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After a cool October and a marginally warm November, a cold December may well loom for the D.C. area. We project temperatures 2 to 4 degrees below the recent 30-year average and would not be shocked if it is even a little colder.

We also lean toward a drier-than-normal month, with overall precipitation (from rain and melted snow) about 1 to 2 inches below average. Even so, the cold air should offer opportunities for snow, and our best guess is around 1 to 3 inches, which is in the normal range.

Our confidence in the monthly snow forecast is very low, as just one substantial storm could easily surpass that amount. We think the chance of a significant snow event is somewhat elevated.

This prediction for a cold December is a reversal from our winter outlook issued on Nov. 15, as short-term influences on the weather pattern have evolved and trended more wintry.

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The big elephant in the room is the development of a large high pressure zone over Greenland, unanimously predicted by computer models. As Capital Weather Gang’s Ian Livingston wrote earlier this week, such a Greenland block increases the likelihood of cold and sometimes snowy weather in the Mid-Atlantic.

‘Greenland block’ could mean cold and snow by mid-December in D.C. area

Weather models show this feature flirting with intensity records set in December 2010. That month, Washington was 7.1 degrees colder than normal. It is too soon to know whether this year’s Greenland block will have the same effect on temperatures, but it stacks the deck for a colder-than-normal outcome, particularly in the second half of December.

For the first half of December, weather models do not show a strong effect from the Greenland block, with temperatures projected to be near or just slightly below normal. But they do show cold building to our north and northwest, which could well sink south during the month’s second half.

The precipitation forecast is a challenge. Greenland blocks are sometimes conducive to big winter storms, but the overarching La Niña pattern tends to favor drier-than-normal conditions. In December 2010, Washington posted slightly above-normal snowfall (2.1 inches), but below-normal precipitation overall. Washington has not seen more snow than that during December in the past 11 years.

Computer models simulate below-normal precipitation for the next two weeks, but confidence in this projection is low because the models have a hard time forecasting storminess (or a lack thereof) more than a week into the future.

November recap

Looking back at November, it was warmer and drier than normal. The monthly average temperature of 52.6 degrees was 2.6 degrees warmer than the 30-year normal. The monthly rainfall total of 2.65 inches was 0.26 inches drier than normal. This was the 11th-warmest November on record in Washington and the 86th-driest (tying 1927).

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November started very warm, turned very cold mid-month and then closed mild.

Here is the daily detail breakdown:

Thanksgiving Day was our only exactly normal temperature day.

The following records were set during the month:

In Washington (as measured at Reagan National Airport):

• Nov. 6: Record warm low of 66 degrees (previous record: 65 from 1938).
• Nov. 7: Record high of 81 degrees (previous record: 77 from 1975 and 1938).

At Dulles International Airport:

• Nov. 6: Record high of 81 degrees (previous record: 80 from 2003).
• Nov. 6: Record warm low of 66 degrees (previous record: 63 from 2015).
• Nov. 7: Record high of 79 degrees (previous record: 76 from 2020).

In Baltimore (as measured at BWI Marshall Airport):

• Nov. 6: Record warm low of 64 degrees (previous record: 63 from 2015).
• Nov. 7: Record high of 81 degrees (previous record: 77 from 2020, 1975 and 1938).
• Nov. 20: Record low of 22 degrees tied, last set in 1951.

Over meteorological autumn (September through November), Washington saw an average temperature of 61.2 degrees, a slight 0.2 degrees warmer than the recent 30-year average. The three-month rainfall total of 7.7 inches was 2.8 inches drier than normal and the driest since 2017’s 5.45 inches.

How did we do?

In an update a month ago, we projected November to be 1 to 5 degrees warmer than normal; the actual number was the 2.6 degrees above normal. We called for rainfall between 2 and 2.5 inches; the actual number was 2.65 inches. These results were very close to our forecast, meriting a grade of A-.

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