We are currently in a strong El Niño which is likely to remain strong throughout the winter even as it slowly moderates. What that means for our winter weather is still questionable.
A strong El Niño usually leads to an active southern jet stream and more storms tracking across the South which leads to wetter and often cooler than normal weather in these areas. By contrast, warmer than normal weather dominates the northern tier of the U.S. during strong El Niño events.
The D.C. area is caught in that pesky zone between the cooler than normal pattern across the south and the warmer than normal zone to our north. Therefore, our temperatures are most significantly impacted by other factors.
Also, not all El Niños act the same way.
This year has been described as a super El Niño since waters in the tropical Pacific are abnormally warm, ranking among the top three warmest on record.
A scatter diagram plotting the winter Ocean Nino Index (ONI), a measure of El Niño’s strength, versus the winter temperature anomalies (from the 1950-2015 average) shows how little relationship there is between El Niño and our winter temperatures (see below).
The vertical blue line on the diagram indicates instances in which the winter ONI values exceed 0.50, reflecting El Niño conditions. Of the ten years in which ONI was 1.0 or greater, indicating at least moderate El Niño conditions, half exhibited colder than normal temperatures and half were warmer.
The four highest ONI years, indicating strong El Niño events like this year, all produced temperatures warmer than normal. But using such a small sample size to make a forecast is risky especially when the correlation between the ONI and temperatures are weak or essentially non-existent. Really toasty years have occurred during both La Ninas and El Niños.
Let’s look at the relationship between snow and El Niño. The average snowfall for all the winters with an ONI of 0.50 or greater is 20.8 inches significantly higher than during La Nina or neutral winters. Six of our 11 snowiest winters since 1950 coincided with an El Niño.
Before snow lovers start celebrating, they should also be aware that of the 10 strongest El Niño winters, half produced snowfall totals well below average and half produced amounts well above.
Essentially, whether the D.C. area totals more or less snow than normal during a moderate to strong El Niño is a coin flip.
Complicating the picture is the fact that more than 40 inches of snow were recorded in two of six strong El Niños while only 0.1 inches of snow were measured during the two of the others. The typical El Niño storm track can help lead to snowy winters but the bottom line is you need a pattern to lock in the cold and also need to have a dollop or two of luck.
The Arctic Oscillation (AO) and North Atlantic Oscillation (NAO)
So what other factors might help us know how cold it will be?
Two important drivers are the Arctic Oscillation (AO) and its close cousin, the North Atlantic Oscillation (NAO). They are so closely correlated that they are often discussed interchangeably. In the D.C. area, the AO correlates a little better with our winter temperatures than the NAO so I’ll focus on the former.
When the polar vortex, the frigid high latitude region of low pressure, is stronger than normal, the AO is in its positive phase (see right hand figure below).
A positive AO usually keeps the surface pressures across Canada lower than the pressures farther south in the mid-latitudes. Since air moves from areas of higher pressure to lower pressure, the cold arctic air gets locked in place across Canada and has a hard time plunging southward into the U.S. During the positive phase, storms usually track to our north putting us on their warm side as they cross the U. S. to the East Coast.
By contrast, pressures across Canada are higher than normal during the negative phase of the AO. As air moves from regions of higher pressure to lower, a negative AO helps facilitate the southward movement of cold air into our region. In this pattern, the jet stream and storm track is often located south of its normal seasonal position giving us a better chance at having a storm track to our south which tends to keep us on the cold, snowy side of storms.
You can get a feel for the importance of the AO on temperatures by looking at scatter diagrams of the ONI index versus the AO for the 17 coldest winters since December 1950 (top image) and 18 warmest winters (bottom image).
The AO was negative for each of the cold years (top image) no matter whether it was an El Niño, La Nina or neutral year. To get a really cold year, having a negative AO is imperative.
A positive AO was present for 14 of the 18 warmest winters since 1950. Clearly, with a positive AO, the probability of a warm winter increases. Eight of the warmest years occurred during La Niñas with five happening during strong La Niñas. However, a strong El Niño was present during three of the really warm winters, and even one that was accompanied by a strongly negative AO.
