It’s a case of déjà vu all over again across the continental U.S.: While the eastern part of the country braces for a deep chill, the West is baking in record-breaking heat.

That pattern was the dominant one over the last couple winters and exacerbated California’s tenacious drought. A strong El Niño finally disrupted that pattern this winter, helping to fuel storms that have brought much needed rain and snow to the parched state.

There are concerns that this encore appearance of a ridge of high pressure and warm air over the West could threaten some of the progress that has been made in building up a healthy mountain snowpack.
 


While similar to the pattern that stuck in place over the past couple winters, the current setup isn’t quite the same. The so-called “blob” of warm waters off the West Coast that reinforced the ridge in recent winters has dissipated, and the eastern half of the country has actually had a very mild winter so far.

The blast of cold that the Midwest and Northeast will see this weekend comes thanks to a warming episode in the Arctic stratosphere. That warming pushed around the so-called polar vortex, the current of fast-moving air that pens in cold air at the pole, according to Weather Underground. That push has allowed frigid air to surge southward.

Some scientists have suggested that such wobbles in the polar vortex could be more likely as the planet warms because of the effects of Arctic sea ice loss on the overlying atmosphere, though that idea is hotly debated and the subject of continuing research.

The Arctic outbreak isn’t expected to set many low temperatures records — such records are becoming less and less likely as winters overall warm up. Windy conditions will make the air feel even colder, though, and are setting off unusual February lake-effect snows around the Great Lakes.
 


Normally those lakes would be too covered by seasonal ice to generate much snow; lake-effect snow generally happens in late fall and early winter, when bouts of chilly air blow over the still-warm lakes, pulling up moisture that then falls as snow on shoreline communities. But as the world warms thanks to the excess heat trapped by accumulating greenhouse gases, lake-effect snows could extend further into the winter season as lake temperatures also warm and reduce ice cover.

Out West, the snow has stopped for the time being, as the ridge of high pressure is shunting storms up over the Pacific Northwest and keeping California sunny and dry.

Daily high temperatures across the state have been hitting 15° to 25°F (8°C to 14°C) above normal with dozens of records broken since Monday, according to Weather Underground. Some of the highs have been more typical of spring or summer temperatures, with Los Angeles International Airport recording a high of 85°F (29°C) on Wednesday. (Unlike record cold, record heat is on the rise nationally and globally.)

That unseasonable heat has set off concerns about the snowpack in the Sierra Nevada mountains, which is trending slightly above average for this time of year. The snowpack is a key source of water for the state in the warm months, as it slowly melts and helps to refill reservoirs.

Experts, though, said there shouldn’t be too much snowmelt from this warm weather, as the melt is caused more by incoming solar radiation than the air temperature. This time of year, the days are shorter and the angle of the sun in the sky is lower, meaning less melting than would happen in the sunnier days of spring.
 


“I think we’ll see some melt, but not catastrophic,” Jeff Dozier, a snow hydrology researcher at the University of California, Santa Barbara, said in an email.

There will be some compaction of the snowpack though. Water from the little melt that does occur will be partly trapped in the snowpack, where it will cause the surrounding grains of snow to grow and become rounder. As they do so, the grains will settle against one another more easily, make the snowpack less deep, but more dense, Dozier said.

The bigger worry in terms of the snowpack is that February is typically when some of the biggest precipitation in California happens during a winter with a strong El Niño.

“The big challenge right now is we should be accumulating snow and with the sunny skies that isn’t happening,” California state climatologist Michael Anderson said in an email.

There is hope, though, that El Niño isn’t done with California yet, and that the stormy pattern will re-emerge later this month and help put an even further dent in the drought.