There's a lot that goes into trying to determine who will get what during a winter storm in New England. Here's a breakdown of the most important factors.
THE THREE BASIC STORM TRACKS
While there are many facets to winter weather in New England, nearly all storm systems can be boiled down to three basic tracks. The track of a storm system through the region can often play the biggest role in the precipitation type; where in New England it will rain, where it will snow and where the transition zone will set up. The track a storm takes can come down to the position of the polar and subtropical jet streams.
The reason the track is so important comes down to the fact that nearly all (non-tropical) storms have two sides: a warm side and a cool side. Winter storms typically have a warm front that extends in a generally eastward direction from the center of the system and a cold front that trails behind the system, extending generally southward.
There can also be an occluded front, which basically occurs when a faster moving cold front catches up to a slower moving warm front. Occluded fronts can divide cold air behind a cold front from cool air ahead of a warm front.
INSIDE RUNNER
There are a few names for this kind of storm, others include inland runner, winter washout and sou'easter, but inside runner is likely the most common name used among forecasters. This name stems from the fact that the center of the system moves well inland across the northeast. These systems move out of the Great Lakes region and often move north of New England.
This pulls the storm's warm front across all of New England, resulting in a strong, mild southerly flow into the region. Temperatures can surge into the 50s and 60s in the middle of winter as the storm approaches, resulting in widespread rain and little (if any) snow reserved only for the highest terrain (hence the name winter washout).
While the bulk of the storm is spent in the system's warm sector bringing rainfall, the system's cold front will get dragged across New England at the end. This will sometimes result in the storm ending as a bit of snow or a flash freeze if the front is strong enough. These storms can also bring widespread damaging winds and spread them well inland, not just at the coast.
While these storms don't pack much snow, they can be among the most impactful if they get strong enough. The December 2023 storm was a prime example of a strong inside runner. This storm brought severe flooding with around a half foot of rain in parts of Maine and a massive power outage to New England as around 750,000 lost power due to hurricane-force wind gusts. Of course, not all inside runners will be nearly as strong as December 2023.
STRAIGHT THROUGH NEW ENGLAND
This storm track is really just an offshoot of the inside runner. The main difference with this one and the above track is that the system's center passes somewhere through New England instead of to the north of New England. These storms are often the epitome of a mixed bag for New England.
In this track, only a portion of New England will get into the storm's warm sector as the warm front only travels so far north. Within the warm sector, it will be mainly rain, to the north, it will be mainly snow and a messy mix of rain, snow and ice will set up in between. These are among the trickiest to forecast since it can be difficult to determine exactly where the battle zone between warm air and cold air will set up and how far north the warm front can push.
These storms can often start as all snow for all of New England before a transition to rain occurs in southern areas. Elevation can often play a big role in precipitation type with this type of storm as well with higher elevations naturally holding onto colder air.
NOR'EASTER
The quintessential New England storm, a nor'easter is a storm system that takes a coastal track. Nor'easters feed off the temperature contrast between cold air over land and relatively warm air over the ocean. These storms move up the coast, often staying offshore of New England. These storms bring New England blizzards.
With the center of the storm to the east of New England, the system's warm front does not play a role for New England, keeping the region on the cold side. With that said, these storms can still bring a mixed bag of precipitation depending on how close they get to the shoreline.
If a nor'easter tracks close enough to land, it can pull more mild marine air over land, causing mixing and rainfall along the coastal plain. If the storm passes far enough offshore, but not too far offshore, much of New England can be slammed with white-out conditions. If the storm passes too far offshore, the heaviest precipitation will remain offshore.
These storms will often bring a strong northeast wind, with the strongest winds typically contained to the coastline. These storms are also capable of producing severe coastal flooding if the timing of the storm lines up with high tide, particularly if tides are at their monthly highs.
THE BENCHMARK
Exactly who gets what during a nor'easter often comes down to the track. The main determining factor in whether a storm comes close enough to land to cause mixing and rainfall or if a storm produces all snow for New England comes down to the benchmark. The benchmark is a singular point in the ocean, located at the coordinates of 40° north and 70° west, or about 80 miles south of Nantucket Island.
If a storm passes inside the benchmark (closer to land), it will draw mild air into the system, allowing for mixing along the coast and keeping the heaviest snow in the mountains. The further inside the storm passes, the further inland and north the mixing and rainfall will push. If it passes well outside the benchmark, the heaviest precipitation will remain offshore. If it passes near the benchmark, southern and central New England will see the heaviest snow with lighter accumulations farther north.
