This feature will see transient resonance that leads to a split the vortex. Here's the earlier GEOS run. Watch how that semi-permanent feature attaches with the upper strat ridge, causing the vortex to split. This is the beginning of a flip to a strong -NAO state ahead. pic.twitter.com/uyFhCuQg6l

— Anthony Masiello (@antmasiello) December 31, 2018