It might be meteorological spring now, but it's still going to feel like winter for a while longer. After escaping wintry weather during the height of the season, the end of the season is looking to make up for lost time. This is mainly due to what is called a "Greenland Block". This weather phenomenon is not only likely to send below average temperatures to New England, it could also usher in more snow chances throughout the month.
Basically, a Greenland block occurs when a ridge of high pressure develops over (or near) Greenland. The high pressure can "camp out" over Greenland for a while, causing persistent colder weather in the northern United States.
So, how exactly does this high pressure over Greenland lead to colder weather in the United States? The high, known as a blocking high, causes troughing in the jet stream. The jet stream is forced to take a southward dip across the United States, which allows cold Canadian air to filter into the northern tier, including New England.
The current Greenland block that has developed is not yet strong enough to allow cold air to hang around in New England quite yet, as seen in this week's forecasted temperatures. With the exception of one cold day (Tuesday 3/7) it will be a seasonable week. The block will be getting stronger over the next couple weeks and as it does, chances increase for more persistent below average temperatures.
Temperatures look to start cooling down this weekend and going into next week. New England is on the edge of the cooler air with Maine looking at more seasonable weather while the rest of New England is leaning toward below average. Note that on these maps, the darker the blue is, the higher the chance of below average temperatures. It does not mean temperatures will be way below average in the darker areas.
Heading into the middle of the month, confidence increases of below average temperatures across New England. At this point, the Greenland block will be firmly in place and strong. New England will very likely be looking at a cool, more wintry week as we approach the end of winter. There's currently a 40-50% chance of below average temps, with a lot of highs in the 30s.
The temperature outlook heading into late March looks very similar to the middle of the month. Currently, the NOAA gives much of New England a 55-60% chance of below average temps with Vermont looking at a 60-70% chance. Now, I don't typically bring up the 3-4 week temp outlook because it's getting far out and there are numerous factors that affect high temperatures. However, in this case, there is greater certainty of persistent below average temps with this Greenland block in place.
As you can see by the NOAA outlook maps, New England is more on the edge of the March cool down. This is because the jet stream will be pushed well south across the heartland and will likely rebound northward again near New England. What this means is that while cool weather is expected, true arctic outbreaks remain unlikely.
This setup also increases chances for late season snow. The way the jet stream sets up could lead to storms more likely to form into nor'easters that track just off the coast. With the colder air in place, these storms will have the chance to produce more snow. The ocean is also warmer than average for this time of year. Nor'easters feed off the temperature contrast between warm ocean air and cold land air.
With the colder air in place, nor'easters will have plenty of energy to feed off of. On top of this, the atmospheric river looks to continue, with storms continually ejecting into the west and tracking across the country. The middle of March looks to feature very active weather for New England. Just how much snow falls versus sleet, rain and freeing rain will remain to be seen as there are many factors that go into what precipitation a storm produces.
Now, when we say that the entire month of March (and into early April) will be below average, that does not mean that every single day will be cold. As stated above, numerous factors go into determining high temps and there will be warmer, spring-like days mixed in. It's just that there will be more cool, wintry weather than spring-like weather.