INSIDE RUNNER EXCEPTION
While inside runners are synonymous with winter rainstorms, there is one critical exception that can occur, and does occur multiple times a winter. This exception is where occluded fronts can become important. Sometimes, a secondary area of low pressure will spawn as a primary low lifts north of New England. This secondary low often forms along the system's triple point, where a warm, cold and occluded front meet. This can trap colder air between the low pressure systems, leading to more wintry precipitation over rainfall.
When a secondary low forms, it can keep the system's warm front to the south of New England rather than the front crossing through the region. Oftentimes, when a secondary low forms, it strengthens as the primary low weakens. The primary low can transfer its energy to the secondary low and the secondary low then becomes the primary low. This can turn what could have been a rainy day into a more wintry day. Exactly where a rain/snow line sets up in this scenario comes down to the track and strength of the secondary low.
This is an actual (not hypothetical) forecast graphic we used this past spring when discussing a potential snowstorm incoming:
NORTHERN AND SOUTHERN STREAMS
The level of impact a coastal storm brings to New England can often come down to the levels of phasing (or interaction) between a northern stream and southern stream system. Main nor'easter systems are typically a southern stream system. These systems dig into the south, sometimes as far south as the Gulf of Mexico, before following the jet stream northward along the coast.
While this happens, a system following the northern stream will sometimes work across the northern tier of the United States, usually across the Great Lakes, before moving through New England. These systems are usually weaker and much less moisture-rich. When this system phases with the southern stream system (the main storm system), it can allow both to strengthen and spread snowfall across a much wider area of New England. When these two systems phase heavily, it can create a powerhouse storm that affects a wide swath of the region.
Below: an actual forecast graphic we used recently when discussing a potential snowstorm:
On the other side, if these systems undergo little phasing and interaction, the main storm system will typically remain well offshore, leading to a much lower-impact event than what could have been. When little interaction occurs, New England will often see much lighter snow from the moisture-starved northern stream system only.
(EXPLOSIVE) CYCLOGENESIS
Another factor in what a storm brings other than the track is how strong the storm gets, particularly a nor'easter. Cyclogenesis refers to the general strengthening of a storm. Explosive cyclogenesis or bombogenesis refers to the rapid strengthening of a storm. The term bombogenesis has gotten a bad reputation for being a social media buzzword, but the term was actually coined in 1980 by meteorologist Frederick Sanders in an MIT research paper.
For bombogenesis to be declared, the central pressure of a storm needs to drop at least 24 millibars in 24 hours or less. Naturally, the stronger a storm, the heavier the precipitation rates can get. Sometimes when a storm gets strong, it can drag cold air down from the upper levels to the lower levels, resulting in heavy snowfall.
SNOWSTORM BUSTS
As everyone in New England knows, snowstorms don't always pan out according to forecasts. These busts can happen for a variety of reasons, but here's a few of the more common ways a forecast can bust. For starters, if models are adamant that a storm will undergo explosive cyclogenesis or at least strengthen quickly on approach to New England, then it doesn't manage to strengthen, precipitation rates will not get nearly as intense as expected.
If temperatures ahead of a storm are mild (say 40s and 50s), but a coastal storm is approaching and is expected to strengthen quickly, the storm can draw cold air from the upper levels of the atmosphere down to the lower levels and turn initial rain into heavy snow.
If a storm doesn't strengthen enough for this to happen, the storm will remain lighter rain rather than switching to pounding snow. This happened in part during the major snowstorm bust this past February when Boston got 0.1" of snow and plenty of rain when a couple days prior the city was forecast to see up to a foot. The storm just couldn't strengthen.
Nor'easters can also bust if the storm tracks farther east or west than anticipated, either keeping the snow too far offshore or pushing the more mild air and rain far inland. During inside runners that run straight through New England, if the storm travels farther north than expected, the rain/snow line will push farther north than anticipated.
SLEET/ICE/WINTRY MIX
Not only can it rain or snow in New England, but sleet and freezing rain can also occur leading to additional impacts. Whether a wintry mix produces more sleet or more freezing rain comes down to temperatures at different layers in the atmosphere. This comes down to the development of a "warm nose". A warm nose occurs when a layer of mild air forms and rises above cold air at the surface.
The depth of the mild air above the sub freezing air determines whether sleet or freezing rain falls. A deep layer of warm air over a shallow layer of cold air produces freezing rain while a shallow layer of warm air over a deep layer of cold air produces sleet, as seen in the graphic below.
This can be among the toughest parts of a winter storm to forecast since subtle depth changes in the warm nose can change a sleet forecast to freezing rain and vice versa. This can be difficult since the impacts between a sleet storm and ice storm are very different. Sleet, while a nuisance to clear, won't cause the level of damage and power disruption as freezing rain.
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