U.S. Cold-Air Outbreak of November 2014: Precursors and Predictability

Following a warm October, a large-scale flow pattern reconfiguration in early November 2014 culminated in a significant cold air outbreak over the continental US. By November 17-18 more than 50% of the continental US was snow covered, and snowpack for the month was more extensive than any November on record. The price of US natural gas rose by 25% from October 29 to November 21 in response to surging heating-related energy demand. November 2014 ranked as the 15th coldest November since 1895, despite NOAA temperature outlooks issued in mid and late October indicating that a colder-than-normal November was unlikely. The 15-day European Centre for Medium Range Weather Forecast (ECMWF) operational ensemble prediction system (ENS) failed to show risks for extreme cold more than 10 days in advance.

The objectives of this study are to understand key synoptic precursors to the November 2014 cold air outbreak and assess whether this event would be better predicted were it to occur today. In part one of the study, synoptic precursors to the pattern reconfiguration – e.g., explosive cyclogenesis over the Bering Sea associated with the remnants of Typhoon Nuri -- are diagnosed using the ERA5 and observational datasets. In the second part, forecasts of the cold air outbreak and key synoptic precursors from the then-operational ENS and extended-range model forecasts are compared against hindcasts from the latest operational configuration of the ECMWF extended-range model initialized with ERA5 data. The forecasts are evaluated in terms of ensemble mean skill and reliability of the ensemble distributions.