This follow-up provides a quick look at the average 500mb height patterns associated with the most severe cold spells in two contrasting periods, 1931-1960 and 1981-2015. To examine this I used data from the 20th Century Reanalysis, courtesy of NOAA/ESRL/PSD. The 20th Century Reanalysis uses historical sea-level pressure and sea surface temperature observations to produce an evolving 3-D estimate of atmospheric conditions back to 1851. Owing to the strong correlation between sea-level pressure and upper-level heights, the historical 500mb height estimates should be broadly correct back to the early 20th century or before; by 1925 there were already well over 1000 daily pressure observations north of 20°N (although not at all evenly distributed around the Northern Hemisphere).
The two sets of maps below show the mean height anomaly (departure from normal) for 1931-1960 cold spells on the left (as defined in the earlier post) and 1981-2015 on the right. The first pair of maps is for 10 days prior to the first day exceeding the cold threshold, and then pairs of maps are shown at 5-day intervals until 15 days after the cold spell onset. Click to enlarge the images.
| 1931-1960 | 1981-2015 | |
| -10 Days |  |  |
| -5 Days |  |  |
| 0 Days |  |  |
| +5 Days |  |  |
| +10 Days |  |  |
| +15 Days |  |  |
With respect to the main question of interest here, it's immediately clear why cold spells in the earlier period tended to last longer than more recent cold spells. In 1931-1960 the average flow pattern remained quite similar from the date of onset through 10 days later, with high pressure over the Bering Sea and low pressure over western Canada; this pattern funnels cold air southward across the interior. In contrast, the 1981-2015 pattern is completely different by +10 days, with low pressure over the Bering Sea and high pressure over western Canada; this pattern brings warmth to Fairbanks.
It's also interesting to note that the precursor flow patterns were quite similar between the two periods at a lead time of 10 days, with high pressure over and south of the Aleutians and low pressure over the Arctic Ocean. However, at 5 days in advance, the modern pattern is much more amplified, with a very strong northerly flow anomaly over the Bering Sea (rather than northwesterly in 1931-1960). It seems as if modern severe cold spells tend to arrive with greater ferocity, but the high-amplitude pattern continues to evolve rapidly so that the cold blast is followed quickly by warmth. In the "old days" the cold-advection pattern gives the impression of being more stable and long-lived, at least as far as the Alaska-centric flow anomalies are concerned.
A possible next step would be to investigate historical sea surface temperature anomalies to see if we can identify a reason for the preferential patterns shown above.