Here we go again. After a brief warm-up this week after what seemed like a month of single-digit temperatures, weather forecasters are calling for a low of 4 degrees Saturday night and a low of 2 degrees Sunday night.

What's causing this roller coaster of frigid weather, followed by snow and rain during brief warming periods and then a plunge into the deep freeze again?

Lee Hendricks, a meteorologist with the National Weather Service's Pittsburgh office, credits an immobile high-pressure atmospheric ridge 3,000 miles away for this yo-yo effect.

He said, “Right now, it's just the overall pattern in North America. There's a ridge of relatively high pressure on the West Coast of the United States.

“And basically that leaves central Canada and the United States open to pulses of cold air coming from northern Canada,” said Hendricks.

Julie Snow, professor of geography and an atmospheric chemist at Slippery Rock University, said, “A ridge is the top of a wave. In the upper atmosphere, the air moves in waves just like a fluid.

“Those waves have troughs and ridges,” she said. “When we have a ridge, in the northern hemisphere, to the right of the ridge, northern air is drawn down and the deeper the wave the more northerly the air will be.”

“That stationary wave is funneling cold air from the north into the United States. We are seeing more and more of these waves,” said Snow.

That's because the jet stream, bands of strong wind in the upper levels of the atmosphere that blow from west to east, is shifting, Snow said.

The jet stream follows the boundaries between hot and cold air, she said and when the contrast between the cold northern air and milder southern air is high, the jet stream is straighter, like a taunt string, she said.

“But because of climate change, the temperature difference is less as the north warms. The jet stream wiggles and those waves are getting deeper,” Snow said.

“It's not going to happen every year, but we are noticing the jet stream is losing structure,” she said.

And what's in store for the rest of the winter?

Well, there's good news and bad news, according to the weather service's Hendricks.

The West Coast high-pressure ridge will begin to break down in the near future.

“It's looking like once we get through this pulse of cold air through Wednesday, the pattern is, we will begin warming up again,” he said.

“There will be a ridge of high pressure forming off the East Coast of the United States that will bring more active weather,” said Hendricks.

“Temperatures will not be quite as cold as we've been getting,” said Hendricks.

“We will have more seasonal highs in the 30s and lows in the 20s,” said Hendricks. That's the good news.

The bad news, he added, “The more active weather pattern means above-average precipitation. That means more freezing precipitation or more snow.”

“It's up to readers to judge if it's a good change. There's no free lunch,” Hendrick said.

Snow said the rest of the winter might be influenced by a weak La Niña phenomenon, a cooling of the water temperature, in the Pacific Ocean.

“Typically during a La Niña, we see colder temperatures in the northern United States and warmer temperatures in the south,” Snow said.

“We can also expect the northern United States to get a little wetter than average,” said Snow.

As for the rest of the winter, the oldest prognosticator of the weather, the 226-year-old Old Farmer's Almanac, predicts that after the next few days, the area will get a break from frigid temperatures for awhile.

“It looks like the next cold snap will be in early to mid-February,” said Sarah Perreault, senior editor of the almanac. “You are going to get quite a bit of snow in early February and again in March.

“At the end of February and the beginning of March, temperatures will be above normal which will be quite nice,” said Perreault, who says the almanac has an 80 percent success rate in its weather predictions made using a formula devised in 1792 by almanac founder Robert B. Thomas.