Differential Cold Air Advection

Hey everyone... Almost 2 years ago, it was my 3rd day working at the NWS Des Moines office. We had a blizzard come up without any lead time, and it closed major roads and killed 7 people in Iowa. However, it certainly didn't come "out of nowhere." This spawned a research study to better understand and forecast these event. As luck would have it, we had a similar event this past week, so I thoughts I'd compare/contrast. First of all, to crudely quantify the impact, I'll show a map of the road conditions. This first one is from Jan 26 2010.The next one is from Jan 11 2012.
the Jan 26th one must have been worse right..? Well, kinda, but in my opinion that is due to one reason. SNOWPACK. Jan 2010 had nearly record snow pack (1-2ft) across Iowa, whereas this year we had a whopping zero. No snow pack means no snow to blow around, and any falling snow will gather in the grass, fields, and ditches. To prove my point, here's the metars from Mason City Iowa (think north central Iowa).

Jan 11 2012
time temp press VISBY Wind Gust Weather

3:17 PM	26.6 °F		29.75 in	1.8 mi	NNW	27.6 mph	38.0 mph	Light Snow
3:53 PM	25.0 °F		29.80 in	1.8 mi	NW	25.3 mph	38.0 mph	Light Snow
4:53 PM	21.9 °F		29.82 in	1.8 mi	NW	26.5 mph	36.8 mph	Light Snow
5:53 PM	19.0 °F		29.83 in	2.5 mi	NW	32.2 mph	46.0 mph	Light Snow
6:53 PM	18.0 °F		29.85 in	3.0 mi	NW	32.2 mph	44.9 mph	Light Snow

Jan 25 2010
time temp press VISBY Wind Gust Weather

10:53 AM	21.9 °F	29.38 in	0.8 mi	NW	33.4 mph	41.4 mph	Light Snow
11:53 AM	19.9 °F	29.39 in	0.5 mi	WNW	29.9 mph	40.3 mph	Light Snow
12:53 PM	19.0 °F	29.39 in	0.5 mi	WNW	32.2 mph	40.3 mph	Light Snow
1:53 PM	18.0 °F	29.41 in	0.2 mi	WNW	32.2 mph	48.3 mph	Light Snow
2:53 PM	16.0 °F	29.45 in	0.2 mi	WNW	36.8 mph	44.9 mph	Light Snow
3:53 PM	14.0 °F	29.49 in	0.5 mi	WNW	26.5 mph	38.0 mph	Light Snow

So whats going on...? First of all, the pressure is rising, which doesn't fit the classic blizzard conceptual model. Second of all, there's only light snow being reported. Thirdly, the northwest winds are impressive. These three things are all common with strong cold fronts. However, the question I had is "how does strong wind mix down to the surface when surface Cold Air Advection is only going to act to stabilize the boundary layer..? The answer my friends is to replace "Cold" with "Differential". Differential Cold Air Advection (usually between 925mb and the Surface) will steepen the lapse rates in the boundary layer and promote momentum transfer of strong winds down to the surface. Here's the 12Z (pre-frontal) and 24Z (post frontal) soundings from Omaha during this last event on Jan 11 2012. The black Xs on the 12Z sounding show the 850 and Sfc temp from the 24Hr sounding (dtemp/dt). This was unusually deep mixing in the boundary layer. I also circled the 0-1km wind shear. (its blurry, so click it)