This December will go down as the warmest December on record--shattering the previous record set in 2006 by over 100 Heating Degree Days (HDDs)! As a matter of fact, if this December continues on its current pace, it will be the warmest winter month (December - February) since 1950, basis normal, from both a gas-weighted HDD standpoint and a U.S. average (area-weighted) temperature departure. In other words, using virtually any metric, this month will be the warmest winter month ever recorded.
The only other month that was more anomalous from an HDD standpoint during the heating season (November - March) was March 2012. Again, March 2012 was the month that capped off the warmest heating season on record during the winter of 2011-2012. Furthermore, if December continues on its current warm trajectory it will go down as the second warmest month – basis normal – from a U.S. average temperature perspective, which takes into account all months back to 1950. The only warmer month, basis normal, was October 1963.
Given the impressive warmth this December and how it compares to the historical record, the question begs, how anomalous is this December compared to all months in either a warm or cold direction? This December actually ranks as the 6th most anomalous temperature departure from normal (3.22 standard deviations above normal), with the four most extreme months coming in as cold months. The most extreme month basis normal was the frigid January of 1979, which came in over 4 standard deviations below normal. Bottom line, this December is not only likely to go down in the record books as the warmest December by a long-shot, but also as one of the warmest months on record. Furthermore, this month will feature one of the most extreme temperature departures of any month, warm or cold.
Continuing on with the warm theme this December, another remarkable feature is showing up on the forecast maps. Throughout this week, the models have indicated a high pressure system or ridge off the coast of the Southeast is likely to strengthen to a point where the 500-hPa height exceeds 5940 m. To put this into perspective, a 5940 m 500-hPa ridge is indicative of strong ridge...in the heart of summer (July - August)! These consistent projections also beg the question: how rare was this system in that location based on the historical record since 1950? If a ridge of this magnitude were to verify, and it’s likely that it will, this would be the latest in the year that a 5940 m ridge would form in the vicinity of the southeastern United States. The previous latest date for the formation of such a feature was December 14th, which occurred in both 1972 and 2000. This is yet another example of how extreme the pattern is that we are currently dealing with this December.
The Polar Vortex (PV) continues to look strong and stable as we progress through the remainder of December and into the new year. However, there was slightly more variability this week in terms of strength, location, and shape of the projected PV in the 11 - 15 day period. This variability is likely the result of the model’s handling of the upcoming wave activity flux (WAF) pulse arriving during the next week. This WAF pulse or PV “Punch” is highlighted in the figure below by the red box immediately to the right of the black vertical line. Again, high WAF or a WAF pulse, is the force that opposes the circulation of the PV, which can ultimately lead to changes in the strength, orientation, and shape of the vortex. When the PV is as strong as it currently is, it typically takes multiple pulses like the upcoming one or a prolonged period of high WAF to cause significant alterations in the vortex. There is a second pulse that is coming into the forecast near the turn of the year that we will monitor, however, we do not believe these pulses together are strong enough to cause significant alterations to the PV.
There are no changes to the forecast today. The eastern half of the Nation will remain the extremely warm spot through the end of the year, while temps out west will be near-to-below normal. In the details, the least anomalous warmth across the Midwest and East will come in the next 5 days with a day of almost near normal heating demand coming tomorrow. Thereafter, highly anomalous warmth will set back in across the eastern half of the Nation through the remainder of the 15 day forecast period, where temperature departures will come close to 20 F above normal. Bottom line, the record shattering warmth and below normal heating demand, shows no signs of waning through month’s end.