What Will Happen After The Polar Vortex Collapse?

I have been heralding the collapse of the polar vortex for days now, and have written several detailed explanations on how the collapse will happen. But what will happen after the collapse? What will be the effects of a collapse of the polar vortex?

We'll start with observed conditions. The graph above shows North Hemisphere snow cover anomalies. There is a proven correlation between the anomalies of snow cover in the North Hemisphere in October and the Arctic Oscillation's phase in the following winter. As you can see, the first half of October brought below normal snow cover, and the month of December brought warmer than normal conditions across much of the nation. Now we're looking to the second half of October to predict what temperatures will be like for the rest of winter. The second half of October looks to have above normal to very above normal snow cover, meaning we could see the Arctic Oscillation blow its bottom out later on in winter. This bodes well for the predicted collapse of the polar vortex and the effects being felt in late January and through February.

Speaking of the Arctic Oscillation, let's look at the forecasts for the AO and the North Atlantic Oscillation (NAO) from a couple of big models.

The Arctic Oscillation's forecast from the GFS model shows the index bombing out into extreme negative territory- and that doesn't even cover the full extent of how negative the AO could get. When the AO goes negative, the polar vortex is naturally weakened and cold air will make a good push to go into the US. However, in cooperation with the multiple sudden stratospheric warmings (whose effects will be seen in late January), the Arctic Oscillation has a good chance to be tanking to an extremely negative area. That would most likely happen beyond January 20th. Again, Siberia supports a tanking Arctic Oscillation, meaning this forecast of a strong negative AO is not nearly off the table.

This is the ECMWF's forecast for the North Atlantic Oscillation (NAO). The model is projecting the NAO to be in a very deep negative phase, below the -4 value, which is extremely negative. In the negative phase of the North Atlantic Oscillation, the jet stream buckles to bring cold and storms specifically into the Northeast US and New England regions. Such a negative NAO would indicate that  cold and snow would be provoked to move east as soon as the negative PNA relaxes and the high pressure commanding the East US leaves the region. If such a strong negative NAO is still around post-January 20, some serious cold could bleed east into the East US, whereas a negative PNA could pose a significant barricade to the cold getting to the East right away. It will certainly get there, but it could be held back a few days due to the persistent negative PNA.

The ECMWF above is projecting potential vorticity (PV) values for the Day 10 forecast timeframe. Raised PV values suggest the presence of lower pressure, in this case the polar vortex. The polar vortex has split into two pieces at this point, with one over southwest Canada and the other in Eurasia. The polar vortex is severely decimated at this point, meaning a large amount of the cold previously held by the polar vortex must now be released, because it cannot be held by two smaller and weaker polar vortices.  Furthermore, a third sudden stratospheric warming is in the forecast, and if such a warming strikes in the perfect place, I find it likely that both vortices will be unable to sustain themselves and may dissipate completely, flooding the lower latitudes with Arctic air.

Models agree that the long range includes good chances for strong cold entering the nation, as shown in the GFS Ensemble temperature departure forecast in the first image below, and the CFS weekly forecast in the second image below.

After the polar vortex collapse, it's looking very cold for the end of January, supported by Siberia, the forecast models, and the stratosphere itself. When the end of January does come around and the US does not experience anomalous cold, I will be extremely surprised. I highly doubt the cold will miss the US; things are looking positive for a sharp Arctic outbreak in the nation, possibly persisting through February.

Andrew

When Will The Snow Come Back?

Snow has dropped off the radar for a while now, and questions are arising- when will the snow be back?

The problem is something called zonal flow. The term zonal flow refers to when the jet stream is displaced north and moves in a monotonous west-to-east line. This monotonous jet stream formation then stirs up above normal temperatures in the East and Central US, and commonly stays until a new pattern comes along.  My weekly forecast for the next 7 days is shown above (see Week 1 and Week 2 explanations by clicking here), and I am expecting a zonal flow formation to set up during this time period. This comes as the Madden Julian Oscillation (MJO) moves into Phases 5-7, which are best known for being favorable for warm weather. So we can pretty much rule out the next 7 days for any snowstorms across the nation.

