It certainly doesn't help matters when we get a twin westerly QBO regime, and the WAFz is not very disruptive (aside from Hurricane Nicole Nicole in October) wrt weakening the polar vortex with equatorward & cross-equatorial rossby wave progression dominating the picture. The Siberian Snowfall Advance Index serves as an approximation for wave activity flux (essentially measuring the intensity of tropospheric rossby waves' upwelling into the stratosphere & depositing westerly momentum which slows down the easterly polar night jet surrounding the Polar vortex), and is useful in a sense that it gives us a general idea of the intensity of mid-latitude rossby & planetary waves and their location wrt the mid-high latitude waveguide (a waveguide refers to features such as jet stream, mountain range, baroclinic zone, or any other feature that changes the amplitude & movement of Rossby Waves). The SAI's timing is such that its measured when the polar vortex observes its most rapid evolution (which usually occurs in October), and the values are dependent on the placement of these Rossby Waves, if they are positioned in a way that enhances the standing planetary wave configuration (Planetary waves are essentially quasi-stationary, large waves influenced by seasonal changes & mountain ranges), SAI values increase (You should take note once again that the Plaentary Wave configuration is dependent on the large-scale mountain ranges, with ridging occurring on the windward (western (in the NH)) side of a mountain range, and troughs on the lee (eastern) side. In North America for example a significant portion this planetary wave configuration can be described by the PNA pattern with a mean ridge over the Rockies, trough in eastern North America & trough near the Aleutians) (hence why you should note that most very high SAI years occur during El Ninos because El Ninos tend to enhance the PNA pattern). There are other parameters that can significantly modify or skew this WAFz indicator such as abnormally low arctic & Barents/Kara Sea Ice, an extreme NINO (as was observed last year) or extreme westerly QBO regime (as we're currently observed), both of these aforementioned features but have dominated the past several years in some way, shape, or form, have occurred with relatively low frequency in the observed record, although one (low arctic sea ice) or more of features may become more prevalent in the coming decades. This just goes to show once again that one-dimensional indicators can't provide a complete picture of a multi-faceted, highly non-linear system & phenomena such as the AO, you really have to look @ the large-scale picture instead of cherry-picking only what you want to see.
As far as February is concerned, I think we may have a decent indication of the probability of February turning out to be above average &/or a blowtorch as most guidance suggests or colder than normal once January's pattern unveils itself. Based on what I've seen wrt the progression of winters with a similar ENSO/QBO background, if an anomalous trough is closer to southeastern Canada & the Great Lakes and high-latitude blocking dominates Scandinavia & the far eastern North Atlantic in January, we might be in business in Feb. However, if the trough is closer to the Canadian Rockies & there's little-no high latitude north Atlantic blocking (as was the case during the warm set of WQBO/ENSO years), we may be in trouble. Even if we observe the former pattern, there's still no guarantee of a colder than average Feb, the probability would certainly increase though...
