As most people in the central US are probably well aware, a powerful winter storm will be moving across the area over the next few days. In fact, it's already pretty well developed. Here's the surface analysis at 17Z this morning (11 AM CST):
Here's this morning's 500mb GFS forecast at 18Z this morning (12 PM CST). Notice that the surface low over western Missouri is in a region between two jet streaks, though the streaks themselves are not too impressive.
We're expecting a lot of different precipitation types with this storm. First--snow to the north. You'll notice on the surface map progression that north of an Iowa-central Illinois-Detroit sort of path the surface temperatures don't look to get much above the low 30s. While surface temperatures would seem to be cold enough to support snow, we need to look at the temperature of the atmosphere above to be sure that the whole mass of air above is also cold enough to support snow. We might suspect this is so, given that the jet stream shown in the maps above stays to the south of this particular line, and usually the jet stream is a good indicator of the separation between cold, polar air to the north and warmer, subtropical air to the south. But, we can still check the forecast for 1000-500mb thickness. Here's the forecast for this afternoon:
Remember that at lower levels (near the surface), southerly winds are bringing up warmer, moister air from the south. That air will only really rise once it encounters a mechanism that will force it to rise, like the presence of a warm front. We saw that the warm front would be moving through central Illinois and the Ohio River valley by this evening. And, sure enough, as we look at 700mb, suddenly we see a band of near 100% relative humidity just north of the warm front. That's where that warm, moist air below has lifted over the warm front, cooled, and is now saturated. This is the best area for precipitation to form, and it so happens that most of this looks to be in the area where it's cold enough to support snow.
In fact, if you look at the National Weather Service watch/warning map from early this afternoon, it outlines much of this same banded area with winter storm watches and warnings, particularly on the northern edge of the moisture:
As this storm moves east, there will also be the possibility of severe weather. At the moment, that warm air advection to the east of the low is rapidly bringing the atmosphere throughout parts of the southeast to a conditionally unstable environment. This is particularly true slightly further north away from the Gulf where the upper-air temperatures are colder, making the atmosphere more unstable. However, until a strong lifting mechanism comes through, this conditional instability will remain untapped.
That cold front looks to quickly blast through the southern plains and is even expected to move across the Mississippi river by the evening. How fast of a blast is this going to be? Notice the large areas of pinks in Oklahoma and Texas on the watch/warning map above. Those are Red Flag Warnings, indicating that they're expecting unusually strong winds accompanied by dry air. This is all the result of a fast cold frontal passage.
As that cold front moves into the southeast, a strong lifting mechanism will be present that can tap into that conditional instability. It's no wonder that the Storm Prediction Center's day one outlook has a slight risk of severe weather for the Ohio River valley down into Tennessee and Kentucky.
So we get two different kinds of "severe" weather with this--snow to the north and thunderstorms to the south.