1. PDO/PNA
The PDO (Pacific Decadal Oscillation) is an index derived from North Pacific sea surface temperature anomalies and it has a high correlation to the PNA (Pacific North America) pattern.
A positive phase of the PDO corresponds with a positive PNA pattern which is generally a ridge over western North America and a trough over eastern North America and the opposite is true of the negative phase of both. The long term phase of the PDO has been positive, but there can be short term oscillations, thus making it difficult to predict. We are currently in a long term positive phase of the PDO (Dec 2006).

2. PDO/PNA
THE PDO (Pacific Decadal Oscillation) and PNA (Pacific/North American Index)
The PNA/PDO are major factors to the global pattern in that they are part of what determines when and where the ridges and troughs will set up downstream, especially throughout North America. Below is a 500mb pattern map of winters where we saw a positive PNA. Blue signifies lower heights (trough), and red signifies higher heights (ridging).
As you could imagine, a negative PNA would do the opposite. The correlation between the PDO (which is more of a SST index), and the PNA (which is a pattern index) is greater than 0.80 in the winter months.

3. PDO/PNA ( Trends)
As you can see, we are clearly heading into, and staying in a negative PDO Fall as the SSTs right now could make the PDO index severely negative. To get a good idea as to where we are headed for the winter, I looked at years where we saw a +PDO Spring and Summer, followed by a -PDO Fall. Out of the years mentioned above (years where we saw a +PDO Jan-July), 9 years saw a -PDO Fall in that the PDO averaged out to be below 0.0 for the Sept/Oct/Nov period. Out of the 9 years that saw a -PDO Fall after a +PDO Jan July period, 7 of these years (78%) saw a drastic rise in the PDO through the early winter making it positive for the winter as a whole (DJFM average). Since the PDO will most likely be rising from negative to positive through the late Fall and Winter, I believe that the odds are in strong favor of a +PNA.

4. PDO/PNA ( Trends)
Below are the PDO readings for Jan-July 2006 Jan: +1.03; Feb: +0.66; Mar: +0.05; Apr: +0.40;May: +0.48; June: +1.04; July: Since 1900, there have been 26 years where each month from January to July saw a positive PDO reading. (1900, 1902, 1906, 08, 28, 31, 34, 36, 40, 41, 42, 58, 77, 81, 83, 85, 86, 87, 88, 92, 94, 96, 97, 2003, 2004, 2005) Of these 26 years, 21 of the following winters (81%) saw a +PDO in that the DJFM PDO average was > 0.0.

5. Interpretation of PNA and NAO
Interpretation of PNA and NAO indices on computer forecasting models.

6. PDO/PNA (DT)
The map below explains HOW and why the PDO affects the pattern across North America. As you can see when the PDO is in the Positive or WARM phase...the warm sea surface temperature anomalies along the West Coast North America strongly supports a ridge developing over the western portions of the continent and locking this Ridge in place. Of course if you have a Ridge over the West Coast then downstream you have to have a trough....(for every action there is the equal opposite reaction)...which in this case is over the eastern third of the US.
This pattern is known as the +PNA and it is the classic signature for a cold weather pattern. The arctic air masses from the Arctic regions and northern Canada follow the Jet stream down into this trough which results in sustained intervals of below row temperatures cold water. On the West Coast the Ridge of High-pressure means clear skies very little storminess and Above normal temperatures.

7. PDO/PNA (DT)
However the opposite is true what we had a negative or cold phase of the PDO. In this case the warm water is located out in the Central Pacific Ocean and cold water develops along the West Coast North America. This results in the jet stream developing a trough along the West coast...and of course downstream a Ridge develops over the eastern half the US. This sort of pattern results in a very cold and stormy West Coast with areas such as a Seattle and Portland saying snowstorms in the very strong cases out of the negative phase PDO...and the central and eastern US saying below normal precipitation and above mobile temperatures.

8. PDO and El Nino (DT)
In addition, the PDO directly affects the ability of the El Nino or La Nina event to reach the STRONG category. When the PDO is in a negative phase the El Nino events tend to be weaker while La Nina events tend to be stronger. On the other hand when the PDO is in the positive or warm phase the El Nino events tend to be longer lasting and stronger while La Nina events tend to be weaker.
-PDO = El Nino Weaker
-PDO = La Nina Stronger
+PDO = El Nino Stronger and Longer Lasting
+PDO= La Nina Weaker

9. PDO Switch and Split Jet Stream
However it is not a strongly negative phase like we saw last year which means that it's possible during the middle or second half the winter the PDO phase could change... from negative to positive. In order for something like that to occur... they would have to be a significant increase in areas of warm either over the Central Pacific that moves eastward or developing over the Eastern Pacific near the North American coast. Began at this time there does not appear to be anything indicating that might be the case but because we are dealing with a weak -PDO phase such a change could occur in the second half of the winter. Again keep in mind that -PDO means a constant stream of Pacific Energy bombarding the West Coast and it also restricts the ability of cold air pour southward from Western Central Canada into the central and eastern portions of the CONUS. However since this -PDO is a weak one ... there is a high probability that the Pacific Jet stream will split into two branches on the West Coast. This sort of split in the jet stream pattern is an indicator that forecasters look for that the pattern is about to become stormy.