In Week 2, we see a significant change occur in the jet stream pattern. High pressure has formed in the Gulf of Alaska, leading to low pressure holding its ground in the West US and the Rocky Mountains. The jet stream accommodates this predicament by pulling the strongest upper level winds south, and then pushing them far north as high pressure builds across the East Coast. This is a good example of the negative phase of the Pacific North American (PNA) index. In the negative phase, low pressure builds over the West US and, as physics dictates, high pressure then forms in the East US to balance out the low pressure. The negative PNA pattern brings about cold weather in the West and warm weather in the East. This pattern is probably the most favorable for the Plains, and that is where I expect to see the next snowstorm.

I anticipate low pressure to enter the Southwest, eject into the Southern Plains and move northeast on a track known as a Great Lakes Cutter. This cutter gets its name for going through the Great Lakes region, hence Great Lakes Cutter. This system should move through Oklahoma to Wisconsin, giving snow to the Central Plains and Upper Midwest. Given the high moisture content of the storm system, heavy, flooding rains would be expected across the lower Midwest, Gulf Coast and Ohio Valley regions. I expect this storm to hit between the 11th and the 13th of January.

This negative PNA pattern continues for several days, with the trough responsible for below normal anomalies in the West beginning to slide east very slowly. When the trough breaks free, it should move northeast into Southern Canada, where a meeting with a piece of the polar vortex could be in order. A strong cold front could follow this trough, and that could very well be what kicks off the brutally-cold Late January into February. This would happen around the 20th of January.

So it's looking dry for the next week thanks to the infamous zonal flow pattern. A negative PNA pattern will then emerge in the wake of the zonal flow, and I expect heavy snow to hit the central Plains and Upper Midwest around the middle of the month, with flooding rains in the Lower Midwest and eastern Great Lakes into the Ohio Valley. Beyond that, things get unusually murky thanks to the incoming Arctic outbreak that will start in the West and bleed East.

Andrew

Weekly Forecasts: Warmth Precedes Sharp Cold Outbreak

Warmth is slated to precede a sharp cold outbreak that will occur in late January and into February. Above is my weekly forecast for January 6 to January 13th. I expect a zonal flow pattern to set up, with the jet stream displaced north and warm air overrunning much of the nation. This will certainly decimate any remaining snow pack, thus warming temperatures even further for areas around the previously snow-covered regions. The Madden-Julian Oscillation will be headed to Phases 5-6 during this timeframe, both of which are favorable for warmer than normal temperatures across much of the nation. As time goes on, the strength of the MJO signal in phases 5-6 is forecast to increase, only increasing the strength of warm air that will be present in the East US.

In Week 2, I expect low pressure to build across the West US and Rocky Mountains, and high pressure will then build on the East Coast in response to such an anomalous low pressure system. Colder than normal conditions will be displaced south, as the Arctic Oscillation goes negative and cold air presses south in preparation for the late January arctic blast. As the week progresses, I expect the low pressure system to move northeast into southern Canada, where it will stick around for awhile and bend the jet stream to provide cold air for much of the Plains. Eventually, it will bleed south and penetrate the entire nation later on in the month, when temperature anomalies as low as 10 degrees below normal could be found- and then some. The North American Oscillation should keep negative through Week 2, allowing for this bleeding cold air to at least try and push east. However, the still-unfavorable MJO and still-negative Pacific North American index (PNA) will inspire a battle to form between semi-warm air and frigid cold air. The frigid air will inevitably win out, but it could be delayed from hitting the East Coast thanks to these other teleconnections.

Andrew

Arctic Trifecta Could Lead To Brutal January-February

It is becoming increasingly apparent that three big atmospheric factors will have to combine in an Arctic Trifecta to make for a brutal end of January into February.

