This fall's weather follows many of the challenges of the Summer Outlook regarding the analogues since we are in a process of change from La Nina phase, to a Neutral and back to a La Nina. This was seen last in 2008 (one of the analogues) however timing is a bit different. That being said, I feel under the present Neutral conditions, the least effect occurs downwind across the country and particularly the Great Lakes. In a word or two, there is no effect and we basically will be at the mercy of the phase North Atlantic Oscillation /NAO/ and the upper level ridge over the south-central portion of the country. Therefore, this is why one must look to these other patterns affecting the region and researching back in time to see similar various patterns under the broad-scale Neutral Pacific SST.
Latest seas surface water temperature /SST/ readings and projections are the most challenging in regards to timing and outcome. The latest data is already showing subtly cooler water temperatures are beginning to take hold in the Pacific (especially Nino 3.4) where phase or state of the ENSO is determined.
In addition, model projections are somewhat narrow but straddle an important area for determining downwind effects across the country this fall and winter. Extreme ranges varying from better than a +1.0 /El Nino/ to less than a -1.5 /La Nina/. Of course these are the outliers but outliers should not be dismissed as they are not always wrong (as my Analogue data has shown, albeit infrequently). In any event, projections and very recent current data suggests a Neutral to weak La Nina scenario is the likely along with downwind effects and subsequent weather for the upcoming Autumn and Winter.
Maybe not surprising, this exact pattern is an infrequent pattern to research after the 1970s. This is one of the main reasons the 1970s have been in the forefront in my Outlooks written in the last decade (and we'll look at this in more detail in my Winter Outlook)
The Pacific Decadal Oscillation (PDO) is the long-term ocean fluctuation of the Pacific Ocean. The PDO waxes and wanes approximately every 20 to 30 years.The cool phase is characterized by a cool wedge of lower than normal sea-surface heights/ocean temperatures in the eastern Pacific and a warm horseshoe pattern of higher than normal sea-surface heights connecting the north. The last time the PDO trended into the negative phase was in the early 1950s, lasting into the late 1970s.
The Pacific Decadal Oscillation water temperatures appear to have reversed to this dominant cooler than average phase. I've spoken about this several time the last few years in my Outlooks. I exhibited this PDO graph in the last few years of my Outlooks and highlighted the change /red line/.
This reversal to cooler water temperatures is more conducive to La Nina development as opposed to El Ninos (which dominated more from the early 1980s to the early 2000s). Between the two dominant phases (El Nino & La Nina), lies a more neutral or "period of change" as I'll call it. This is the time period where one phase weakens and the other begins to show strength. In my previous Outlooks, I felt that we started this "period of change" during the early-mid 2000s, as seen below, in our ENSO chart.
Indications are temperatures this fall will average near normal (or -1.5 to +1.5 degrees of normal) but getting there will be interesting. The data in our analogue falls remains quite mixed and looking at that, along with the upper wind projections, suggest end results will be near normal. The thing that is worth noting were the sharp contrasting cold snaps that periodically erased the longer warm spells. However, in spite of those sharp cold snaps throughout the fall, it also should be noted that October and in some years, early November had the best chance to contain some of the nicest weather of the three months with a couple of periods of Indian Summer weather thrown in (see below).
Our last two analogue fall years, 2008 and 2000 both had wet early falls with 2008 also having tropical influenced rains. Preference was for normal to above normal rainfall in our analogues with four wet and three normal falls out of the ten. September was likely to be the wet month with October the drier relative to September and November. Overall, this trend for normal to locally above seems to carry over from the summer rain pattern.
2011 Autumn *Analogues
once the fall pattern unfolds.
Look for the Winter Outlook early November!