A huge, unusually deep and persistent dip in the jet stream developed over western North America - indicated by the deep purples in the map above (left side). While it was springlike in the East, Arctic air flooded the Pacific Northwest.
Consider what happened in Seattle as described by KOMONews:
As the calendar turned to 1950, region became gripped in a 6-week arctic blast that wiped out nearly every cold temperature record we have -- and almost all of those records stand today as we have yet to have even come close to approaching them since.
Incredibly, 63.6” of snow fell in Seattle that month, including 20” on January 13.
On the flip side of that trough in the jet stream, a huge ridge bubbled up over the East Coast, bringing the anomalous warmth.
This trough/ridge configuration (left side) is a classic example of the negative Pacific North American pattern which characteristically brings cold to the West and warmth to the East.
Unlike January 2012, the 1950 flow across the northern hemisphere was highly amplified and oriented north-south (this is known as meridional) leading to the extremes in warmth and cold. Washingtonians in 1950 were lucky (or unlucky, depending on your perspective) enough to ride the warm side of the see-saw.
This year, the flow has been more zonal - or west to east. Recall, we’ve frequently discussed a lack of blocking flow, which has kept the cold air at high latitudes rather than forcing it south to interact with moist, tropical air and form big storms.
The zonal flow in 2012 is apparent in the right panel above by examining the elongated strip of yellows and greens extending from northern Europe across northern Asia and then across the Pacific into western North America (as well as the strip of elongated blues to the south).
Zonal flow inherently brings more moderate weather conditions than the amplified (north-south or meridional) alternative. Under zonal flow, Arctic air and tropical air are basically staying at their latitudes of origin, swirling around the world in a straight line, rather than bouncing up and down.
Snow in January 1950?
As you might expect with all of the warmth in 1950, snow was virtually non-existent. Just 0.3” fell. Februrary 1950, which had near average temperatures, recorded just a trace whereas March received 3.1”.
(Thanks to Steve Tracton for providing the 1950 map above)