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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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!

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.