The Pattern Change After the Pattern Change

If you were to look at the 7 day forecast you’d probably fall asleep due to boredom. For weather lovers it doesn’t get much more dreadful than this.

WVIT Forecast 7 Day 2012

Even though this is about as dull as it gets for January there are tons of changes going on across the globe. In fact, the jet stream is about to undergo a really monstrous change between now and 10 days from now. This is an (overly) simplistic way of looking at it.

WVIT_Jet Stream

There’s a whole lot that will go into this pattern change and virtually every signal is banging the drum for a wholesale change of the pattern by January 15th. The odds of a significant Arctic outbreak over a portion of the country are good and the odds of a significant snowstorm in Connecticut during the last 2 weeks of January are better than normal.

– Meteorological Discussion Below –

If you had a box of Crayolas and had to draw medium range weather porn – this is what you’d draw.

18z GEFS Day 11-15 500mb Height Anomalies

18z GEFS Day 11-15 500mb Height Anomalies / Courtesy: Alan Huffman

I mean… does it get more beautiful??? A gorgeous -EPO ridge dislodging the already split PV (see yesterday’s post) and a phenomenal looking ridge bridge from the -NAO region straight across the North Pole over into Canada.

Not surprisingly, the D11-D15 850mb temperature anomalies are approaching -10C over Saskatchewan and Manitoba! The GEFS today aren’t alone. By D15 the Euro Ensembles are equally impressive with a beautiful +EPO/-AO/-NAO combo. This is a FRIGID COLD setup for portions of the lower 48 with some impressive cold not too far from southern New England.

m500z_f360_bg

While it’s too early to get into specifics regarding where the core of the coldest weather will setup I’m excited. Cross-Polar flow will deliver the goods to this side of the globe and the signal for strong ridging over Greenland (-NAO) starts raising the specter of a decent snow pattern for the northeast.

While the stratosphere is doing some good things upstairs near the Arctic Circle, tropical forcing from the MJO is also working its magic. There’s renewed vigor to the somewhat sleepy MJO thanks to a burst in MJO-driven convection near Indonesia.

ECMF_phase_51m_small

The ECMWF Ensemble MJO forecast shows the tropical convection rounding the equatorial Pacific after leaving Indonesia and winding up toward phase 7 by 1/20. Many of the dynamical and statistical MJO forecasts are similar.

These plots, courtesy of Alan Huffman, show the 500mb anomalies that correspond to each MJO phase in January.

Phase 5 / Phase 6 / Phase 7

Notice how mild phases 5 and 6 are for the northeast – and how cold (and stormy) phase 7 can be. While the initial Arctic dump may be to our west in the D11-D15 time range as long as the MJO keeps trucking toward the dateline we shouldn’t have much problem getting into the fun stuff. The change is coming – we just may need to be a bit patient!

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Not All NAOs Are Created Equal

Even if you have a passing interest in weather you’ve probably heard about the North Atlantic Oscillation. During the winter if you want a big snowstorm you need to watch the NAO. The NAO is technically defined as the difference in pressure between the Icelandic low and Azores high.

500 mb Height Anomaly Composite for a +NAO / Courtesy: NCEP

In general, below normal 500mb height anomalies over Greenland correlate to a positive NAO while above normal heights correlate to a negative NAO.

While a -NAO in the winter typically means colder than normal weather in the northeast and a storm track that’s favorable for snow not all -NAOs are equal. For example the location of strongest ridging in the north Atlantic means the difference between a pattern that favors storms tracking to our west (cutters) or storms that travel near or to our south.

Today’s model runs are a great example of the importance of the NAO ridge’s location. The 12z op Euro and Euro Ensembles show a pretty intriguing pattern. Let’s start with the Euro Ensemble 500 mb mean height forecast for 12z Wednesday 11/28 (216 hour forecast).

12z 500mb Euro Ensemble mean 216 hour forecast

The first thing to look at is off to our northwest. You can see strong ridging from the Aleutians to the Bering Strait which effectively dislodges the cold/polar vortex from the North Pole. Looking chilly! But the storm track would likely set up to our west. The reason why is the NAO is very east based. Strong ridging over Iceland doesn’t do much to help us get a good storm track from snow – we want the ridging over Greenland.

