It is not often when an 8-12″ snowstorm can be considered a “surprise”, but that was the case Sunday in SE Iowa and parts of NW Illinois.
A shortwave from Missouri moved across SE Iowa Sunday afternoon which produced warm advection over a cold, dry airmass that was sitting over a snowpack. As the moist layer ascended, it quickly saturated the cold, dry airmass. At the same time, the moist layer undercut a cold pool aloft from an upper level trough, which destabilized the atmosphere and produced convective snowfall (thundersnow) that produced 1-2″/hr. There were no snow advisories issued for any of the affected areas until well after the snow started falling. Here are a couple of maps that show the snowfall totals (central IA and eastern IA). The Sunday morning forecasts issued by the NWS were mainly for 1-4 inches of snow in Iowa and for less than 1 inch of snow in northern Illinois.
However, this was not a situation where the NWS can be blamed. All forecast guidance and QPF models showed very little moisture associated with this system. Only an experimental mesoscale model correctly forecast the snow burst.
On a side note, a similar meteorological set-up occurred again this morning, although this time the atmosphere was warm enough that the precipitation mainly fell as rain. Severe thunderstorms were reported across northern Illinois in the same areas affected by Sunday’s snow burst.