While a negative AO increases the chances of a colder than normal winter, there are other factors that also help determine whether a winter will end up cold or warm. Statisticians will be quick to point out that the AO only explains only a little more that 26 percent of the variance of the winter temperatures in D.C.
In terms of snow, the importance of negative AO for big amounts is apparent in the scatter diagram of the AO index versus the ONI index for all the years when more than 20 inches of snow was recorded in D.C. since December of 1950 (top image below). All but two of the 20 inch or greater winters were associated with a negative AO.
Seasonal prediction researcher Judah Cohen has found a rapid increase in snow in Siberia during October very frequently portends a negative AO during winter, and this year the snow advance was faster than normal. If Judah Cohen’s call for a negative AO is correct, the chances of a snowy winter increases, but it’s not a sure thing.
In 7 of the 19 seasons with below normal snow or 36.8 percent of the time, the AO was negative. In other words, even if a forecaster is correct with a call that the winter will experience a -AO for much of the winter, the AO is no panacea for long range seasonal prediction. The winter of 1997-1998 experienced a strong El Niño with a strongly negative AO and NAO but only produced 0.1 inches of snow in D.C.
The Eastern Pacific Oscillation (EPO)
The EPO index has a correlation with our winter temperatures only slightly lower than that for the AO. A negative EPO can sometimes offset a positive AO and produce a colder than normal winter.
The last two winters are prime examples of the importance of a -EPO or blocking pattern in the northeast Pacific. Most remember the series of cold air masses that fed into the region despite those winters being under the sway of a positive AO. The EPO was the culprit.
Last winter, the AO and NAO were strongly positive because of the below normal pressures from Iceland across Greenland into eastern Canada. But the EPO was strongly negative (see below).
The oval annotated on the map roughly indicates the area where the EPO index is calculated. When pressures are above normal across the northern portions of the area and below normal to the south, a negative EPO is present. Last winter, the strong high pressure ridge across Alaska and northwestern Canada helped drive cold air southward into the eastern U.S.
By contrast, when the EPO is positive, the pressures over Alaska are below normal with above normal pressures to the south. Under these circumstances, the jet stream across the Pacific is stronger than normal and those strong westerly winds help drive Pacific air masses into Canada cutting off the cold air supply for the eastern U.S. A positive EPO tends to produce warmer than normal temperatures across the country especially the northern Plains and upper Midwest.
Putting it all together
Cold lovers should be hoping that the EPO and AO indices both stay negative this winter as that is the best combination for getting really cold air in the area. Those who like warm weather should root for positive readings of the AO and EPO indices.
Snow lovers may want to know: Does a cold winter support a snowy winter? Your grandmother would’ve said so and that common sense answer is the right one.
Cold temperatures correlate with a snowy winter better than any of the indices I just discussed.
Below is a scatter diagram of winter temperature anomalies and seasonal snowfall.
The best predictors of a snowy winter are, in this order, cold temperatures, the AO, NAO and the ONI (El Niño).
As I don’t have a lot of confidence in the temperature forecast this winter since the dominant phases of the AO and EPO aren’t obvious, I don’t have a great sense as to how the winter will ultimately pan out in terms of snowfall.
Moving ahead, my focus will be in analyzing the weather patterns out two weeks, which are more predictable. I will publish two week outlooks on CWG starting later this month.
In constructing the two week outlooks, I pay attention to the forecasts of the AO and EPO but I put more stock in model forecasts of pressure and temperature and their uncertainty (conveyed by ensemble predictions which show how well the various models agree with each other). I also look at what similar patterns in the past have produced.
Usually the best you can hope for with a two week outlook is to tell whether the pattern looks wetter or drier and warmer or colder than normal. Occasionally, the model forecasts are advertising so much uncertainty even that is impossible. Anyway, winter is coming soon. We’ll have to wait to see what it brings.