Much of the nation is in the grip of a fierce cold blast, but here in D.C., things haven’t been so bad. That’s about to change in a big way over the next week, with Sunday’s disruptive wintry mix and a punishing punch of polar air landing here by Tuesday.
As much as this December seems unusually cold – and it is, at this early stage – it could be worse. Some in the meteorology field are drawing comparisons to the early winter of 1983-84, which set a slew of cold records primarily over the Central U.S. and got bitterly cold in D.C. a week before Christmas. However, while the current and upcoming polar plunge shares similarities with the pattern from December 1983, there are enough differences to suggest that D.C. avoids the worst of this winter wickedness.
To be sure, temperatures are poised to drop 15-20 degrees below average during the middle of next week.
But, the magnitude of that cold anomaly (number of degrees below the average temperature) will be barely half of that which is currently in place over the western High Plains.
What’s behind the polar plunge?
One of the key drivers to the Arctic outbreak is a large upward bulge in the jet stream – what meteorologists call a ridge – over Alaska.
Surface high-pressure areas, which contain frigid air, build over far northern Canada and slide down the eastern side of the Alaskan ridge, thanks to the southward movement of the jet stream. Stronger high-pressure cells pack colder air, and the very dense nature of Arctic air often means it will undercut and eventually replace an existing warm air mass. We will experience this locally on Sunday as cold, dense air sinks to the surface and warmer air “overruns” the surface, causing a wintry mix. The Arctic boundary will push through completely on Tuesday.
Now turn your attention to the forecast map for this afternoon (the top graphic above). Note that the pressure reading associated with the surface high over Alberta is near or slightly above 1050 mb. That is certainly high! It does not come close to the pressure measured on Christmas Eve 1983, however, in Miles City, Mont., which Wunderground’s Weather Historian Christopher Burt says set the record for highest pressure in the U.S. at 1064 mb.
Just how cold did it get in December 1983?
It was so cold around Christmastime in 1983 that Reagan National remained below freezing for 86 consecutive hours between December 23-27. The airport set record lows of 5 and 3 on Christmas Eve and Christmas, respectively. Dulles’ lows hovered around zero on the 24th (1), 25th (-2) and 26th (-1). As a whole, December 1983 was nearly four degrees below average (with a mean temperature of 36F).
More impressive record-setting cold plagued the Plains and parts of the Midwest that month. Amarillo recorded subzero lows on seven days mid-late month (the average low is 23 for that part of December) and set, in total, six daily low maximum temperatures and five daily minimum temperatures. On Christmas Eve 1983, Amarillo’s high/low showed little spread (2/-3).
Sioux Falls, S.D. set four daily low maximum temperature records, with the lowest high occurring on December 23, 1983 (-14). The average high on that date is 27, which meant that the town was an astounding 41 degrees colder than normal! According to Joe Sheehan of the National Weather Service Forecast Office (NWSFO) serving Sioux Falls, temperatures fell below zero on the night of December 15 and didn’t rise above zero again until Christmas. Also, according to Joe, a temperature of -22 in the early morning hours of Christmas Eve combined with average wind speeds of 30 mph combined to produce a wind chill temperature of -82. More can be read here.
Finally, in the midst of its coldest December on record at nearly 13.5 degrees below normal, Chicago sank to -25 on Christmas Eve 1983, a reading low enough for third coldest in the city’s recorded weather history. With wind speeds that morning ranging between 29 and 41 mph, the Windy City also experienced a wind chill temperature of -82, the coldest wind chill in Chicago’s recorded history. Highs remained well below zero for three consecutive days on the 23rd (-6), 24th (-11), and 25th (-5), each a daily cold maximum record. The NWSFO serving Chicago wrote more on this and other notable December weather events here.
How the current pattern is different from that in December 1983
A look at the upper levels of the atmosphere – around 18,000 feet above the surface, or at the 500 mb level – helps to explain why December 1983 was so cold nationwide. That same large ridge over Alaska responsible for the current and coming Arctic attack was a very prominent feature 30 years ago.
The center of the upper-level high was situated midway between Alaska and Greenland early-mid month, but migrated south into mainland Alaska during the second half of December 1983. That southward realignment coincided with a crash in the Eastern Pacific Oscillation (EPO) index, an atmospheric signal that describes the pressure pattern over the central North Pacific and northeast Pacific basins. When the EPO index turns negative, a high-pressure ridge builds over the northeast Pacific and extends into Alaska; the jet stream dips well to the south across Canada and the Northern U.S.
Oftentimes, the anomalously cold air will spill over into the eastern U.S., though its duration is limited without a high-latitude blocking pattern in place. A negative phase of the Pacific North American (PNA) pattern, described by CWG’s Wes Junker as a pattern that favors a high-pressure ridge over the East and a storm track to the northwest, is a resisting force to the northern-based cold, and usually results in warm southerly flow and above normal temperatures here in D.C.
The PNA index was only slightly negative in December 1983 and, especially mid-late month, was absolutely overwhelmed by a flood of Arctic air downstream of the Alaskan ridge. Thirty years later, the PNA is trending more negative. It has been negative more often than not since late September.
What does this mean moving forward, beyond next week’s cold shot from Siberia, and another possible Arctic blast early during the week of December 16? Well, just as with anything in weather and climate, all factors should be considered and evaluated. Much of the latest ensemble model guidance features a westward shift in the ridge over Alaska to a position closer to Siberia. This shift appears to also draw the core of anomalously cold air westward, perhaps into British Columbia and Alberta. That may enable the warm ridge along and off the East Coast – beaten down temporarily by the cold shot next week – to exert more influence on D.C.’s weather toward the last third of the month.
With so much cold air remaining in Canada, we will have to monitor the risk for colder interruptions. Anomalously cold spells would seem to be more likely across the northwest and north-central U.S. toward the last third of December, but the cold could slosh south and east from there at times. It’s important to state that an overall warmer-than-normal period – perhaps slightly so – would be susceptible to colder variability (highs in the 30s possibly). And if you time the colder air with a favorable storm track just right, then wintry precipitation events would remain in play.