NYC Mayor Bill de Blasio said at a press conference Sunday, "This could be the biggest snowstorm in the history of this city.” The National Weather Service (NWS) and Weather Channel meteorologist Chris Dolce have both said the impending storm is “potentially historic." So, what does historic mean, and how strong is this “potentially”? It depends on your definition, but this storm could be one for the record books, and not just in the highest-3 point-shooting-percentage-in-the-third-quarter-with-two-bench-players-on-the-court-on-a-Tuesday type of statistic.
Based on a new experimental forecast from the NWS, as of Monday morning there is an 80 percent chance that NYC will receive at least 12” of snow. Since record keeping in Central Park began in 1869, there have been 35 events exceeding a foot of snow, so 12" wouldn't be a big record. But there is a 62 percent chance for at least 18” of snow, and there have only been 11 events reaching that marker. Despite the seeming endlessness of last year’s winter, only one event (on February 13th and 14th) made the 12”+ snow event list for New York City. New York has only seen snowfall totals above two feet twice, first in December 1947 and more recently in February 2006.
De Blasio announced Monday afternoon that NYC public schools will be closed on Tuesday. Although one might imagine school closures as a pseudo-metric for a storm's impact, the correlation between snowfall and closed schools is not always strong. The only time schools were closed last winter was for the January 3rd snowstorm that dumped just 6.4" of snow. The city had three larger snowstorms later that season but no school closings. Schools were closed in March of 2001 for an over-hyped storm that only produced 3.5 inches of snow in Central Park, but they remained open in February 2006 following the largest snowstorm (26.9") in NYC history.
In addition to the heavy snow, NWS says this Nor’easter could produce sustained winds of 35 mph over Long Island, with gusts across the region in the 40-50 mph range, and potentially up to 60 mph over eastern Long Island. Although temperatures Monday night will remain relatively warm in the upper teens, the high winds will make it feel much colder, and the temperatures will not rebound much during the day on Tuesday. The high winds and heavy snow will also put power lines at risk of falling trees and branches. NYC's Office of Emergency Management has tips here for what to do if you lose power.
Another major factor affecting the storm's impact is timing. Storms that strike on weekends or overnight tend to cause less disruption, but this storm is expected to begin intensifying before Monday's evening commute. In February of 2013, hundreds of drivers were stranded on the Long Island Expressway for up to 12 hours. To avoid a repeat event, Governor Cuomo announced Monday afternoon that they are considering a ban on all main roads beginning tonight, and that a decision will be made by 4 p.m. He declared a state of emergency in Bronx, Dutchess, Kings, Nassau, New York, Orange, Putnam, Queens, Richmond, Rockland, Suffolk, Ulster, and Westchester Counties, and asked all non-essential state employees in those counties to leave work at 3:00 p.m. He also said that MTA subways will begin running limited service around 7 p.m. and Metro North and LIRR may close at 11 p.m. The governors of New Jersey and Connecticut also declared states of emergency. With the exception of emergency personnel, a travel ban is in place in Connecticut after 9 p.m. tonight, and in NYC after 11 p.m. (De Blasio clarified in a press conference Monday that food delivery bicycles do not qualify as emergency vehicles. Tip your Seamless deliverers well!) De Blasio also announced the suspension of alternate side parking rules through Wednesday, although meters are still in effect.
One of the tricky parts of forecasting snowstorms is something called the snow to liquid ratio. Basically, the warmer it is, the more dense the snow and the lower the snow totals. In New York City, temperatures during the storm are expected to be between 25-30ºF, so the snow will probably be on the denser side and totals will not be as dramatic as they would if temps were in the teens or lower 20s. At two inches of liquid equivalent (the amount called for Sunday by the National Weather Service), the difference between temperatures in lower-mid 20s and the upper 20s/low 30s is 10 inches of snow.
Other sources of uncertainty in forecasting include the speed and exact track of the storm. “There is some uncertainty in the exact path, speed, and intensity of the storm,” said Tony Barnston of the International Research Institute for Climate and Society at Columbia University (full disclosure: I work there). “Although the three main weather models agree in general, there are subtleties that can make the difference between getting only about one foot on the lower end to more than two feet on the upper end. The speed may be the most important one of the three, since it governs how much time we will be under the high snowfall rates of two or more inches per hour,” said Barnston.
If the storm hits in the lower range of the forecast, say 20”, we could call it about a once-in-25 year storm, since there have been six such events in the past 145 or so years. If we get 24" it would be a once-in-70 year storm by the same logic. But if storm comes at 30” of snow, the upper range of the NWS forecast, the math gets a little trickier since we haven’t had a storm exceeding 27” since record keeping began. Although unlikely, another record on the line is snowiest month. That record is held by February 2010, with 36.9 inches of snow. We’ve already had 4.5” of snow this month at the Central Park weather station, so if we get 30", we could come close.
To be recorded in official weather history, what matters most for NYC is the official snowfall in Central Park. This is where the longest period of record is for the city, so it's what is used for most of the statistics on weather events. While the NWS is calling for 20-30" in most areas around NYC, local bands of snow will likely cause several more inches in some places. Scientists have difficulty predicting where these bands will occur, but whether such a band forms over Central Park could be the difference between a nuisance-maker and a history-making nuisance.