St. Louis and much of the viewing area, it'll be a mix of clouds and sun...but mostly likely more clouds than sun for most. Farther south and southeast - the cloud cover may be less. Farther north and northwest, the cloud cover will most likely be higher due to storms. We will know much more by early Monday morning when we can see extent and location of storm development and movement of those storms.
There is also a slight chance for a few storms in the afternoon in St. Louis - but hopefully those will hold off until after the eclipse is over.
I wish there was more clarity in the forecast right now. Stay tuned!