There may be no monthly temperature record in southern New England as impressive as the December 1989 Arctic outbreak. It was epic. It was a relentless assault of cold!
At Bradley, the daily high temperature was below average every single day. The daily low temperature was at or below average every day except for the 31st.
Looking back in the record books the average monthly temperature was 18.1º which is a full 13.5º below average. December 1989 was the coldest December on record beating out the previous record holder, December 1917, by 2.7º. According to the 1981-2010 normals from NCDC the monthly average temperature in December is 31.6º at BDL with a standard deviation of 4.2º. December 1989 was a full 3.2 standard deviations below normal (assuming a normal distribution that means the cumulative probability for a monthly December average temperature <18.1º is 0.065%).
It wasn’t just Connecticut. Across northern New England the anomalies were even more impressive! Significantly below normal temperatures stretched from Texas to North Dakota and east to Maine and Florida.
While December and January feature the largest variance in temperature of any month in Connecticut (largest standard deviation) the records in 1989 were quite extreme. In fact there was a 19 day stretch in December with sub-freezing temperatures at BDL which is the longest such stretch on record. The month is also the 7th coldest month on record which is quite a feat to accomplish in December – a full month before the climatological temperature minimum.
So why did it get so cold? The 500mb anomaly for the month shows a very cold pattern.
A few things jump out at me on this plot. One is the strong -EPO/+PNA pattern across the Pacific. Positive height anomalies forced the Arctic Air off the top of the globe and down toward the U.S. The +PNA kept the cold east of the Rockies.
What is interesting is a lack of a NAO signal and an only somewhat negative Arctic Oscillation. If you look at the progression from beginning to end of December you can see an initially powerful -NAO that began to retrograde toward Baffin Bay during the mid-month and by Christmas was replaced by a +NAO. Even so, the end of the month remained cold with snow on the ground and a flexing +PNA ridge that kept us in the ice box including a sub-zero morning low on Christmas (though not as cold as the record breaking 1980 bone chilling Christmas).
Once New Years came the pattern flipped. And boy did it flip. January 1990 was was the 5th warmest January on record with a mean temperature of 34.7º or about 9 degrees above normal!
December 1989 is likely the most extreme month we’ve seen temperature-wise. In a month known for large variations in average temperature from year-to-year the 3+ standard deviation in 1989 is remarkable.
The hemispheric pattern evolved through the 31 days in a way that set the stage for a January torch. Pretty neat turnaround.