UPDATE, 3:20 p.m.: National Weather Service director Louis Uccellini, in a press call this afternoon, said his agency did not do enough to communicate the uncertainty in the forecast for New York City and Philadelphia, where snowfall predictions were far too high.

“It is incumbent on us to communicate forecast uncertainty,” Uccellini said. “We need to make the uncertainties clear.”

He added: “We’re going to review this [issue of communicating uncertainty in weather forecasts] very carefully and assess a different approach as we deal with these types of storms.”

However, Uccellini stressed erring on the side of caution and planning for the worst was “the right decision” given the potential for “extraordinary” snow totals.

Craig Fugate, administrator for the Federal Emergency Management Agency, agreed forecasts helped preparations.

“The NWS forecast is just that – a forecast. And we need to be prepared to respond to incidents based on the best available information,” Fugate said. “The most important aspect of this event is that people responded and lives were saved.”

Original post from 12:20 p.m.


Snowfall forecast for the New York City metro area made by the National Weather Service as of 3:34 p.m. Sunday. (NWS)

For the Big Apple, the great Blizzard of 2015 was forecast to rival the paralyzing 1888 storm, dubbed the White Hurricane. Up to three feet of snow was predicted. Reality: About 10 inches fell.

The forecast in Philadelphia wasn’t any better – and arguably worse. Up to 14 inches of snow were forecast. The City of Brotherly Love tallied roughly 2 inches, about the same as Washington, D.C. (where the forecast was actually right).

Why were the forecasts so bad?

It’s simple: Many forecasters failed to adequately communicate the uncertainty in what was an extremely complicated forecast. Instead of presenting the forecast as a range of possibilities, many outlets simply presented the worst-case scenario.

Especially for New York City, some computer model forecasts were extremely dire, predicting upwards of 30 inches of snow – shattering all-time snowfall records. The models producing these forecasts (the NAM model and European model) had a sufficiently good enough track record to take them seriously.

However, some model forecasts (e.g. the GFS model) signaled reason for caution. They predicted closer to a foot of snow.

The more conservative model forecasts proved correct. They predicted the storm to track just slightly too far out over the ocean for the blockbuster snow totals called for by the models that tracked the storm closer to the coast.

When a forecast is so sensitive to small changes (eastern Long Island, not far away, received 30-plus inches), it is imperative to loudly convey the reality that small changes could have profound effects on what actually happens.

Unfortunately, the hype surrounding the historic possibility drowned out the very real scenario that the storm could underwhelm.

To their credit, both the National Weather Service (NWS) offices serving New York and Philadelphia have owned up to their flawed forecast information.

The New York City forecast office wrote on Facebook:

The storm has moved further east and will be departing faster than our forecasts of the past two days. The result is much less snow than previously predicted for the western half of our region. The heaviest of the snow will be over Long Island and southern Connecticut with lighter snow elsewhere through the morning hours.

The science of forecasting storms, while continually improving, still can be subject to error, especially if we’re on the edge of the heavy precipitation shield. Efforts, including research, are already underway to more easily communicate that forecast uncertainty.

The meteorologist-in-charge of the Philadelphia forecast office, Gary Szatkowski, posted to Twitter:

My deepest apologies to many key decision makers and so many members of the general public.

You made a lot of tough decisions expecting us to get it right, and we didn’t. Once again, I’m sorry.

These mea culpas are sincere, and there is no absolutely no doubt these NWS offices did their best to issue a good forecast in support of their mission to protect life and property. But the general lack of information provided about the forecast uncertainty is a major disappointment considering both the state of weather forecasting and the efforts some have made to improve how we communicate the forecast.

For many years, the need to express forecast confidence and communicate different scenarios during complex, high-stakes forecast events has been discussed and stressed in the weather community.

The National Weather Service has been slow to develop products and communication techniques to do this more effectively.

Some NWS offices, including New York and Philadelphia, have begun to issue maps showing various snowfall scenarios – including minimum, maximum and most likely. This is an important step forward, but these pages are difficult to find on their Web sites, and this information is not included in public forecasts.

Unfortunately, the presentation and delivery of the National Weather Service forecasts has not evolved in decades. The primary forecasts issued to the public give one answer, when in important situations, many answers are possible.

This week’s forecast for New York City is a black mark for the National Weather Service and, unfortunately, could diminish public trust in weather forecasting. The NWS must respond by decisively and rapidly evolving the way it presents forecast information during complex, potentially high-impact events. This cannot happen quickly enough.

***

Post script: For readers of the Capital Weather Gang who rely on our forecast for Washington, D.C. – we have attempted to develop prominent ways of communicating forecast uncertainty by including “boom” and “bust” scenarios and confidence levels on our snow forecast maps. We also explain the range of possibilities and scenarios for complicated weather events.

We appreciate that this is challenging, because it is easiest for people to focus on one number and the worst case, but we continue to try to experiment to find ways to help people understand difficult weather forecasts.

Finally, I should point out it is much easier for us to do this for one city than it is for the National Weather Service to do it for the entire country. But this week’s forecasting underscores the importance of moving forward and learning by doing.


Snow accumulation forecast for through Monday afternoon. (Capital Weather Gang)

Related: Snowquester: when forecast information fails