One of the hardest tasks in weather forecasting is projecting snow totals. We saw its perils in January 2015, when meteorologists wiped egg off their faces because New York City didn’t get anywhere near the foot and half or more that was forecast.
This weekend, the exact opposite is happening: New York City is getting far more snow than the forecast of 7 to 12 inches from the National Weather Service, which was issued less than 24 hours before the storm hit the city.1 It’s the perfect example of why forecasts that vary based on probability are so important, and why outlier predictions should not be dismissed.
The National Weather Service in New York City did a good job expressing the doubt in its forecast through the use of minimum and maximum amounts. You can see that in its Friday morning briefing: Even though the agency was forecasting 7 to 12 inches at the time, its maps listed a minimum potential of less than an inch and a maximum potential of 23 inches in New York City. In this case, the maximum potential looks like it will be closest to the truth.
You might be saying to yourself, that’s a wide range. But the breadth was well warranted. As I wrote on Wednesday, New York was right on the edge of heavy snowfall. A 50-mile shift in the wrong direction (wrong for this snow-lover, anyway) would have left the city with very little snow, while the same shift in the other direction would have given it what we’re seeing now.
There were other forecast models suggesting that New York City would get heavy snow. In particular, the North American Mesoscale model showed New York getting hit hard run after run. The run late on Thursday evening, which now looks prescient, showed 2 feet of snow falling in New York City.
Now, the NAM has been known to forecast too much precipitation (see the aforementioned faux blizzard in New York last year), and it definitely overdid snow amounts north of New York City in this case. Still, as I said on Twitter, you simply couldn’t feel too confident of lower snowfall totals in New York City because the NAM was consistent in calling for big snows. It turns out my warning was justified.
Of course, it could have gone the other way. The Global Forecast System was on the other end of the spectrum and had only about 3 to 4 inches falling in New York City in its run late Thursday night. It forecast amounts of less than an inch only a few miles away.
Weather buffs might remember that the GFS was one of the models that correctly predicted that last year’s January snow would underperform in New York City. It could have easily been right this time, and many New Yorkers probably would have laughed that forecasters had hyped up another storm.
Instead, it turns out that the GFS was very wrong this time. New York is experiencing a blizzard that blows away most expectations. Fortunately, people who were paying close attention to all the possibilities weren’t caught off guard.