New England has reached the heart of winter once again. New England's average high temperatures continue to fall, reaching their lowest point of the year toward the end of the month before very slowly beginning to creep back up heading into the second half of winter. Here's a look at the large-scale patterns that will be setting up throughout the month this year.
Summary: This month is looking to feature persistently cold temperatures with multiple chances for very cold air to push into the region through the first half of the month. The cold may gradually begin to ease up heading for the second half of the month. This cold air mass looks pretty dry, at least for the first half of the month with large storms potentially being pushed away. Read below for the in-depth discussion.
TEMPERATURES
After an unsettled stretch of weather helped usher in mild temperatures to end December and start January, temperatures will now be falling into this weekend. This will come as strong cold advection behind New Year's Day's departing storm. This is evidenced by the strong winds across New England today (Thursday). An expansive trough in the jet stream over the eastern two thirds of the United States will set up as this week comes to an end.
The cold temperatures setting up this weekend will likely remain in place through much of the first full week of the month (January 5-11). A rather strong Greenland Block will set up, leading to persistent troughing over the northeast. In addition, ridging over the western United States and Canada will also be in place. These two features are expected to begin to gradually merge heading into next week. This supports cold air getting locked across the eastern half of the United States amid the troughing.
Below: 500mb height anomaly for mid-week next week showing the Greenland Block, western ridge and eastern trough:
A persistently cold first half of January for the entire east coast of the United States is supported by large-scale flow patterns. There are a handful of oscillation patterns around the Earth, but the three that most directly impact New England are the North American Oscillation (NAO), Pacific North American Oscillation (PNA) and Arctic Oscillation (AO). While there are many factors that go into what temperatures will be, a negative NAO can support colder weather over the east while a positive phase is the opposite.
A negative NAO doesn't automatically mean cooler weather is coming (if it did, long range forecasting would be much easier), but in this case, other factors are also lining up. AO will continue to turn deeply negative while NAO remains negative. This negative AO phase is expected to peak around -4 standard deviations. In addition to this, PNA is expected to remain positive. PNA can work the same as NAO for the west. A positive PNA and negative NAO is indicative of a ridge-in-the-west-trough-in-the-east pattern.
Below: NAO and AO Index values for the last four months. The solid black line represents observations dating back to mid-September while the dotted line represents the upcoming forecast through mid-January:
So basically, a positive PNA and negative NAO support a cooler east coast and a milder west coast. Add in a deeply negative AO and the fact that it's now the middle of winter, and the large-scale flow setup favors blasts of very cold air to move into the eastern United States. The question becomes just how cold it will get for New England. As of now, the core of the coldest air relative to average is expected to move to New England's west and south.
There could be a historically cold Arctic blast for the deep south around late next week (January 8-12). For New England, this could result in a more or less a standard Arctic blast that dives through the region a few times a winter. The southward center of this cold air can be seen on the Climate Prediction Center's 6-10 day outlook. Much of southern New England has up to an 80% chance of below average temperatures while northern Maine has just a 20-40% chance.
Heading toward the second half of the month, a slow moderation is expected with the troughing over the east. Troughing is forecast to remain in place and only slowly retreat. This can be seen by looking back at the NAO and AO Index charts above. You can see both are forecast to gradually climb back near neutral conditions, but still remain negative, heading toward the middle of the month. Overall, trends have been toward a faster moderation of temperatures across the Plains, which could lead to a faster moderation and at least a slight warming trend for the second half of January for the east.
Overall, we're predicting a cold January with persistently below average temperatures. There will likely be multiple blasts of very cold air, especially through the first half of the month. A slow and gradual moderation may take shape for the second half of the month, bringing temperatures closer to average, but this will likely be a below average January all together.
PRECIPITATION
A persistently cold January would support more by way of snowstorms for all of New England rather than winter washouts, which have been the story for southern New England in December. With that said, the pushes of cold air may suppress storm systems to the south and well offshore of New England. This looks to be the case early next week as a storm system dives south of New England. This will happen as this weekend's push of cold air will also push the storm away, though a glancing blow remains possible for the southern tier.
Below: Current weather map for Monday morning, showing the storm system moving south of New England:
The coldest air masses tend to be the driest ones, and the air mass that will be persistently cold is poised to keep precipitation at bay for the most part. Greenland Blocks tend to support nor'easter development for New England, the issue is that the jet stream may get pushed so far south, it keeps developing storms well away. For a snowstorm during this time, New England will need the right balance of a cold air intrusion and jet stream position.
The first half of the month looks to be drier than average amid the dry, cold air mass with a persistent northerly flow. The second half of the month may be able to get wetter (or whiter, depending on numerous factors) as the air mass begins to gradually let up. Overall, we're thinking the month will end up below average with precipitation, with things ramping up toward the end of the month.
LOOKING BACK
December's large scale pattern played out about as expected with a colder first half of the month followed by a more mild end to the month. We predicted a below average month overall temperature-wise, this didn't really pan out when looking at the month all together. The month ended up right around average, mainly because there were days that were both very far above average and very far below average. These days ended up canceling each other out.
For example, the day that was most above average for Boston was December 30th, which was 19° above average. The farthest below average was December 22nd, which was 19° below average. In Burlington, December 22nd was 23° below average and December 30th was 27° above average. Overall, there were six days that were at least 10° above average and 5 days that were at least 10° below average for Burlington this month. Boston's split was 5 above and 3 below.
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