Why Northeast Winter Storms Can Be Difficult To Forecast

Nor’easters, Explained

When a winter storm is threatening the East Coast, you'll hear the word "uncertainty" from meteorologists in the days leading up to the storm.

There are signs in the models for a potentially significant snowstorm, but the important details on track, timing and strength remain unclear and will determine where and how much snow falls.

That's probably frustrating, since you want specifics. However, it's the nature of the setup along the East Coast that can often 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 that make forecasting Northeast winter storms a challenge.

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 draw in the Northeast 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. These impacts not only affect 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., where the amount of scrutiny and impacts are far less.

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

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.

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

Which air mass wins out and for how long can be the difference between a walloping snowstorm near the coast or a messy mix of precipitation.

Another hurdle: intensifying low-pressure systems can create their own cold air through a process called dynamic cooling. This can counteract the warmer Atlantic air, allowing precipitation to remain in the form of snow.

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 coastal storm.

Meteorologists often talk about the so-called 40/70 benchmark, which means 40 degrees north latitude and 70 degrees west longitude, which is roughly 80 miles south of Nantucket Island.

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 major snowstorm to at least parts of the Northeast.

If computer models consistently paint this low-pressure track, it raises the confidence of a Northeast snowstorm forecast.

4. The Problematic Tracks

Many forecast situations have a lower-confidence track of coastal low-pressure systems that are either west or east of the 40/70 benchmark. What's more, forecast guidance can be erratic in the days leading up to a storm, painting several different low track scenarios.

When this occurs, forecast confidence can be much lower.

A track to the west 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 of 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 event and that can be the case in a large metro area such as New York City.

A storm that tracks east of the 40/70 benchmark can result in a close call with only a brushing of snowfall on immediate coastal areas and little to no snow in the Northeast interior.

5. The Finer Details

There are some 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 the Blizzard of 2006 (Feb. 11-12, 2006) that had an arcing band of heavier snow that set up right over New York City. It dropped 11 inches of snow on the Big Apple in three hours, vaulting the storm total to 26.9 inches at Central Park.

Another important detail, sometimes not resolved until a storm is underway, is what meteorologists call a 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 greatly reduce snowfall amounts in some areas compared to what was originally forecast.