I have decided to do a detailed post on what is probably the most certain indicator of a coming cold winter that I have found in all my years of research. This is based on a climate index I developed a few years back that I call the North Pacific surface pressure index (NPS). Nine out of ten years this index has been +2 or above for the month of October, the following winter was cold with at least one major Arctic outbreak. Even the one year that didn’t deliver a great winter (2001-02) had some cold air intrusions in late January and March, with areas from Everett northward having a solid winter. While it is not certain this month will achieve the +2 threshold, it is very likely as the month to date number is sitting at over +4 with prospects for the next week to 10 days being strongly positive.
A few years ago I developed this index which denotes the surface pressure anomalies over the NE Pacific at two locations (35N – 135W and 45N – 150W). The former is close to the climatological center of the semi permanent NE Pacific surface high pressure cell, and the latter location is a region that is very important to modulating temperature anomalies over the Pacific Northwest. The index is derived by taking the surface pressure anomalies at both locations and taking the average of both readings. I have calculated the monthly totals of this index for every month back to 1948. The map below shows where these coordinates are.
The cold / positive phase of the NPS is marked by abnormally high surface pressure over the region in question. The composite map below shows a typical example of the NPS in its positive phase. This phase is often associated with northerly or NW flow and below normal temperatures over the PNW.
The warm / negative phase of the NPS is marked by abnormally low surface pressure over the region in question. The composite map below shows a typical example of the NPS in its negative phase. This phase is often associated with south or SW flow and above normal temperatures over the PNW.
The tables showing the monthly NPS values are below. They were derived from monthly reanalysis data of monthly surface pressure anomalies at the aforementioned locations.
The years listed below are the seasons that featured an October NPS reading of +2 or higher and the extreme minimums for the following winter at SEA, and for Landsburg (figures in parenthesis). All winters featured significant snowfall in at least parts of the Puget Sound region, including 2001-02 which had decent snowfall from Everett northward. Quite an amazing feat considering this group of years is based on just one criteria. It’s also remarkable that many of these winters had 2 or more significant Arctic outbreaks.
1949-50 = 0, (1)
1956-57 = 9, (2)
1961-62 = 13, (5)
1971-72 = 12, (6)
1972-73 = 13, (4)
1985-86 = 10, (7)
1990-91 = 12, (6)
2001-02 = 25, (25)
2006-07 = 19, (14)
2013-14 = 19, (12)
Average = 15.1
(Average) = 8.2