Winter storm breaks New York City’s record for most daily snow in 2 years

New York City has recently been in a lengthy snow drought. Last winter was its least snowy on record and, until 1.4 inches fell Jan. 16 this year, it endured a record 701 calendar days without an inch of snow.

But moderate to heavy snow poured down Tuesday morning. Amid the snow, the visibility in the Big Apple dropped to a quarter-mile for three hours. The snow was intense but brief; all warnings for the Tri-State area were dropped by midafternoon.

Tuesday’s measurement fell a bit shy of National Weather Service forecasts for 5 to 7 inches, but nearby areas saw more. JFK Airport tallied 4.2 inches, and 6.9 inches fell on Coney Island. Farther east on Long Island, 8.7 inches fell in Locust Valley. Yonkers, on the Hudson River just outside the city to the northwest, got 8 inches.

Several forecast challenges overlapped to make predicting what would happen in New York City a headache.

Challenge 1: It was hard to forecast narrow ‘bands’ of heavy snow

In large storms, precipitation doesn’t fall uniformly everywhere. Instead, “mesoscale banding” occurs with narrow “bands” of heavy snow — about 10 or so miles wide — embedded within large areas of light precipitation. Air rises inside the heavy snow bands and sinks in “moats” of lighter surrounding snow.

In Tuesday’s case, the more significant bands parked just west — and east — of Central Park.

“How far inland the best banding would get” was a challenge, said Jay Engle, a meteorologist at the Weather Service office serving New York City.

Still, eastern Pennsylvania, northern New Jersey and the lower Hudson Valley saw between 6 and 10 inches. That’s where a “deformation” band set up. That means stretching of the air at the mid-levels of the atmosphere pulled air from below upward, enhancing snowfall rates.

Then over eastern Long Island, Connecticut, Rhode Island and southeastern Massachusetts, the heavier snow totals — including up to 15.7 inches in Hartford — were caused largely by “frontogenesis.” That’s the process by which clashing air masses result in the formation of a localized front and narrow zone of particularly heavy snow.

Challenge 2: Temperatures were barely cold enough for snow

Another big challenge? The temperature in Central Park stayed above freezing as it snowed, limiting accumulation.

“That’s the thing,” Engle said, “with temperatures being so marginal, you get a compaction of the snow that’s already fallen. Basically, you need to keep snowing to accumulate and compact less.”

He explained that this was especially true in urban areas, where asphalt and cement help radiate heat from the ground. Ground temperatures were around 33 or 34 degrees, which are borderline at best for accumulating snow.

Engle also mentioned one more limiting factor — the lighter snowfall in New York City did not cause enough “dynamic cooling” of the atmosphere. The air would have cooled more had the snow been heavier and fallen into the slightly drier air and evaporated.

Challenge 3: Models forecasts waffled

Many residents may have noticed swift changes in predictions made between Monday morning and evening.

Boston, which was forecast to see 7 to 13 inches just 36 hours before the snow began falling, wound up with a paltry 0.1 inches. Albany — originally pegged to get 8 to 12 inches — didn’t see a flake.

The forecast in New York City didn’t bounce around as much but still fluctuated. On Monday morning, high-resolution models projected about 10 inches; later models depicted closer to 6 or 7 inches.

Forecasts changed because the storm was following a region of changing temperatures that models weren’t able to accurately pin down until about 12 hours before the snow was to begin. The result was some of the biggest shifts in predicted snow amounts in recent memory in several locations.