December 1989’s Epic Cold

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.

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Pattern Change Coming Into View

What a change! The atmosphere across the northern hemisphere is going to undergo a major transformation as winter finally appears ready to arrive. The current pattern (which has been remarkably persistent) features low heights over Alaska, the Gulf of Alaska, the Bering Strait, and Kamchatka. This +EPO pattern is essentially flooding the United States with mild Pacific air.

In addition the Arctic Oscillation has been strongly positive since October but is finally falling to near neutral numbers.  You can see the changes over the North Pole over the last 7 days. Here’s a look at 500mb height anomalies from 1/1/12 across the northern hemisphere.

You can see the negative height anomalies over the North Pole and across the Arctic with huge positive height anomalies across the mid latitudes. Here is the composite for Arctic Oscillation phases and you can see how closely the pattern fits the +AO.

Fast forward to 1/7/12 and you can see the huge changes underway in the Arctic. The negative height anomalies over the North Pole are gone and heights are rising thanks to a ridge north of the Caspian Sea across Russia and into the Arctic Ocean.

As we move forward the neutral AO state we have presently will become negative with a huge ridge developing over the Aleutians/Bering Strait by 1/15.

The height rises over the Aleutians is very impressive with a large omega block developing. Winter lovers rejoice! This will effectively dislodge the polar vortex and the cold from the Arctic and send it south.

With little upstream blocking this will not be a prolific snow producing pattern. It will, however, be a chilly pattern and with an active northern stream of the jet stream I expect some snow threats to be around.

The question becomes how long does this pattern change stick around? I’m fairly confident a strong +AO will  not return. One reason for this is a stratospheric warming event that’s ongoing. Temperatures are beginning to warm in the Arctic stratosphere and zonal wind anomalies are beginning to drop and in some cases reverse.

With the Arctic Oscillation on our side I doubt we’ll see a return to the December and early January torch. That said, I’m not sure the Pacific remains favorable for cold. For example the omega block over the Bering strait may retrograde into Kamchatka which will lower heights over Alaska and bring a return to the +EPO.

GFS Ensemble Mean 500mb Height Anomalies Jan 21st 00z

Though this may result in a warmer pattern overall by January 20th without the +AO I doubt it is anywhere near as warm as we’ve been. In addition renewed MJO convection over Indonesia (as forecast by the Euro weeklies) may help skew the pattern warmer over the U.S. by February 1 even with a neutral or negative AO.

Bottom line is that winter weather is coming with seasonable to below normal temperatures and some threats for snow. This won’t be an “epic” stretch of winter weather by any means but I do expect more winter than we’ve seen in since Halloween. What’s to be determined is how the pattern shakes out by the end of the month. Will stratospheric warming lead to a sustained -AO? Will a renewed push of MJO convection teleconnect to a warmer pattern overall? Will we see the +EPO vortex return? We shall see!

Winter May Finally Be Ready To Move In

After a seemingly never ending stretch of mild and snowless weather there are signs of a major change to the hemispheric weather pattern by next weekend.

The Arctic Oscillation has been strongly positive with a very powerful polar vortex over the North Pole. This has effectively kept cold weather bottled up in northern Canada. The ice cold stratosphere north of the Arctic Circle is beginning to warm and that warming is going to dislodge the polar vortex from the North Pole and bring winter to the United States by 1/16 or so.

Here’s the 500mb height anomalies on the European computer model for next weekend (day 8-10 ending on 1/16).

This is a total change from the pattern we’ve been dealing with since Thanksgiving. The +EPO Alaskan vortex is gone and replaced by a strong ridge over the Bering Strait which is helping to dislodge the PV over the North Pole. In addition a brief +PNA spike with ridging on the west coast helps send cold south.

Besides changes in the troposphere the changes in the stratosphere are impressive with a notable warming over the North Pole and disruption of the polar vortex. Here is the 10 day 30mb Euro temperature forecast. Between D0 and D10 there is a nearly +25C warming at 30mb at the North Pole. Definitely a change.

Beyond Day 10 the pattern remains chilly for the northeast with the +EPO nowhere to be seen and a -AO. The presence of a -AO has a big impact on temperatures across the U.S.

For people who want snow I don’t think this will be a prolific snow pattern with little upstream blocking (i.e. +NAO) but I do see seasonable to at time below normal temperatures for the second half of January. No question this is a major pattern change across the northern hemisphere. The question I have is how long does this change last – will it stick around through February? I don’t know.

If I had to guess I could see two wintry threats one around 1/16 with a clipper-type system and another around 1/21.