This is a monster El Niño, and having such a large area of very warm water increases the potential that El Nino will dominate the weather pattern in North America this winter. It is fueling strong model consensus as well as higher-than-normal forecast confidence from the National Weather Service for a widespread warm December with wetter-than-normal weather in the South and East. I’m less confident on the precipitation forecast, but more on that below.
Here is the current forecast estimate for Washington, D.C. (National Airport) for December 2015:
December average temperature forecast
42 to 44 degrees, which is 2.3 to 4.3 degrees above the December average of 39.7 degrees.
Confidence: Medium to high that it will be warmer than average, just not sure how much warmer. There’s certainly a potential that it could be warmer!
For reference: The warmest Decembers on record were both 45.6 degrees, tied between 1889 and 1984. We came close to the record in 2001 (45.4 degrees) and 2012 (45.3 degrees).
December precipitation forecast
2.75 to 3.25 inches of rain, which is right around the average of 3.05 inches.
Confidence: Low overall
December snow forecast
Zero to a trace (normal is 2.3 inches). I’m sorry, snow-lovers.
Confidence: Medium to high
December snow in D.C. has had mixed results, with some years delivering more snow than average, and others having little to none. Interestingly, the 2000s have slightly outpaced the long-term 30-year average. I think this year will drag the average down, though, thanks to this warm-influencing super El Niño.
Here are some factors involved in our December forecast:
Big warm start
Just like November, we are getting a super-healthy head start on a warmer-than-normal month. Temperatures in the first two weeks of the month are expected to be well above normal more often than not. Highs are frequently in the 50s to even a few 60s, while lows stay mostly above freezing, especially right in the city.
We are seeing some signs of cooler volatility possible by the third week of December, but a super-warm pattern across southern Canada, as Angela blogged about yesterday, will significantly reduce cold air availability for us going into the middle third of the month.
We are also getting a wet start, but we did that in November too, and it didn’t persist through the month.
This current El Niño is a powerhouse. I looked at four other strong or super El Niño cases and what they reveal (as shown in the chart below) is strong support for warmer than normal temperatures (all four showed this), but a mixed result on precipitation (two wet and two dry). Three out of four strong or super El Niño years were also below normal on snow (which is 2.3 inches in December for National).
I worry that the drier answer could eventually verify since the two super El Niño cases were also dry — and this year’s El Niño is looking super. So far, the upper level weather patterns through the first half of December are not supportive of snow, but we will have to keep an eye on the situation from week-to-week ahead.
I’ve been very impressed by consistency of the long-lead models. The National Weather Service’s CFS model, the European long-lead model, and even the Canadian long-range forecast models are all favoring a warm-dominated month. But interestingly, they also lean wetter-than-normal.
The one thing I’ll say is that the CFS offered an excellent forecast for November temperatures in Washington, D.C., but it was too wet on precipitation. That recent failure and the mixed results on the historical El Niño analogs above are a source of my current caution (which could be negated if today and tomorrow’s precipitation out-performs expectations!).
National Weather Service
The National Weather Service outlook is in strong support of the warm and wet scenario offered by the long-lead modeling. In fact, this is probably the best month-ahead forecast consensus I’ve seen since the very warm March 2012 forecast. You can read the Weather Service’s December forecast discussion here.
Review of November’s forecast — strong on temperature and weak on precipitation
November’s forecast did very well on temperatures, but missed on precipitation. We experienced moderate to strong El Niñ0-based Pacific forcing in the first 2/3 of November, especially before a bit of mixed weather just recently at the end of the month. This bolsters our confidence on December temperatures, but offers some hesitation on precipitation.
November average temperature forecast: 50.6 to 53.6, 1 to 4 degrees above normal (normal is 49.6 degrees).
Result: 53.7 degrees (missed the range by 0.1 degree!)
November precipitation forecast: 3.5 to 4.5 inches of rain, 0.33 to 1.33 inches above normal (normal is 3.17 inches).
Result: 2.1 inches (drier-than-forecast)
I’m thinking that since the month wasn’t super-dry (12 days of rain occurred), I can get away with a B- grade given the near right-on temperature forecast. I’m aiming for a better score in December thanks to an assist from El Niño!
Let us know what you think in the comments!
More winter outlooks: