After above normal temperatures graced most of the region over the Summer, it’s now time to look ahead to what we can expect for the Autumn months of 2016. Autumn is a transition period between the heat and humidity early on, to the snow and cold that awaits in the next season. It is a time when plants cease to grow, and trees change their colors and eventually lose their foliage. Some of the most extraordinary and breathtaking photos are captured of nature’s natural beauty each year during the Fall months.
Autumn of 2016 Seasonal Outlook
The outlook will break down the three month period of September, October, and November with temperatures and precipitation. This will be preceded by a technical discussion of how this outlook came about, followed by the monthly breakdown.
State of the Climate:
It has been well documented that Sea Surface Temperatures, hereinafter referred to as “SSTs” in this discussion, drives global climate. So it is fitting that we start the outlook with a look at the current state of SSTs and where we are headed. The area of most interest that has the greatest influences on our weather is along the equatorial eastern Pacific regions. The measure of these SSTs in that region are compared to long-term climatological “normal” values, and are anomalously warmer than average (El Niño), near average (ENSO neutral), or cooler than average (La Niña). All three play very distinct roles in shaping and evolving our climate with both temperatures and precipitation in the big picture.
However, no two are alike, and there isn’t a finite “If A then B” conclusion of how a season’s forecast will transpire based on past climate analogs. There are many factors other than SSTs in the equatorial Pacific. But this provides a good starting point. For example, 2007-2008 was a La Niña year for this region, but that doesn’t mean this year will be exactly the same. Some are stronger than others, and even the ones that hold similar ONI values (which is the measure of El Niño/La Niña strength) have had different additional climate considerations… such as the Pacific Decadal Oscillation, hereinafter referred to as “PDO” in this discussion, whether that would be in a warm, cool, or neutral phase. And there are others, but this is just one example of an “additional” factor.
Current SST progression – past 4 weeks in the equatorial eastern Pacific
Current SST anomalies compared to normal, 4 week progression in the same region
We can see from the above that not only have we reversed the strong El Niño that was present in this same region last winter into last Spring, but has also neutralized it over the summer, and is now transitioning into a weak La Niña state. Using climate model projections, most see this transition continuing into the Autumn months:
The mid-August plume of model ENSO predictions shows the current state at the brink of a weak La Niña. Typically “ENSO neutral” is measured in a range of +/- 0.5°C, and a weak La Niña begins when SST anomalies reach or exceed -0.5°C, which is where we are now. Using the graph above, there is still a considerable spread on where we go from here, but the mean or average places the upcoming Autumn and even Winter months in a weak Niña state, between -0.5°C and -0.8°C.
So for all intents and purposes, we will base this outlook on the assumption of a weak La Niña state for the Autumn months, and will see where we are at again in November for possible Winter implications down the road.
Going into the summer, we weighted the 1983 analog to correctly predict a hotter than average summer. 1983 is still a leading candidate as we head into the Autumn months, but there are others. Important to analog forecasting is to match up not only current ENSO state, but also looking at both past months leading up to where we are now, and where we are projected to go in the coming months.
That being said, the closest matches to this year are highlighted below with a red box next to the year. They are 1964, 1966, 1983, and 1995. There were other considerations, but these stand out as the closest matches of an El Niño transitioning into a weak La Niña the following year:
Important to note here, the 1983 analog remains a frontrunner, not only with ENSO state, but also with other factors, such as the state of the PDO. The main reason is we transitioned from a strong El Niñ0 the prior year, to a weak Niña the following year, and that is the case this year. 1966 is probably the least favored in this group of analogs. I see no reason to abandon the 1983 look at this time for the Fall months, and climate models are backing this up.
For copyright reasons, I cannot show the EPS/monthlies, but I can show the JMA climate models. Below are a side by side comparison between September, October, and November 1983 and the JMA projections for those same months this Autumn:
September of 1983 featured warmer than average temperatures across the region, with the month as a whole finishing generally +1°F to +2°F above.
October 1983 showed much of the country slightly above normal, but our neck of the woods had enough of a cool air intrusion mixed in to keep temperatures near to slightly above average.
November 1983 saw the western US begin to cool a bit, and anomalous warmth continued from Texas into the Ohio Valley. Our area returned to slightly above average temperatures.
The JMA 500mb heights for September would favor above normal temperatures in our region, while a trough dominates the Plains states to our west.
The JMA 500mb anomaly for October projects the best anomalous warmth to remain to the Pacific northwest to Northern Great Lakes. Our area more likely near to at best slightly above average.
The JMA 500mb anomaly for November shows the ridging in the east, while a trough establishes itself in the western US and western Plains states. This would be above average for our region.
EPAWA month by month projections
The following is a breakdown in three monthly increments of how we see the Autumn unfolding for our region. Overall, we expect a warmer than average three month period, with the highest anomalous warmth occurring in September, and again in November.
We do feel that the warmest anomalous warmth for both September and November will occur in the 1st half of the month, with more troughiness returning for the 2nd half of both months. October will likely be the opposite. Cooler in the beginning of the month, warmer in the 2nd half.
Precipitation will be a commodity this Fall, and we are projecting below normal precipitation to continue through the first half of October at least. Beyond that, analogs would suggest a return to at least near normal values. Ridging persisting well into November as the JMA projects will have implications on dry conditions continuing longer than projected. The Euro weeklies suggest a wetter period in the 2nd half of September, but part of that looks like it incorporates tropical systems and remnants. That is always a wildcard in precipitation forecasts, similar to the fluke blizzard in winter. It can always throw off a seasonal forecast.
About EPAWA seasonal forecasting:
We take great pride in our seasonal outlook product accuracy, and take it very seriously. Many argue that seasonal forecasting is done without a high degree of accuracy, but we have had a streak of dependability and success which began in the Spring of 2012. Research is compiled using a combination of analogs, current climate, observational data, and lastly, model projections. Seasonal forecasting with more detail is offered through our private client services… please visit http://epawaweather.com/private-forecast-services/ for more information.
Forecaster: Meteorologist Bobby Martrich
Contributions to the outlook by Meteorologist Dom Carmella
August 31st, 2016