A warm December in Washington, D.C., was somewhat boring for winter-lovers, with only a few minor traces of snow early in the month.

The prevailing jet stream pattern throughout the month favored warm weather, which contrasted with our colder-than-average November. That pattern also helped to scour out the cold air supply to the north in Canada. At the same time, we did see an active pattern with fairly frequent weather systems and many days reporting precipitation.

Now the pattern seems to be shifting to favor cold, snowy weather in early January, though it’s uncertain how long that will last, as long-term indicators continue to show signs of more warmth to come.

Let’s dig into the details.

Temperatures

December 2014 averaged four degrees warmer than normal, which makes it the fifth warmest of the 2000s and the 14th warmest on record in Washington. Below, you can see all the years ranked from warmest (1889/1984’s 45.6 degrees) to coldest (1876’s 27.1 degrees) with this year in the upper segments of that range.

This year’s mean December temperature of 43.7 degrees was the warmest since 2012’s warmer 45.3-degree outcome.  We hit 70 degrees once and dipped into the 20s only once at Reagan-National Airport. The table below illustrates daily December detail including only six days of below normal temperatures at National.

Precipitation

The first chart below shows all the outcomes for precipitation in December for D.C.’s official reports from the super-wettest 7.56 inches in 1901, to the driest just twelve years earlier, 0.19 inches in 1889. December 2014 lands on the higher side of the averages greater than 30 years, with its 3.5 inches being the 48th wettest of all-time, but not as much as 2013 when we went above 5 inches for the month.

The second chart below shows the daily details. There were several days when we traced with clouds and very light precipitation not listed there — it was quite a cloudy and frequently damp month.

Records

There were no records noted for any of the region’s three airports — Reagan National, Dulles, or Baltimore-Washington International.

The weather pattern

The last several weeks of measurements have shown the central tropical Pacific Ocean to be in a weak El Niño state (it won’t be official until we have seen three months’ worth of El Niño conditions, but it essentially is one at this point as other met agencies have noted).  El Niño patterns can be cold and snowy in their weaker state, but they can be warm and wet in stronger versions.

What was interesting about December was that while water temperatures were on the weak side of an El Niño, the atmospheric response in terms of global momentum and wind was stronger-than-expected.

The December weather pattern over North America was therefore a warm-dominated one with frequent weather systems including the classic onslaught of West Coast precipitation. The pattern was too active from a Pacific standpoint to allow blocking patterns to build in high latitudes to transport cold air south. In fact, much of Canada’s cold air supply was also scoured out.  We have only just recently seen it return.

2014 overall

The warmer-than-normal December helped push Washington, D.C., to be slightly warmer than normal on the whole for the year, but its positioning among the 2000s held steady as the 6th coolest and 6th wettest (just barely scraping ahead of 2013 on the precipitation front).  Other stations around the area were estimated to end 2014 just cooler than the long-term 30-year normal instead.

On the whole though, the cold first quarter of 2014 combined with the relatively mild summer outcome helped keep 2014 on the lower end of the 2000s range and very close to the 30-year normal mean as shown below.

Meanwhile, a reversal in the Pacific Decadal Oscillation (from negative to positive) and the emerging El Niño helped DC to run wetter-than-normal on the whole.

decannual2014precipJanuary 2015 outlook:  Lower than normal confidence

The new year is kicking off with some actual winter-like weather. Expectations are growing for cold weather to return next week for much of the eastern 2/3 of the U.S., including the possibility of light snow for the D.C. region.

That will surely be welcome news for snow-lovers, but it’s not clear how long it will last. A surge of high pressure ridging over Alaska is responsible for much of the change we’ve seen this week and next week, but alas, it seems like its foothold of dominance is weaker this year versus last. Already, extended range guidance is starting to waver, and we see no significant signs of more traditional cold air mechanisms kicking in to keep the winter party going beyond the next week or two.

In fact, some very latest extended range guidance from the European tools favor a warmer pattern for the middle third of January before a cold pattern attempts yet another comeback late month.

The National Weather Service (NWS) final January outlook favors equal chances for any temperature or precipitation result for the Mid-Atlantic reflecting their very low confidence on the month ahead. In their discussion, they note the complicating factors of the current pattern change, the discrepancies in the El Niño component, and their preference for increased caution. I tend to agree that the chaos level seems higher right now (and also in general this winter), and the Weather Service correctly forecast a warm December outlook for the Eastern U.S.

You can look at the NWS forecast here and see their discussion about the forecast here.

For further information

The National Weather Service publishes nice monthly assessments about five days into the start of the next month. You can click on your closest airport location here: