Discussion: Given the indication by the GFS and ECM of stratospheric warming over northwestern North America in the 11-15-day period (Fig. 1-2) consideration of arctic air evolution is required. The mega-cluster ensemble second most-likely scenario is arctic air intensifying over Alaska in the day-15 forecast (Fig. 3). There is increasing support for this expansion and intensification of the arctic air. Ahead of the cold air mass, the Central U.S. is warm. The third most-likely scenario is that the arctic air shifts farther southeastward reaching the northern states (Fig. 4). Arctic air is very shallow and at 15 days out is very difficult for operational models to identify. So…the “potential” cold scenario (into the U.S.) around Dec. 8-9 must be considered.
Fig. 1-2: GFS ENS and ECM ENS 10 MB temperature anomaly forecast for day-15 ahead indicates stratospheric warming.
Fig. 3-4: Mega-cluster caveat forecast no. 2 and 3 temperature anomaly projection in 15 days (Dec. 8-9, 2021).