Will El Niño Bring a December Warm Wave to North America?

If you’re looking for an old-fashioned holiday, you may be out of luck across large parts of the U.S. and Canada, at least when it comes to December cold. El Niño climatology and seasonal forecast models are pointing toward high odds of a very mild December across most of the continent east of the Rockies and north of the Deep South. We wouldn’t expect every day to be unusually balmy--and in December, “warmer than average” can still be quite chilly--but the analogue years and the model forecasts do raise the possibility of at least a few days of record-melting weather across a vast area.

How the effects of a strong El Niño unfold during autumn
WU contributor Eric Webb (@webberweather) tweeted a powerful image the other day (Figure 1 below). It’s a composite showing the likelihood of above- and-below-average temperatures for each month during strong El Niño events (11 in all since 1895). Notice how the composites change dramatically from October to November to December, before settling into the prototypical mild-north/cool-south pattern for January through March.



Figure 1. Month-to-month variations in average temperature during strong and “super” El Niño events between 1895 and 2014. Temperature departures are shown in blue/green colors (cooler than average) and red/orange colors (warmer than average), as calculated against the long-term average for the period 1895-2000. The El Niño events in these composites (peak Niño3.4 indices of at least 1.5°C above average for at least three overlapping three-month periods) include 1896-97, 1902-03, 1930-31, 1940-41, 1957-58, 1965-66, 1972-73, 1982-83, 1987-88, 1991-92, and 1997-98. Image credit: Eric Webb, @webberweather, using a mapping/analysis tool from NOAA’s Earth System Research Laboratory.


Figure 2. Departures from average in U.S. temperatures for October 2015. Image credit: NOAA National Centers for Environmental Information.


Thus far, the 2015-16 El Niño is behaving much as you would expect a strong event to behave in terms of U.S. temperature effects, with a spoonful of long-term warming and a few sprigs of natural variability mixed in. Figure 2 shows that we saw a warmer-than-average western and central U.S. during October 2015--in fact, record-warm in some areas--with mostly near-average temperatures near the Ohio Valley and along most of the East Coast. This roughly matches the west-to-east contrast depicted in Figure 1 for October, provided you bump up the temperatures from Figure 1 across the board. For the first few days of November 2015, we’re again in the groove for the overall pattern one would expect based on the composites in Figure 1: cooler-than-average readings in the far West and mild readings in the East. The biggest departure from the El Niño composite so far this month is the marked mildness over the Midwest. We still have more than two more weeks of November to go, but NOAA’s latest 6-to-10 day and 8-14 day outlooks strongly support the cool-West/mild-East pattern that fits with the Figure 1 climatology.

What do seasonal climate models say?
NOAA’s Climate Forecast System model (CFSv2) has been singing a consistent tune over the last few days, and it doesn’t sound like your typical holiday song. For more than a week, daily runs of the CFSv2 have called for an unusually mild December across nearly all of North America, with the possible exception of Texas, Mexico, and Alaska. “With such persistence in the CFS, I don't see much room for deviation from this forecast,” says WSI’s Michael Ventrice, who adds that the ECMWF seasonal model paints a similar picture. Overall, says Ventrice, this is “some of the most anomalous warmth I've ever seen” in monthly model output.


Figure 3. Surface temperature forecasts for December from NOAA’s CFSv2 model, produced each day from November 4 (top left) to November 12 (bottom right). Temperatures are shown as departures from the monthly average in degrees Celsius, with reds and tans indicating warmer-than-normal readings for the month.

The caveats
Every El Niño event is different, and we only have two “super” El Niños in modern records that compare to the current event in strength (1982-83 and 1997-98). Even across the 11 cases encompassed in the Figure 1 composites, not every December shows the breadth of unusual mildness implied in the composites. In each of these 11 cases, the warmth shows its own distinctive fingerprint. However, very few parts of the country experienced below-average monthly readings during any of these Decembers. The exceptions are 1896 and 1902, which run cool mainly because they fall toward the cool end of the 1895-2000 comparison period, and the odd case of December 1972, which came in below average for most of the contiguous U.S. west of the Appalachians.

At NOAA’s Climate Prediction Center, forecaster Stephen Baxter provides some more perspective. Although the warm-West/less-warm-East pattern shown in Figure 2 does appear to roughly match the El Niño composite in Figure 1, October 2015 was clearly a lot warmer than the composite. Baxter points out that, by itself, the El Niño/Southern Oscillation (ENSO) doesn’t tend to produce the type of dramatic nationwide warmth seen in both September and October. Thus, he said, “To a first order, ENSO is not to blame for the warm autumn thus far." Baxter added that the CFSv2 isn’t extremely skillful predicting monthly temperatures this far out, because of the role of month-to-month climate variability. NOAA will issue its next set of long-lead climate outlooks on November 19. “We will likely have lots of red on our December outlook," said Baxter, "but with more modest probabilities than those inferred from the CFSv2. So while the odds of a record or near-record warm month are substantially elevated across parts of the northern contiguous U.S., it is still not the most likely outcome.”

The dramatic month-by-month evolution seen in Figure 1 for autumn settles down by January. At that point, the Pacific jet stream is much more likely to fall into the midwinter pattern typical of strong El Niños, with storm systems barreling into California and across the South while mild air spreads across much of the northern U.S. and eastern Canada. These atmospheric effects can persist through much of winter even if the warm sea-surface temperatures associated with El Niño begin to cool. One potential fly in the ointment is the possibility of a negative North Atlantic Oscillation, which is highlighted in the latest Arctic Oscillation outlook produced by Judah Cohen and colleagues at Atmospheric and Environmental Research. Snow cover advanced more quickly than usual over Siberia during October, and Cohen’s work has related this variable to an enhanced risk of negative NAO conditions by early winter, especially in January and February. Running counter to the warming effect of a strong El Niño, a period of negative NAO would favor intrusions of cold air over the northeast U.S. In the past, the combination of a negative NAO and a strong El Niño has led to some noteworthy winter storms along and near the East Coast, so it will be interesting to see if this pairing actually develops.

What about the ski season?
Milder-than-usual winter weather is anathema to skiers and snowboarders, but snow hounds now have a place to find winter outlooks specific to their favorite resorts. The website opensnow.com unveiled its first-ever nationwide winter snow outlooks in October (see map below), with the forecasts just updated this past Tuesday. To the best knowledge of company founder Joel Gratz, this is the first long-range seasonal snow forecast specifically tailored for ski areas. The outlooks were produced in collaboration with Amato Evan, an assistant professor at Scripps Institution of Oceanography. Evan’s technique includes the precipitation and temperature outlooks common to most seasonal forecasting efforts, but it goes a step further by translating these two variables into the likelihood of snowfall at various locations and time periods. The technique also incorporates the tendency of El Niño years to skew snowfall toward the southern half of the western U.S. ski areas, as evident in Figure 4.
Gratz adds a cautionary/encouraging note: “If we are forecasting that a ski area will see below average snowfall over a 6-month season, they will still likely experience plenty of snow storms and powder days, so don’t be too picky.”

Have a great weekend, everyone!

Bob Henson