It would be good to see some evidence. Perhaps average winter temperatures in North America (I know it's a very big area) during recent decades would be helpful. 

With global warming it should had been lmiited the such cold appers and not go further south but seem happening now it gone further south than normal even Madrid got deep cold and snow cover that by pass the UK which which we missed out few years back that enraged all of us here.

Once again the cold over there in the past had no effect to the UK 1980's winters which we still get it regardless what USA/Canada weather doing.  Most suspect is the HP that preventing us to get some cold air in, always parked in the wrong place all year around that prevent us getting seasonal weather pattern.

Was it the case in the 1980s that the States were less prone to widespread and powerful cold plunges? There seem to have been more recently, and once a plunge is in place, it would seem that we are guaranteed a powerful jet with windstorms being wound up across the Atlantic. Conversely were the 1980s US snowfalls from LPs guided southwards being guided but only temporarily by a waving jet stream, the wave propagating eastwards across the Atlantic and generating in turn LP from the north for Britain.

All very speculative, and subject to the weakness of a failing memory!

Yes we used to hear that al lot back in the day didn't we

When I looked into this once I also discovered that the troughs in the jet stream (Rossby Waves) tend to precess from west to east as a slow moving standing wave and to me this added extra weight to this old piece of "folk lore". I have no idea how long these waves take to precess across the Atlantic or how reliable that info is but when the trough is right above then it would be a southerly portion of the jet stream and encourage colder outbreaks as the depressions would often be to the south.

There was a also a reference the other day from the Met Office to the fact that the Jet Stream is getting stronger as Global Warming increases so perhaps this has caused a change we have not yet fully evaluated yet

1935:

October. Very wet in Scotland. On the 19th an intense depression passed east north of Scotland. A severe gale killed 11 people; 92 mph was recorded. There was heavy rain too, and the winds turned northerly briefly afterwards to produce a cold snap. The pressure near the centre of the low was 950 mbars.

November. Unsettled and wet.

December. Dense fog over parts of Scotland, particularly around 20-21st. There was a maximum of only -9C at Paisley.

1936:

January. Wet, except in the north-west, and mostly mild, although there was a cold, wintry spell from 12th to 23rd.

February. Cold and wet.

https://www.trevorharley.com/1935.html 

https://www.trevorharley.com/1936.html