To dig into this a little deeper, I examined the evolution of the 500 mb height pattern in the years that saw a significantly negative (bottom quintile) PDO index on December 21, which is the peak of the anomalous "warmth" in the figure above. Fortuitously, all 17 of the years occurred between 1948 and the present, which is the era of the NCEP/NCAR reanalysis data; none were found in the period 1930-1947.
The sequence of maps below shows the 500 mb height pattern at 10-day intervals for the 17 negative PDO years, from November 1 to January 30 (left to right, then top to bottom). Blue shading indicates where the height was below normal in the majority of years, and red shading corresponds to above-normal heights. The upper-level circulation appears to have followed a somewhat consistent sequence in these years; for example, an upper-level trough was frequently in place over Alaska on November 11. The interesting feature in connection with the late December warmth is the ridge that develops south of Alaska in December in the majority of years; the December 21 map shows that above-normal heights tend to encompass the southwestern half of Alaska on that date. However, the anomaly is quickly erased by the end of the year.
I also created a parallel set of maps showing the reanalysis temperature anomaly patterns in the (static) negative PDO years, see below. The cold anomaly over the interior on December 1 and January 10 agrees with the Fairbanks chart above, and we see that on December 21 warmth is common over the Bering Sea. The overall impression of the weather pattern is of volatility, with cold episodes quickly giving way to normal or even warmer than normal conditions over Alaska. This is consistent with what we saw in the previous post, that winter temperatures are more variable during the negative PDO phase than during the positive phase.