In late December 2009, shortly before Christmas, a dynamic and strong closed upper level low moved out of the Central Plains into the Midwest with several short wave troughs pivoting around this system. Blizzard warnings were posted for a wide swath of the nation's midsection, and the resulting snowstorm severely hampered holiday travel. The 2009 Christmas Eve Blizzard will be remembered mostly for the deadly conditions it posed to holiday travelers on December 24th and 25th. Although, even as this storm was weakening, it still maintained enough energy and moisture to produce a widespread area of 3 to 6 inch snow amounts across northern Illinois. This system and the broad area of moderate snow were well anticipated in the forecasts leading up to this event. However, a small pocket in the Chicago suburbs where snow totals nearly doubled was not. This presentation investigates the factors that contributed to this missed forecast.
While synoptic forcing related to smaller scale vorticity maxima revolving around this upper level low along with isentropic ascent were the main culprits, mid level frontogenesis was also a key feature in this event. Enhanced lift due to this mesoscale forcing coincided within a deep dendritic growth zone adding another key element to this event. The higher snow amounts observed were partially the result of extremely high snow to liquid ratios, estimated near 40 to 1 in some locations. While snow ratios of this magnitude are not unheard of, they certainly are not typical. In addition, a closed upper level low with its surface reflection passing across northwest Illinois does not typically produce 12-inch snowfall amounts across the Chicago area. This surface and upper-air pattern was not recognized as one that would traditionally be associated with such large snowfall amounts in northeast Illinois presenting yet another interesting aspect to this event.