The 12z / 25-DEC-14 run of the ECMWF continued its current campaign for a sudden stratospheric warming (SSW) event come JAN.
Progs suggest the birthing of daughter vortices around 04-JAN-15 induced by the upward propagation of a large tropospheric Rossby wave in a QBO-east environment. A 'major' SSW event is indicated.
"A major SSW occurs when the 10 mb 60°N zonal mean zonal wind reverses from westerly to easterly and the 10 mb zonal mean temperature gradient increases poleward of 60°N." (.pdf)
Another view of the impeding split at the 475K level from a potential vorticity (NASA) perspective ...
The downward propagation of the stratospheric easterly circulation (or a significant weakening of west wind) and the establishment of the Arctic Oscillation (AO) in its negative phase usually follows major SSW events.