Winter Storm Forecast Tripped up by Fine Line Between Rain, Snow

New Yorkers just got a lesson on how notoriously difficult it is to predict the severity of an East Coast snow storm.

All it takes is a wobble in a low-pressure system’s trajectory or a sudden shove of warm air and the best-laid forecasts of human and computer can go right out the window. That happened Tuesday in New York, which braced for more than 20 inches (51 centimeters) and got 4 in Central Park. U.S. natural gas futures rallied on Monday only to slide on Tuesday.

A shift in a storm’s path of just 10 to 15 miles from what meteorologists call the rain- snow line makes all the difference. People outside New York can attest to that. While Manhattan was spared the worst, New York’s Westchester and Putnam counties got more than a foot of snow, while Orange County, a little farther afield, logged 17 inches. Bergen County in New Jersey got 13.

“Forecasting the snow storms along the East Coast can be tricky because it all comes down to the exact path,” said Sean Ryan of the Weather Prediction Center in College Park, Maryland.

The storm was definitely powerful – wind gusts of up to 60 miles (97 kilometers) an hour in Massachusetts felled trees and utility poles.

“The low pressure was very strong, basically the equivalent of a Category 1 hurricane,” said Dave Samuhel, a senior meteorologist with AccuWeather Inc. in State College, Pennsylvania.

But for really heavy snow to fall in Manhattan and other boroughs, the low-pressure system’s center has to pass about 150 miles southwest of the city, he said.
‘Forecast Uncertainty’

This one traveled about 50 to 75 miles out and crossed eastern Long Island on its way toward New England. That’s why the heaviest snow cut across the Hudson Valley and Berkshire Hills of Massachusetts. Some of those areas could still get as much as two feet of snow.

National Weather Service Director Louis Uccellini has often stressed the uncertainty in every forecast for a snowstorm, just as there is in presidential elections and Super Bowls. And New Yorkers have seen massive snowfall threats wimp out before. In January 2015, early outlooks were for almost three feet, and only 10 inches fell. Boston got hammered in that storm but what most people remember is the miss to the south.

The computer models that forecasters use look at the atmosphere as it is at the moment of prognostication and then try to calculate what it’ll be going forward. “With this system, there were two pieces of energy, one from the Great Lakes and one from the Southeast, combining to form the nor’easter,” Ryan said. When that’s the case, “the forecast uncertainty can be enhanced.”

The storm packed a punch, snow or no. Ryan said its central pressure dropped from 1007 millibars at 8 p.m. New York time on Monday to 978 millibars as of 2:30 p.m. Tuesday just off Long Island. A drop that quick in such a short period of time meets the definition of bombogenesis, a term used to describe a storm that has become super strong.

But the system picked warm air up off the ocean and wrapped that around itself in a counter-clockwise twist as it moved, Ryan said. Warm air aloft melts snow before it can fall. That’s not easy even for computers to pick out days in advance.

Meteorologists think about storms in terms of how much water they produce. This is where warm and cold air make a difference. The same inch of water that falls as rain will produce two inches of sleet or 10 inches of snow depending on how cold the air is, Samuhel said.

Inject a little more chill into New York’s Tuesday and city dwellers would have still been shoveling out their cars Wednesday morning.