Why Winter Storm Avery Produced More Snow Than Expected In the New York City Area

(NEWS: Latest on Winter Storm Avery) 

The snow was heavy and arrived in the late afternoon and early evening. Drivers navigating the first snow of the season clogged roads and got into accidents. Trees came down on streets and added to the gridlock. Public transportation slowed to a crawl. The Port Authority Bus Terminal had to partially close because of delays and overcrowding. Rail switches froze for trains. The commute home took up to 10 hours for some.

Beyond the Tri-state, drivers in Lehigh Valley, Pennsylvania, were stranded on an interstate overnight for 12 hours.

(MORE: Why Northeast Winter Storms Can Be Difficult to Forecast)

For Central Park, where 6.4 inches of snow fell, it was the second-snowiest November day on record. The forecast on Thursday morning was for a few inches of snow.

As is often the case, forecasting the precipitation type and amount near the Interstate-95 corridor was tricky.

But why did the city receive several inches more than predicted earlier Thursday?

Blame the cold air that stuck around longer and delayed the changeover to rain and the storm's slow progress.

The area of low pressure that developed near the North Carolina coast tracked northward slower and was also weaker than expected. As a result, it was not able to scour out the cold air as early as predicted.

The change to a colder and snowier afternoon and evening in the New York City area became evident by midday Thursday.

Just after 12:30 p.m. the NWS made a significant update based on the radar trends, as well as the colder temperatures in the region and said that "period of steady snow looks to be heavier and of longer duration than initially expected."  It also mentioned that the timing of the snow "should cause significant disruptions to the evening commute."

The wind direction was from the northeast and temperatures in southern New England were chilly which would not result in much moderation. In addition, dew points during the morning were only in the teens. This meant that there was a higher chance of temperatures dropping when moderate precipitation arrived. 

Based on this information a winter weather advisory was posted for locations near the coast, including New York City, with winter storm warnings a little farther inland. 

Snow began to fall in the New York City area shortly before 2 p.m. and by 2:30 p.m. the NWS issued a special weather statement for snowfall rates between 1 and 2 inches per hour through the late afternoon.

The drop in temperature as the precipitation moved in did occur. The temperature in Central Park was 35 degrees as just before 2 p.m. and fell to 33 degrees by 3 p.m. and continued to drop to 28 degrees between 5 and 6 p.m.

Heavy snow fell during the late afternoon and into the early evening. The change to rain did not occur until after 8 p.m.

The cold air that was in place was to the north set record lows for the date, including in Massena, New York (9 degrees), Montpelier, Vermont (4 degrees), Caribou, Maine (6 degrees) and Bangor, Maine (11 degrees).

What resulted was an increase in snowfall totals, especially closer to the coast and near the Interstate 95 corridor where a quicker change to rain was originally expected.

Small changes in the area of low pressure (strength, track and speed) and temperature make big differences when forecasting wintry precipitation and those forecasts are updated frequently to account for those changes. It is important to keep up to date with the latest forecast, especially during the winter.