Why Northeast Winter Storms Can Be Difficult to Forecast

Chris Dolce
Published: March 7, 2018

When a winter storm is threatening the East Coast, you probably hear the word "uncertainty" uttered by meteorologists in the days leading up to the storm.

While that's probably frustrating and you want specifics, it's the nature of the setup along the East Coast that can sometimes create difficulties in determining whether you'll see a foot of snow or a slushy inch followed by rain.

Here are some of the factors on why forecasting Northeast winter storms can be a challenge.

(MORE: Winter Storm Central)

1.) Major Population at Stake

First and foremost, the population potentially impacted by a winter storm in the Northeast Megalopolis is huge, topping 50 million people.

The amount of attention and scrutiny snowfall forecasts in the Northeast draw is equally as large given the massive impact on schools, businesses and commerce.

With so much at stake, subtle differences in the track and evolution of a storm can be the difference between a high, moderate or low impact event.

That not only affects those in the Northeast but can also send ripples of disruption into other parts of the country by disrupting transportation.

Compare that to a similarly difficult forecast situation in a lower population region of the U.S. and the amount scrutiny and impacts are far less.

(MORE: Most Challenging Forecasts in the U.S.)

An overlay of city lights across the Northeast Megalopolis illustrating the dense population.

2.) Blame the Atlantic Ocean

The Interstate 95 corridor's close proximity to the Atlantic Ocean can take some of the blame for the difficult snowstorm forecasts in the Northeast region.

In coastal storm setups, the Megalopolis can be a battleground between cold air over land and the relatively mild air over the Atlantic Ocean waters.

Depending which one wins out is the difference between a walloping snowstorm near the coast or a messy mix of precipitation.

Throwing another monkey wrench into the forecast is that intensifying low-pressure systems can create their own cold air through a process called dynamic cooling. When this occurs, it can counteract the warmer Atlantic air, allowing precipitation to remain in the form of snow. That just adds another layer to the forecast for meteorologists to weigh.

The I-95 corridor can be a battleground between cold air in place over land and relatively warmer air pulled in by the coastal low-pressure system.

3.) The Track of the Low and the 40/70 Benchmark

To determine if the cold air or milder Atlantic air wins out, meteorologists examine the track of the developing coastal storm.

The track is critical, and forecast guidance can have trouble honing in on this until within 24 hours of a storm's impact.

Meteorologists often talk about the so-called 40/70 benchmark, which means 40 degrees north latitude and 70 degrees west longitude. In general, when cold air is available over the Northeast, a low tracking through this general point on the globe is in the sweet spot for delivering a thumping snowstorm to at least parts of the Northeast.

If forecast guidance consistently paints this low-pressure track, it typically results in a higher than normal confidence snowstorm forecast for the Northeast.

The black dot represents the 40/70 benchmark. A low passing near this point typically results in a significant snowstorm along the East Coast when cold air is available.

4.) The Problematic Tracks

Many forecast situations feature a lower confidence track of coastal low-pressure systems that are either west (left) or east (right) of the 40/70 benchmark. In addition, forecast guidance can be erratic in the days leading up to a storm, painting several different low track scenarios.

When this occurs, forecast confidence sometimes drops like a rock.

A track to the west (left) of the benchmark typically ushers in warmer Atlantic air. How far inland that warmer air penetrates can be difficult to diagnose and makes for a frustrating rain/snow line forecast.

Storms can sometimes have a snowfall gradient that ranges from a couple inches to a foot over a distance spanning tens of miles. Where that sets up is the key difference between a high and low impact storm in a large metro area such as New York City.

Conversely, a storm that tracks east (right) of the 40/70 benchmark can result in a close call with only a brushing of snowfall on immediate coastal areas.

The black dot represents the 40/70 benchmark. If the low tracks west of that position, then the rain/snow line may set up near or inland from the coast. A track east of that point typically only brings a brushing of snow to the coast.

4.) The Finer Details

There are a couple finer details that sometimes can't be determined until a storm is actually ongoing.

First is the potential for banding snowfall during storms where the coastal low-pressure system is undergoing significant intensification. When these bands of heavy snow exist, they can bring snowfall rates of 1 to 3 inches per hour, locally enhancing snowfall totals in a storm.

One such example is a Feb. 11-12, 2006 winter storm that had an arcing band of heavier snow that set up right over the New York City metro. It dropped 11 inches of snow on the Big Apple in three hours, vaulting the storm total to 26.9 inches at Central Park.

The lighter blue shadings depict heavier snowfall rates on Doppler Radar in a band that developed over New York City during a Feb. 12, 2006 snowstorm.

Another detail sometimes not resolved until a storm is underway is what meteorologists call the dreaded dry slot.

As its name implies, the dry slot is a punch of dry air on the eastern flank of a developing coastal storm. This dry air can cut off moisture and reduce the snowfall amounts in some areas compared to what was originally forecast.

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