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Winter leaves forecasters with lots of explaining to do
Few came close to predicting the season accurately
By Jason Samenow
Washington Post

WASHINGTON — Predicting the weather for winter many weeks before it begins is hard. If any season proved there is a long way to go in perfecting such long-term outlooks, this winter was it.

It appears no outlet in the government, media, or private sector nailed the 2016-17 winter outlook. In fact, some forecasts predicted the opposite of what actually happened. The National Weather Service and Weather.com probably had the most accurate outlooks, though they were far from perfect.

Forecast errors were linked to a Pacific Ocean that did not behave as expected. Storms traveling across it were supposed to mostly pass to California’s north. Instead, time and time again they hit the Golden State head-on, unloading historic amounts of rain and snow, while flooding much of the rest of the nation with abnormally warm air.

 Temperatures: Forecasters presented two schools of thought on winter temperatures: Warmer than normal air would dominate except for parts of the North as the jet stream hovered near the Canadian border. Or the jet stream would dive south east of the Rockies, and colder than normal air would prevail over much of the North and East. The Southwest would be warm.

The first school proved closest to correct. Temperatures over the entire nation were warmer than normal, except in the Pacific Northwest. It was the nation’s sixth-warmest winter on record.

The National Weather Service and Weather.com fell into the first group, which best captured the temperature pattern, although their predictions missed some important details.

The weather service wrongly predicted below-normal temperatures in the Upper Midwest and didn’t capture the extent of the warm air in the eastern and central United States.

The Weather Co. also missed the warmth that prevailed over the eastern third of the nation.

But weather service and Weather Co. forecasts were quite good compared to those from the Commodity Weather Group, Atmospheric Environmental Research, and WeatherBell Analytics, all of which called for a cold winter in much of the northern and eastern United States — the opposite of what happened.

■ Precipitation: Forecasts I reviewed missed the onslaught of rain and snow in California, which turned out to be the winter’s biggest weather story.

The weather service confessed that its California forecast was a ‘‘big dud.’’ AccuWeather, the only other prominent group that offered a public precipitation outlook, wrongly said the southern two-thirds of California would be dry.

■ Why forecasts failed: The Pacific Ocean fire hose that blasted California not only messed up the precipitation forecast; it reflected a jet stream pattern that wreaked havoc with some of the temperature forecasts for the East.

Before winter began, forecasters expected a La Niña event to develop and influence weather patterns. Typically, La Niña produces a jet stream that keeps the storm track to the north of California, passing through Canada before dipping over the central and eastern United States, allowing arctic air to spill south.

Often, California sees drier than normal weather during La Niña. But this La Niña, which was weak, didn’t behave like a typical La Niña. Instead, a powerful Pacific jet stream frequently swept storms into California. This same jet stream helped draw mild air eastward over the Lower 48, meaning substantially warmer than normal temperatures.

The raging Pacific jet formed from a configuration of sea surface temperatures that few forecasters anticipated. Sea surface temperatures “were much colder in the Northeast Pacific than I thought and much warmer in the tropical eastern Pacific,’’ said Joe Bastardi, at WeatherBell Analytics.

Judah Cohen, of Atmospheric Environmental Research, traced the cold waters in the Northeast Pacific to frigid air over Siberia in November. ‘‘The models missed that impressive buildup of cold air in Siberia.’’ Cohen said.

The contrast between the chilled North Pacific waters and much warmer waters in the tropical Pacific helped establish the rip-roaring jet stream that ultimately collided with the California coast.

Bastardi, who called his winter outlook ‘‘horrible,’’ stressed that many atmospheric factors converged to make the forecast a difficult one. The Pacific Ocean temperatures were ‘‘just the tip of the iceberg,’’ he said.

Matt Rogers of Commodity Weather Group blamed ‘‘a complex interaction of variables.’’