Part I: The Stratosphere

The stratosphere is one of the three big factors needed for a brutal back-end winter, as it is where the polar vortex is based. If we want cold weather, we need that polar vortex to break down.

Observed temperatures are shown on the two graphs above, with the 1 millibar level being shown in the top image, and the 10 millibar level temperatures depicted on the bottom image. These two images show a spike in temperatures in recent days, and this is in response to the big sudden stratospheric warming (SSW) that is in India and spreading across much of Asia as an extreme SSW. The 50 millibar ongoing SSW has yet to be recognized at that level. Nonetheless, any progress is good progress as we will now see a round of damage be inflicted upon the polar vortex. This is the first warming of three that I am concerned about as far as implications on the polar vortex go, so we will move right along to the second warming.

The second warming event should take place in the next two weeks, and the ECMWF model is pinpointing its appearance in the 9-10 day range. Shown above is a forecast image of temperatures across the stratosphere and troposphere, the latter layer shown in the bottom part of the image. We want to pay attention to the upper stratosphere in this case. We see significant warm air anomalies prevalent in the upper stratosphere between 70 N and 90 N. A look at Days 6-8 forecasts reveals this warm air intrusion is only beginning on Day 10, and such an intrusion is signifying that, yes, the second warming event should be ongoing through Days 9-12. I cannot be confident on that timeframe, considering the ECMWF only goes as far out as Day 10. Any way you put it, this significant warming in the upper stratosphere bodes well for extensive damage being put on the polar vortex.

The negative zonal wind anomalies at the top of the right hand corner also suggest a second warming event is in the making. Notice the now-weakened polar vortex between 30N and 50N. This will only get weaker with this second warming, if the second warming turns out to be near the significance that the currently-ongoing one is having.

Skyrocketing heat flux forecast values and a downward-propagating geopotential Wave 2 forecast enhance the chances of a good second warming in the stratosphere:

Geopotential Wave 2 Day 10 forecast.

Heat Flux Day 10 Forecast.

If the geopotential Wave 2 does propagate downwards with its higher heights (high pressure, observed by the warmer colors in the top image), one can expect lower stratosphere disruption in the polar vortex. It doesn't matter if the polar vortex is gone in the stratosphere- it only matters if the troposphere feels the effects, and that's what we need right now to make sure this trifecta can happen. As far as the heat flux goes, it is nowhere as strong as the ongoing SSW, but again, progress is progress, and after a fairly dismal December, we will need all the help we can get.

And then there's the chance for a third warming. This third warming, if it were to happen, would come at a time when the polar vortex has already been split into multiple pieces and absolutely decimated on all levels as far as strength goes. Suppression south is beginning to occur with the remaining polar vortex pieces, meaning Eurasia and North America would begin to feel an Arctic chill.

This suppression is well observed in this long range GFS forecast, which shows the polar vortex entering the Great Lakes region. Temperatures as low as -30 are observed in the upper Midwest, and cities like Chicago, Quad Cities and Gary would experience temperatures well below 0. This is just one of many possibilities facing us if this third warming event comes to fruition.

I should note that the lower stratosphere is predicted to have a good piece of the polar vortex in southern Canada during this same timeframe, meaning that the stratospheric effects are propagating down to the surface. Cold air would certainly be in order for much of Canada and possibly the US if the polar vortex made its way close to the US. If it ENTERED the US, record-breaking cold is more than possible, but I want to wait several more days to let the GFS sort itself and any forecast errors out.

Part II: The Cross-Polar Flow

There is an atmospheric pattern called the Cross-polar flow that is widely acknowledged as the pattern that brings the coldest air to the US the easiest. The term originates from the way a strong ridge of high pressure builds into the Gulf of Alaska to the point that the jet stream actually has to cross the North Pole to get north of the high pressure in the Gulf of Alaska. From there, the jet stream collapses south into the Rocky Mountains, transporting frigid air as it goes. We are seeing good signs of the cross-polar flow setting up in the long range.

