A Diagnostic and Predictability Case Study: Was the Atlanta 2014 Snowmaggedon Rare?

A Diagnostic and Predictability Case Study: Was the Atlanta 2014 Snowmaggedon Rare?

Robert Haynes¹, Violeta Toma¹, James Belanger¹ and Laura Belanger²

¹ Georgia Institute of Technology, School of Earth and Atmospheric Sciences

² National Weather Service, Peachtree City, GA

High impact snowfall events in North Georgia are thought to be uncommon for the region, but their presence is not negligible.� For example, the 28�29 January 2014 snow event had severe consequences for Metro-Atlanta area residents, leaving Georgia highways gridlocked for hours and causing schools and businesses to close for several days after approximately 2.6� fell. In this study, we first investigate the uniqueness of this snowfall event.� Return periods are calculated using both annual maximum series and partial duration series of 24-hr snowfall accumulation using historically long weather station records in North and Central Georgia.� Preliminary findings suggest the 24-hr accumulation of 2�3'' is not uncommon, with a recurrence interval of four-to-ten years, depending on assumptions.� The January 2014 winter event proved to be challenging to forecast in regards to lead-time, positioning of snow gradients, and total accumulations.� This raises a broader question regarding how predictable snowfall events are in North Georgia.� The original ensemble runs from the ECMWF Ensemble Forecast System for the event demonstrate the struggle to determine, at sufficient lead-time, the correct snowfall accumulation for Snowmaggedon.� To place the forecasts into proper context, we examine the predictability of snowfall events with a similar return period ranking that occurred within the 20-yr ECMWF hindcast period.� The analysis considers the hit-rate, false-alarm ratio, and reliability of snowfall in the reforecasts, to provide a recommendation on how to interpret future real-time ensemble snowfall forecasts for North Georgia.