Welcome back, La Niña.
The weather phenomenon last seen three years ago has returned and if it acts like it has in the past, it could mean a cold start to winter in Wisconsin followed by heavy snow storms in January and February.
While it’s always difficult to predict weather months in advance, meteorologists have some arrows in their quiver for seasonal forecasts and this winter, La Niña will likely dictate just how much snow will fall in Wisconsin.
La Niña developed in early September when ocean surface temperatures in the middle of the Pacific Ocean cooled and it is expected to continue through the winter months, according to National Oceanic and Atmospheric Administration’s Climate Prediction Center.
One thing La Niña has been known to do is split the jet stream when high pressure builds off America’s west coast, pushing the jet stream into Alaska and Canada, said Paul Pastelok, lead long range forecaster for accuweather.com.
That will allow more cold air to push into the upper Midwest but high pressure could prevent moisture from the Gulf of Mexico from mixing with cold temperatures, meaning fewer chances for snow early in the meteorological winter, which runs from Dec. 1 until the end of February.
“I think there will be some cold early on and then it will kind of back off and you’ll get normal cold,” said Pastelok.
Last winter Milwaukee recorded 37.8 inches of snow, almost 10 inches below the 30-year average of 47.2 inches. Pastelok said it’s likely Milwaukee could see more than 50 inches of snow this winter.
“Mid- to late season I think you’ll get a promising storm track as the high (pressure) in the Gulf starts to weaken, coming up to the Ohio Valley and Great Lakes and piling up snow mid-season, January and February,” said Pastelok.
Scientists monitoring winds in the upper stratosphere over the equatorial Pacific noted they have shifted and are moving in the opposite direction, which could mean a weaker polar vortex. That could result in warmer than normal winter temperatures later in the season but also allow more storms to move through.
“We think the polar vortex will get bullied around from air masses way up in the upper atmosphere around Alaska,” Pastelok said. “That could lead to more disruption of the polar vortex this year. Last year was one of the strongest (polar vortices) we saw in a while.”
That also means there’s a good chance Milwaukee could see a white Christmas because of the increased frequency of storms in a La Niña.
Because La Niña affects the patterns of tropical rainfall from Indonesia to South America’s west coast, the effects are generally strongest during the winter. That’s also when the jet stream is strongest over the U.S.
“You could be in a snow machine at least for a little bit,” Pastelok said. “It is a La Niña, so we’re not looking for a record snowfall, but likely above normal.”
The Old Farmer’s Almanac is predicting above-normal snowfall in the Upper Midwest with the snowiest periods from late December through late January. The almanac is also forecasting winter temperatures warmer than average with the coldest weather likely in December plus late January and late February.