For example, hour 288 of the GFS control model run shows a fantastic cross-polar flow ongoing, with extreme ridging happening from the Gulf of Alaska to the Arctic Circle. The jet stream is then observed shooting south into the West US, with significant cold air following the jet stream. This pattern also opens the door for a McFarland signature, which is the death-bringer to crops everywhere near the Rio Grande. As observed by McFarland, an atmospheric pattern will set up in which high pressure will build in the East, and low pressure in the West. A severe arctic outbreak will press east across the nation, and anomalous high pressure will build in its wake, holding the nation in a deadly Arctic outbreak. This forecast certainly shows some McFarland characteristics, but for now we will just focus on the cross-polar flow.

The ECMWF model and ECMWF Ensembles also try to build some cross-polar flow, albeit to a much weaker degree than the GFS control:

ECMWF 500mb height anomaly for Day 10.

ECMWF ENS 500mb height anomaly for Day 10.

Don't immediately bet against the GFS control. The ECMWF Ensembles naturally have to be weaker than the individual model forecasts because the ensembles are just that- ensembles. They are composed of multiple forecasts and are only showing an average of the members' forecasts. It is entirely plausible that 50 of the 51 ECMWF ensemble members show extreme Gulf of Alaska ridging, but one member shows record-breaking low pressure anomalies in the Gulf of Alaska. While that's most likely not happening, just remember this is an average of dozens of individual forecasts.

Part III: The Teleconnections

The third and final part of this Arctic Trifecta is the teleconnections, particularly the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO). In the negative phase of the NAO, the jet stream buckles south to bring cold and storm systems to the Northeast, while the negative AO brings about a cold weather pattern across the nation. Both of these oscillations ca be detected by models, so let's see what they show.

Let's bring back the ECMWF Ensemble image for a moment. This is still valid at Day 10. We can clearly see a strong negative NAO and AO. How? High pressure is building over Greenland in this forecast. The negative NAO is achieved by seeing high pressure build over that land mass, in turn bringing about cold weather to the Northeast. The negative Arctic Oscillation is then found by the presence of multiple high pressure systems dominating the Arctic Circle, the typical domain of the AO. The negative AO and negative NAO would typically combine to bring cold air to the East in such a situation, but as you can see, high pressure is building across the East Coast. Why? The dreaded negative Pacific North American index, or negative PNA. The negative PNA, while a common trait of cross-polar flow and the McFarland signature, causes high pressure (and thus warm air) to build across the East US. We will need to see an equally strong negative NAO or a weakened negative PNA to see that high pressure get out of the East Coast. Eventually, by the end of January, this should very well happen.

This Arctic Trifecta can work out very easily, I have no doubt about that. The question is, can the negative PNA turn into more of a McFarland signature to push the cold east, or will it persist in the West? If persistance occurs, cold prospects are dimmed. However, the breakdown of the polar vortex practically assures that some degree of colder weather will be forced into the US- it's how cold that is the question.

Andrew

What I Expect For The End Of January

I have been talking a lot about the developing situation in the stratosphere, and have not really talked about what the end of January could be like. I think now's an appropriate time to do so.

The first thing to discuss is the stratosphere. As observed in the above animation, the month of January experienced two sudden stratospheric warmings- one centered over the Bering Sea, and the other (currently ongoing) centered over Canada. The ongoing SSW is the more significant warming between the two, with stronger and more widespread warming happening. With these sudden stratospheric warmings comes the movement of warm air. The basis of the SSW is anomalous warm air being forced up into the stratosphere, typically in response to some convection or torque-based event (there was a strong mountain torque event observed immediately prior to the ongoing SSW). With the forcing of this warm air is the weakening of the polar vortex, the driving force behind the Arctic Oscillation (AO), but that's a piece for later on. When you have large amounts of warm air being forced into the polar vortex, the vortex tends to go on a weakening trend. Depending on the strength and placement of the SSW, this weakening can be minor or catastrophic. This current warming looks to be enough to cause a split of the parent vortice into multiple daughter vortices.

