A "typical" winter storm

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):

We see a very well-developed low over western Missouri, with strong pressure gradients to the northwest and southeast of the low.  Strong pressure gradients mean strong winds, and already we've had damaging winds reported across eastern Colorado and western Kansas.  The tightening pressure gradient on the southeastern side of the low is going to help increase the winds out of the south, helping to advect very moist air out of the Gulf of Mexico and into the southeast.  Right now, the cold front doesn't look too impressive behind the low--in fact, as the low is analyzed now, it looks to be in the middle of the warm sector with only a weak frontal wave evident in the temperature field.  As more cool air pours down the high plains, though, this low should only become better organized.

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.

However, as cool air continues to move down the high plains to the west of the low, it's going to encounter warm air in the 60s and 70s over Oklahoma and north Texas.  This is going to greatly strengthen the temperature gradient there, sharpening the cold front even further.  We see a response to this sharpening temperature gradient below in the 500mb wind forecast.  Notice how just 6 hours later in the GFS forecast at 00Z tonight, the jet streak over the southern plains looks much stronger:

As that jet streak strengthens, the upper-air flow becomes favorable to support the surface low deepening even more as it moves eastward across central Illinois.  By tomorrow morning, the surface low is forecast to be well-organized, with clear warm and cold fronts:

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:

The "critical" thickness line (the 5400m thickness line) runs right through southern Iowa, central Illinois, and into northern Indiana and Ohio.  North of that line, the atmosphere should be cold enough to support snow.  near and south of that line, we're looking more at rain.  So we can start to narrow down the areas we'd expect to see snow.  First, it's only cold enough north of this critical thickness line.  Secondly, we're pretty sure there will be a lot of lift, as a well-defined warm front and a surface low are moving northeastward from western Missouri into central Illinois.  The only remaining question is the availability of moisture.  This gets back to that strengthening pressure gradient on the southeastern side of the low that I mentioned a while ago.  As southerly winds increase to the east of the low, more moist air will be brought up from the south.  This air should rise over the warm front, cool, and reach saturation in a band just north of the warm frontal boundary.  We can see this in the GFS forecast for 700mb relative humidity for this evening:

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:

The thinking prevalent in most of the forecast discussions I've seen is that by the time we get into southern Iowa and central Illinois, we'll be too close to that "critical" thickness line, and the atmosphere may not be cold enough to support snow.  Further north there's a better chance of snow.

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.