The 12z op Euro is very close to the ensemble mean. So this is a relatively high confidence forecast (for one that’s 200+ hours out).

12z Op Euro 216 Hour Forecast

The op Euro shows an impressive omega block/-NAO over the north Atlantic but don’t expect early season snow with that setup. In fact, you can see the storm tracking through Chicago on the 216 hour op Euro forecast.

I do think that there is some threat for wintry weather during the middle of next week. If we’re able to change the location of the NAO block we may see a storm track a bit farther south and east. Mix? Snow? Ice? Possible.

Regardless of the storm’s track there’s no question that we’re going to get colder as we dislodge the cold from the Pole. The northern tier of the U.S. looks chilly past this weekend for sure.

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.

No Festivus Miracle – No Sign of Winter

What a dull stretch it’s been! The weather pattern across the northern Hemisphere has returned to where it was in the beginning of December. Low pressure over Alaska and Greenland means an exceptionally warm +EPO/+NAO signal for the northeast.

December so far has been near 6 degrees above normal in greater Hartford and though we’ll shave a bit off that over the next week there’s no question the month has been an unmitigated torch.

Here’s the GFS ensemble mean anomalies for 1/1/12 (the Euro ensemble means are fairly close).

The substantial negative height anomalies over Alaska and Greenland show this pattern isn’t going to get much better for snow lovers. Transient ridging in the +PNA regions (western NOAM) may deliver brief cold shots and keep New England a bit cooler but nothing too cold. I fully expect the next 15 days to average above normal and likely most of January.

To show you how bad the +EPO/+NAO (negative heights over Alaska and Greenland, respectively) here are the sfc T correlations.

Put them both together and that’s a pretty lethal combination for cold. At the same time there doesn’t appear to be a mechanism to dislodge the current pattern. For example the MJO has entered a dormant phase and does not look like tropical forcing will be able to dislodge the Alaskan death vortex.

Torch on!

Will It Stay This Warm?

The easy answer is no. Monday was a full +20 above average in the greater Hartford area which is an impressive departure for normal. The month as a whole so far is +3.1 in metro Hartford which is nothing to sneeze at either.

To figure out if it will stay this much above average we have to look back and see why it has been so warm.

November 500mb Departure From Normal

The 500mb height anomalies show substantial negative departures over Alaska and the Pacific Northwest. Ugly if you like cold and snow in New England. This -PNA/+EPO combination is in general a very warm pattern for the northeast. Inversely, across Alaska, this has been a record cold pattern!

In addition to the cold vortex that’s been spinning near Alaska the stratosphere has become exceptionally cold over the Arctic.

30mb Height Anomalies

Up in the stratosphere the polar night jet has gone berserk with a powerful polar vortex. This +AO configuration effectively will mean no sustained high latitude blocking for the foreseeable future. We will need some type of stratospheric warming (possible later in the winter – a SSW?) or some other mechanism (like Rossby wave train breaking) to break up this vortex. Until that happens it will be a challenge to get sustained cold and wintry weather in the northeast unless the Pacific cooperates.

Here’s an example of what we saw last year from Dec 15, 2010-Jan 30, 2011 with that epic 45 day stretch of weather.

You can see the high latitude blocking with an impressive -NAO that brought the record snow into southern New England for nearly 2 months last year.

So where do we go from here? Winter lovers will have some reason to rejoice heading into December as it appears our substantially above normal weather pattern will be changing to something more average.

Finally we’re beginning to see some improvement in our computer models for those who like snow and cold. Ridging is showing up in the PNA region and some ridging over Alaska to help flip the EPO and bring some cold out of northern Canada. Since we can’t rely on blocking in the North Atlantic seeing changes out in the Pacific are important.

Although many of these features appear to be transient this is a reversal from the sustained warmth we’ve been having. We’re getting there. I expect the Day 6-10 (Dec 5-9) and Day 11-15 (Dec 10-14) to average near normal with some transient bouts of below (maybe even much below) normal temperatures.

As for snow chances it doesn’t appear like Connecticut will see anything significant through December 10. It’s possible after that point we become a bit more wintry. The good news for skiers is that there may be several snow chances across northern New England over the next 1-2 weeks which, when combined with colder temperatures, should help boosting the base.