New England December Weather Outlook: A Cold One

We are now officially into meteorological winter (December 1-February 28). Average temperatures continue their tumble, but the rate of the tumble does begin to slow. November sees average highs drop 10-12° while December sees averages drop 7-9° from the 1st to the 31st.

Summary: This December is looking like a cold one for New England overall. A ridge-in-the-west-trough-in-the-east pattern and the potential for a cross-polar flow will likely bring multiple shots of very cold weather to the region. Outside of these blasts, persistent cold is generally expected to hold tight. Precipitation may skew above average with troughing in place as well. Read below for the in-depth discussion.

December in Boston after a dusting of snow. Photo by Denis Tangney Jr.

TEMPERATURES

This month has started off on a cold note, and that will continue through the first week of December. The first Arctic blast of the season will be arriving tonight into Friday. The overall setup the northeast has seen since mid-November has left the region very vulnerable to cold snaps, and it was only a matter of time before a full blast of cold air crashed into the region. The overall flow aloft will be zonal (when the jet stream lays flatter across the region), which will help temperatures rebound this weekend. Zonal flow can lead to up and down temperatures, and this will be the case through the next week.

Below: Flow at the jet stream level, showing a zonal flow across the eastern United States:

Moving into next week, a more amplified will develop. This will come as a classic ridge-in-the-west-trough-in-the-east pattern takes hold across the United States. This will keep generally below average temperatures around for the east coast, with the highest confidence in colder weather for New England.

This will bring very mild weather to the United States west of the Rockies, with the highest confidence in warmer weather for the Desert Southwest. Temperatures will stay persistently below average, with another full Arctic blast possible early next week (December 8-9). Overall, temperatures next week may remain generally up and down, bouncing from very cold to moderately cold.

Heading toward the middle of the month, the ridge-in-the-west-trough-in-the-east pattern is expected to remain stubbornly in place. This pattern will likely remain very pronounced through the end of next week (December 12-13). This will keep the northeast generally on the colder side. The Climate Prediction Center's 8-14 day outlook (which currently covers December 11-17) looks very similar to the 6-10 day outlook seen above, just with slightly lower confidence (and a milder southeast US).

Below: 500mb height anomaly late next next week, showing a very pronounced ridge-in-the-west-trough-in-the-east setup:

Moving beyond next week, the general trough over the east is expected to gradually shift away. Despite that, an established cross-polar flow will keep a cold northwesterly surface flow around. Whenever surface flow favors winds out of the northwest through the middle of December, New England will be susceptible to Arctic air (winds won't always be out of the northwest, which will allow for moderation between cold snaps).

A cross-polar flow often involves a highly amplified ridge in the west and another ridge farther east extending toward the North Pole. General troughing between these two ridges sees a corridor open up, allowing flow from across the Polar-region into the central and eastern United States. This type of setup can bring the United States its coldest Arctic Blasts of the season, if other factors line up correctly as well.

Below: 500mb height anomaly early next week, during the cold snap expected around December 8-9. You can see what's described in the paragraph above setting up:

Moving toward the latter part of the month, the gradual shifting of the trough away from the east coast may allow for somewhat of a recovery from the multiple Arctic blasts coming into New England over the first couple weeks of the month. This can be seen as the North Atlantic Oscillation Index (NAO) moves toward neutral conditions toward the middle of the month after starting off in negative territory this month. With that said, general troughing could quickly return and NAO isn't a tell-all when it comes to weather in the eastern United States (if it was, long-range forecasting would be much more concrete).

Below: NAO Index in negative territory to start off December, then slowly rising back toward neutral conditions:

While there's no such thing as an easy monthly outlook to put together, predicting a below-average December this year with multiple shots at Arctic air is rather straight-forward. Even if cold air generally relents at some point later in the month, the Arctic blasts will likely counter that and keep the average monthly temperature on the colder end overall.

PRECIPITATION

With a generally zonal flow to start out the month followed by more pronounced troughing, bouts of unsettled weather will likely remain in place. There's currently no organized storm system ready to impact New England after the storm earlier this week. Overall, precipitation is a tough call this month as the troughing would inherently lead to more storm chances, but continuous pushes of cold air could keep the storm track suppressed south of New England.

This occurred for much of the second half of last winter, where large-scale storms were pushed south of New England as the region dealt with persistent cold (i.e. a cold, dry pattern). This could end up being the case this month as well. If suppression isn't the name of the game, it could be a rather snowy month with cold air in place. Whether snow or rain falls during storms will almost always come down to a storm's track, so this really can't be predicted in the long-term. Overall, we're going to predict a slightly above average month for precipitation.

LOOKING BACK

November played just about as expected in the temperature department for New England. We predicted that temperatures would generally be up and down, and that these ups and downs would cancel each other out and lead to a near-average month as a whole. Guidance was certainly leaning toward generally more chilly days than warm days overall. There were generally more below-average days, but the warm-ups did counter this somewhat, which led to a near average month. Those cooler days did help skew things slightly cooler than average, but it was slight.