Over the first half of December the Northern Hemisphere has seen some fairly wild weather. Our part of the world has seen some record cold weather, whereas Alaska and Florida have seen record warmth. Over Europe and the Middle East there have been wild temperature swings and early-winter snows in places that rarely see snow. What has been causing this wild weather? Well, it seems as if the jet stream has been meandering a little more than usual so far this month.
What exactly does this mean? The jet stream is a ribbon of high-speed winds that flow eastward across the Northern Hemisphere and form along the boundary between warm air in the south and cold air in the north. As the jet stream flows along it develops waves — that is, the path it takes begins to curve north and south, much like a meandering river. If one of these waves pulls the jet stream north of a location, that region will see warm weather; if the jet stream moves to the south of a location, colder air moves in. In the summer, the jet stream is usually well to the north of our region and we stay mostly on the warm side. In the winter the jet stream sags to the south and we tend to be on the cold side.
The waves or meanders in the jet steam do not usually get that large — that is, we don’t see the jet stream going really far north and then diving way south over a short distance. Instead, the meanders tend to cover a fairly large distance, bringing periods of warm or cold weather to a region. This December the jet stream has not been behaving as usual; instead, the meanders have become exaggerated and as a result, there has been some very unusual weather.
Let’s start in North America. Earlier this month we saw the jet stream become very exaggerated as a strong series of ridges and troughs forced it to climb way north over Alaska, then dive deep into the south-central U.S. before climbing well to the north over the Atlantic. This brought some of the coldest weather central North America has seen since at least January 2011 for Canada and all the way back to 1990 for regions farther south. Record lows were set in several locations across the Canadian Prairies and the northern U.S. As far as I was able to find, only one location set an all-time record low during the cold snap. Lakeview, Oregon hit -32.8 C on Dec. 8, breaking the previous all-time cold record of -31 C set way back on Jan. 15, 1888.
While our region was freezing its butts off, Alaska and the southeastern U.S. were breaking heat records. The north coast of Alaska was particularly warm, with temperatures near +4 C being reported near Prudhoe Bay, with rain falling! This reading may be the warmest temperature ever recorded in December along the Arctic coast of Alaska. Farther south, temperatures also soared, but to a different level. As the jet stream swung north, warm air flooded into the southeastern U.S., pushing temperatures to near 30 C in some places; this came close to or tied some all-time December records.
Over Europe a pattern similar to ours developed and brought with it some extreme temperature patterns. In Sweden, temperatures on Dec. 3 were well above freezing, with temperatures in the north-central region being in the +5 C range. By Dec. 9, temperatures had plummeted to near -40 C in this region. But wait, it gets better: by Dec. 11, temperatures soared once again and to new heights, with high temperatures pushing close to +10 C in some regions! This broke several all-time records for December. In Umea, Vasterbotten, the official high was +9.5 C, which broke the all-time December record of +9 C set back in 1932.
This large northward swing in the jet stream meant that it also had to swing way south. This southward dip allowed cold air to pour into the eastern Mediterranean. Snow was reported at low elevations in Jordan, Israel and Egypt. While it is not super-unusual to see snow in these regions, it is unusual to see it this early in the winter. As can be expected nowadays, some of the reports from these regions have been exaggerated. Cairo was reported to have received snow for the first time in 112 years, but weather historian Christopher C. Bert of the Weather Underground points out that no snow actually fell in Cairo; instead, it was small hail, with the Cairo airport reporting rain and an air temperature of +5 C.
So, what does this mean for us and the weather for the rest of this winter? Well, the real answer is “Who knows?” If this pattern of unusually large meanders in the jet stream continues, then we should expect these large swings and variations in temperatures to continue — along with the unusual weather that comes with them.