The PNA, the acronym for the Pacific North American index, is based in the northeast Pacific Ocean and has tremendous effects on the weather observed in North America. The PNA has two phases: positive and negative. In the positive phase, cold air tends to flow south and the storm track is shifted so it favors the Midwest/Great Lakes for precipitation. In a negative PNA, warmer weather is favored in the nation, while the storm track tends to miss much of the Eastern US. For example, in the Chicago Blizzard of 2011, there was a positive PNA happening that enabled the blizzard to go where it went.
The PNA is enhanced by sea surface temperatures. In above normal SST anomalies, the Pacific North American index tends to favor a positive phase over negative. In negative SST anomalies, the negative phase of the PNA is supported.
In the above one month change of SST's, we can see that warming has taken place across the northeast Pacific. Cooling has also taken place across the north central and northwest Pacific, but that is another post in itself.
The fact that we are starting to see warming in the Northeast Pacific is exciting. If such a warming trend continues through the month of November, the chances of a positive PNA will only continue to increase. This would, in turn, lead to more precipitation in states like Colorado and the east central Rockies, as well as the Midwest and Ohio Valley. A sturdy negative NAO would only help the Northeast as well.