2-meter temperature anomaly, in °C, via Tropical Tidbits

If you're in the eastern half of the U.S. or Canada, doesn’t it feel a little strange to wear a T-shirt without a jacket and throw open your windows just a week before Christmas? We should have snow and cold by now, but instead these regions are knee-deep in a record-breaking warm spell—a complete flip in the pattern we’ve grown used to in recent years. If it’s hard for you to remember such a comfortable stretch of temperatures so late in the year, the data is there to back you up; in many cases, we’ve never recorded this kind of warmth so close to the end of the year.

There are certain places in the U.S. where comfortable temperatures in the cold season are normal. California is beautiful this time of the year. Florida is a haven for northerners looking to escape the deep chill, and aside from an occasional shot or two of cold air, the entire Gulf Coast is pretty temperate through the winter.

GFS model forecast for the jet stream on December 13, 2015, via Tropical Tidbits

What isn’t normal, however, is experiencing spring-like warmth as far north as the Great Lakes and Canada, which is exactly what we’re seeing this year. The jet stream has been flipped from its all-too-familiar orientation of late. As recently as this past summer, we saw a sharp, persistent ridge parked over the west coast of the U.S. and Canada, resulting in abnormally warm, dry weather that exacerbated the region’s devastating drought and raging wildfires. Downstream, the opposite side of North America saw a string of troughs, ushering forth blasts of very cold air and occasional bouts of snow and ice.

The pattern we’ve seen for the past month or two is almost a mirror image of what we’ve seen for so long, creating a steady trough out west and a persistent ridge out east. This reversal is likely aided by the intense El Niño in the Pacific Ocean—a phenomenon that creates abnormally warm sea surface temperatures for an extended period of time, temporarily altering global weather patterns. The ridge over eastern North America is allowing warm weather to prevail, shattering hundreds of long-standing records in the process. 

Graph: Dennis Mersereau | Data: NWS/xmACIS2

Case in point is Buffalo, New York, a city in the far western part of the state that sits on the eastern shore of Lake Erie. Up until the afternoon of December 18, Buffalo had only seen a trace of snow so far this season—a ‘trace’ constitutes snow that melts when it hits the ground—when the city historically averages more than a foot and a half of snow through December 18. This was the latest Buffalo had ever gone without recording measurable snow (at least 0.1”) since reliable measurements began; according to USA Today, these snow records stretch as far back as the 1870s.

The burst of snow on the afternoon of December 18 only produced a light dusting of snow—the city’s airport reported 0.1”, just enough to be considered “measurable.” The last time the city saw so little snow by this point in the season was back in 1998, when they only recorded 0.4” by December 18. (The Buffalo Niagara International Airport ultimately went on to record 75.6” of snow during the 1998–1999 season.)

Adding insult to injury for snow lovers, just last weekend Buffalo tied its second-warmest temperature reading ever recorded in the month of December. The high temperature at Buffalo Airport on December 14, 2015, was 71°F, a whopping 35°F above the city’s average high of 36°F for the date.

Buffalo’s lack of snow is noteworthy because the area falls victim to many feet of lake-effect snow every year during the late fall and early winter. Lake-effect snow is a localized phenomenon—generated by the heat of a nearby lake—that can drop an incredible amount of snow in a short period of time. The size of the Great Lakes allows each body of water to retain heat long after bitterly cold air masses begin to dig south from Canada. The cold air directly above the surface of the lake heats up through conduction and rises through the atmosphere via convection, just like the process that creates a thunderstorm. 

This convection creates heavy bands of precipitation that align parallel to the prevailing winds, usually coming ashore downwind and, when the air is cold enough, dumping very heavy snow on the unlucky cities in its path. The orientation of Lake Erie allows thick, single bands of lake effect snow to develop, creating a sort of fire hose effect, blasting a small area with extreme snowfall totals and leaving the immediate surroundings with a relative dusting. Lake effect snow season comes to an abrupt halt once ice finally covers a significant portion of the lake’s surface, which is slow to happen in warmer years.

The city’s lack of snowy weather is a stark comparison to what happened just last year. In November 2014, Buffalo’s southern suburbs saw nearly seven feet of snow in a few days after back-to-back lake effect snow events. Not only did the storm break records, but the onslaught of snow closed most roadways for an extended period of time, and many stranded residents were further isolated when the snow drifts piled up to their roofs, forcing them to tunnel their way outside.

The shift in atmospheric fortunes is actually more good news than bad, giving easterners a break from the cold of previous winters (though it’s hurting the ski industry), and this type of pattern is bringing much-needed rain and snow to the west coast. 

Above is a comparison of the U.S. Drought Monitor on September 8, 2015 and December 8, 2015. Darker colors indicate a more severe level of drought, culminating in the “exceptional drought” gripping much of California and parts of Nevada. The constant rain has done wonders for the drought in parts of the west, eradicating it around Seattle and significantly beating it back in western Oregon and parts of the northern Rockies. If El Niño continues at its current intensity, it’s entirely possible that the winter will continue on with warmer/drier weather out east and cooler/wetter weather out west. That’s not to say it’ll stay that way—we’ll probably see a few big snowstorms east of the Rockies, and at least some lake effect snow in Buffalo—but the overall pattern points to more warmth for the beginning of the new year.

Editor's note: This post has been updated to include recent snowfall in Buffalo.