In situations like these, I like to look at the polar vortex as an egg (bear with me on this one). When the SSW occurs and warm air is forced up, the polar vortex split can occur. Think of this as cracking the egg open, but not so far as to completely break it apart. The split of the vortex lets out a fair amount of cold air to lower latitudes but not the entire Arctic, similar to how a partial cracking of the egg leads to fair leakage of the inside contents but not the entire yolk.

Back to the relatonship of the SSW and anomalously cold air in the lower latitudes, there is typically a 1-3 week lag time. I prefer to use a 2-4 week lag time, because you're talking about thousands and thousands of feet to move air, and that takes time. Using that timeframe, and looking at the upcoming zonal-flow pattern ahead, I feel that the cold weather will be best felt beyond the third week of January, pretty much beyond January 20th. This will be of direct result from the ongoing sudden stratospheric warming, and the cold outbreak ought to last a good couple of weeks.

(Originally posted January 2) The GFS model is then pinpointing a third stratospheric warming in the long range, shown below:

10mb temperature at Hour 300.

10mb temperature at Hour 384.

Hour 384 is notorious for never verifying, but this warming forecast has been appearing on several GFS model forecasts, meaning the model appears more certain that this will actually happen, rather than not happen. The warming looks to originate over central Canada, right in the heart of a daughter vortice at that same 10 millibar level. That would only bring further weakening to the polar vortex/vortices.

If there is in fact a third sudden stratospheric warming (which I believe is definitely a possibility), there is no doubt that the amount of damage done to the polar vortex will lead to another, possibly stronger cold outbreak. The polar vortices can only sustain so much damage before another split into multiple vortices would result in a loss of the polar vortex altogether. That said, if a third SSW does occur, February would be quite a chilly month for many, as I have been hinting at for some time now.

Models also are tending to agree with the prospects for a sharp cold outbreak towards the end of January. The above forecast is of the CFS v2 long range model's weekly forecasts, with the top image valid for January 16 - January 22, and the bottom image valid for January 23 - January 29. In both images, we see sharp cold overpowering the nation, and significant warming ongoing in northern Canada. This significant warming bodes well for a ridge of high pressure in east Canada, something that could provoke a west-based negative NAO, which brings the party to the East Coast as far as cold and snow goes. However, considering the high variability between these weekly forecasts, I am hesitant to trust that negative NAO forecast. The cold has been trending on the CFS weeklies for a while now, so I am not as concerned for a bust potential in the cold as I am for the negative NAO.

My concern rests in the Madden-Julian Oscillation (MJO). This is a forecast chart of the MJO, with the forecasts in colors and observed conditions in a gray-ish color. Note how all members are in fair agreement with the MJO cycling into Phases 5-7 with increasing strength as time goes on. This does not bode well for cold weather in the East US. On the contrary, Phases 5-7 are very favorable for warm conditions across the nation. Now, if the forecasts were weaker with Phases 5-7 (closer to the center circle), I would not be as concerned. But alas, the models are in moderate strength with the dreaded phases of 5-7. The MJO only tries to reach pro-cold phases 8-2 by the beginning of February, and even then the MJO starts to die off.

We have the ball rolling very well here, as far as getting the cold down to the US. The stratosphere is experiencing a very formidable sudden stratospheric warming, and that will get the cold down to the surface. The question is, can the stratosphere overcome the MJO to get that cold air to come into the US rather than other parts of the world? My answer is yes. I have a good feeling we will have a good negative NAO setting up in mid to late January as the split in the stratosphere is reciprocated in the troposphere by pieces of the polar vortex being displaced away from the Arctic Circle and allowing high pressure to build in that area. Greenland would also be affected by high pressure formation, and the negative NAO would form. I buy more into an east-based negative NAO than a west-based negative NAO at this time.

Andrew