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What's With New England's December Rain Storms?

The first month of meteorological winter in New England has been marked by stormy weather. Despite this weather, New England has basically no snow on the ground, outside of the region's mountaintops. Even northern Maine, which is the one place of all areas in New England that can be counted on to have a snowpack all winter, is bare as of December 27th.

Caribou, Maine experienced its first Christmas since 2010 without snow on the ground. 2023 became only the fifth year since 1953 to feature a completely snow-less Christmas. Northern New England was starting to build a snowpack in mid-December before last week's rain storm stole pretty much all of it amid record high temperatures. Southern New England has barely seen flakes this winter as all storms have come through as rain makers. With another mostly rain event on the way this week, here's why this precipitation type has dominated.

So, despite a few storms rolling through in December, the ground remains mostly bare and muddy across New England. One of the biggest players in this setup has been the presence of a strong El-Nino. We published an article at the beginning of October laying out how strong El-Nino tends to favor above average temperatures and below average snowfall in New England. El-Nino's influence was expected to really show up in December, and this month has been basically the definition of a standard strong El-Nino in the United States.

During strong El-Nino events, cold air is mostly contained well to the north of the United States. This has been the case throughout most of December. The position of the polar jet stream has mainly stayed well to New England's north, in Canada. This has been supported by a positive NAO Index for much of the month. A positive NAO supports more mild weather in the east while negative NAO supports colder weather.

On top of this, the jet stream off the Pacific Ocean has been strong. This strong pacific jet spreads mild air over the Pacific Ocean across the United States, which is why much of the United States, particularly the northern tier, has been generally mild this month.

Temperature departure from average on Christmas Eve. While this is only for the one day, the basic pattern can be applied for much of December:

Naturally, more mild weather will lead to precipitation falling as rain over snow. The average jet stream position and strength is an indicator of the big picture, but there are many other smaller-scale factors that go into what a storm produces in New England. Another reason for the rain is the track of the storms as they move through New England.

Our storm systems have generally tracked inland across New England rather than just offshore. The further inland a storm tracks, the more mild the storms tend to be and the further north snow is pushed. New England's biggest snowstorms occur when nor'easters track just offshore, close to the 40/70 benchmark. That has not been the case so far this season.

New England's most recent two storms, which have been potent, have tracked across western New England or upstate New York. This track allows a strong southerly flow into New England, which helps transport mild air from the south into the region.

Graphics we used in forecasts for the December 10-11 and December 18 storms:

Something else to consider when looking at these rain events is not just the fact that it rained, but how much rain has fallen during the storms. New England has seen rainfall rates this month that are typically seen at the height of summer, when humidity is high and the atmosphere is moisture-rich.

The two most recent storms featured a very strong southerly low-level jet. This transported very moist southern air into New England. This low-level jet (about 2,500 feet above sea level) saw wind speeds of 80-90+mph. This not only allowed the high moisture, dew points and temperature, but also brought the strong, damaging winds parts of New England saw.

During the December 10-11 storm, 3-5 inches of rain fell across a large portion of New England. The December 18 storm brought upwards of 4-6 inches of rain. This amount of rain (and especially the rate at which it fell) in New England is atypical for winter. Combine this with snowmelt from what snow there was before the storm, and a flooding disaster came to be in northern New England, particularly Maine.

WPC graphic issued in the afternoon on December 18:

The December 18 storm originated in the very warm and moist Gulf of Mexico before shooting up and soaking the entire east coast.

As always there are exceptions to the rain. Precipitation changed over to snow in Vermont and the Berkshires during the December 10-11 storm, allowing up to eight inches to accumulate at the end of the storm. This came as the storm trended further east, allowing colder air onto the backside of the storm. Still, snowfall wasn't as much as anticipated for most of Vermont in that storm.

Looking ahead, another rain storm is in store tonight into Thursday, although the storm will be much weaker than the previous ones, leading to less rainfall. Another weak system will likely slide through on Friday night into Saturday morning, bringing the chance for more light snow in northern New England. Southern New England, again, will likely see more by way of light rain showers. New Year's weekend will likely see very seasonable temperatures.

Probability of snow accumulating at least 2 inches by Saturday morning:

Looking further ahead, NAO is forecast to dive negative after the start of the new year, as seen on the NAO graphic above. This would support colder air getting into New England, but it's important to remember that many factors go into what the region sees for temperatures. A negative NAO doesn't automatically mean cold weather and a positive NAO doesn't automatically mean mild weather. It just supports those things happening. A continued strong pacific jet is expected to persist into the new year.



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