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Posted at 12:30 PM ET, 12/19/2012

December warmth in D.C. and its signal for temperatures the rest of winter


December 2012 has featured plenty of warmth and plenty of cloudiness. Photo by John J Young, via the Capital Weather flickr pool.

If you’re a winter lover, it’s been a long December. At this point, you may be convinced winter will be a no-show like last year. But does a warm December really signify anything about the heart of the cold season still to come?

To set the stage, through yesterday, D.C. was running 7.5 degrees above normal this month. Given an expected run of seasonably cold weather to end of the calendar year, it seems unlikely we’ll build on that number, or even hold at such a lofty height going forward.

Let’s take a look at what the climate record for D.C. has to say about the rest of winter after a warm start...

Note: In this post, winter is defined as the meteorological season of December through February. Snow and cold can occur into March, but it’s not as common and the lion’s share of D.C.’s winter is during December-February.

In the full climate set spanning back to the early 1870s, there have been 52 warmer than normal Decembers. That’s a long-term average of 37 percent of years. Of course, “normal” is now defined by current (1981-2010) standards -- or 39.7 degrees for December.

Since measured temperatures have generally warmed in D.C. during the period of record, cooler than normal Decembers outnumber warmer than normal ones by a good margin.

Looking at all 52 winters with warm Decembers, only one ended up with what today would be an exactly normal January/February temperature of 37.5 degrees. 27 are categorized as above normal and 24 as below normal. No outwardly apparent signal there it would seem.


Warmth has dominated the country in December. Via the High Plains Regional Climate Center.

If we shrink the sample down to the winters of 1980-81 through the most recent in 2011-12, warmer than normal Decembers outweigh cooler than normal ones 18 to 14. Of the 18 warm Decembers, two-thirds of the time D.C. was treated to a warmer than normal final two months of winter.

Even here, the evidence one way or another remains muddled, and that’s not the only unknown. Given there are over 10 days left in December, it’s hard to even say exactly where we finish the month.

CWG’s Winter Weather Expert Wes Junker expects a near normal end to December on average, which would chop a bit off our current extreme and record-setting pace. But, temperature averages are still dropping as well, and we’ve yet to realize any super-cold air masses.

I asked Matt Ross, CWG’s seasonal forecast lead, his thoughts on the final number for the month. He said, “I think +4.5 is a good guess .. 50-50 chance of a top 10 [warmest] on record, and a 90% chance of a top 20.”

In the current climate period, including last winter, eight Decembers have finished at +4.5 (44.2 degrees) or warmer. Of those eight winters, five saw January-February periods with warmer than normal temperatures.

Worth noting, a CWG winter outlook analog (1996-97) falls just one spot behind those months with a +4.5 or greater departure in December, holding a +3.3 departure. After that warm December in 1996, January-February combined for a +3.4.


D.C.’s warmest Decembers and the rollover to January-February as ranked by temperature departures from the current normal. *Jan-Feb temperatures correspond to the year following that shown (for December) in the graph. Data source: National Weather Service Baltimore/Washington.

When looking at just the top 20 warmest Decembers in D.C., we don’t find many answers either. Of the January-February periods that followed, nine were above average, 10 below and one equal to normal.

In the 2000s there have already been three top-20 warmest Decembers. Two occasions saw continuation of December warmth through the rest of the winter and one did not. Just last year, a scorching December set the ball rolling for the 3rd warmest winter on record.

While there’s likely much more to dig into, including the snowfall angle, this quick examination of the “signs” seem to mainly point toward consistency in our winter outlook. The December forecast understated the warmth a bit it would appear, but the idea of a “back loaded” winter with snow and cold to come is still very much a possibility.

By  |  12:30 PM ET, 12/19/2012

Categories:  Local Climate, Latest

 
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