It’s that time of year again! When everyone from the 13 year old weather fanatic with a Facebook following to the National Weather Service puts out a winter forecast. Unlike any season, the winter forecast is of the biggest demand simply because the winter has so many impacts on a variety of facets in our economy, life styles, and even emotional state. If you don’t believe me about the emotional state, just talk to a snow lover after a potential snow storm turns to rain. It’s not pretty.
As with the past 7 years (time flies) I will walk you through my thoughts on what to expect this winter based on the data I am seeing. I will explain why certain features will be important for the forecast and just as important, what could make the forecast go wrong. Then there will be a monthly break down of what I expect overall for this winter to help fine tune how this winter should unfold. So let’s get down to it!
ENHANCERS AND DRIVERS
First, I want to explain to you the term of drivers of a weather pattern and enhancers of a weather pattern. Understanding which factors drive a weather pattern and why is key to being able to forecast for any season. There is typically a rush to go with climatology based on one factor from some sources, like the state of ENSO which can feature La Nina, Neutral, or El Nino conditions. However, there is more to forecast for the winter than just the state of ENSO. Man, I wish it was that easy too!
So what is the difference between and enhancer and a driver. An driver in the weather pattern has a direct influence on the 500 MB pattern and alters wind patterns. For example, thunderstorm development around the date line. There can be more than one driver in a weather pattern which can interact with each other.
An enhancer in the weather pattern can enhance certain features in the weather pattern that is in place. The enhancer can be a clue into when more active periods of the weather will evolve. A perfect example of this feature is the Madden-Julian Oscillation or MJO.
There will be three fundamental drivers in the weather pattern this winter. The first is the obvious, Sea Surface Temperatures in the Pacific. Note I did not just say El Nino, but the entire Pacific. The second is the Sea Surface Temperature Anomalies in the northern and western Atlantic. The third is the state of the stratosphere which will influence the development of the Arctic Oscillation and high latitude blocking over the northern Hemisphere.
We will break down this discussion around these three drivers and also discuss the enhancers involved in these drivers of the weather pattern.
THE PACIFIC OCEAN
The pattern in the Pacific is going to be a dominant influence on the winter’s weather pattern. Obviously we have El Nino to consider which is a very warm body of water between 170E and 80W. However, there is also the positive Pacific Decadal Oscillation (PDO) which is a very warm body of water in the Gulf of Alaska down the West coast.
El Nino’s of Years Past:
Let’s just get to the elephant in the room here first and foremost. There is a lot of talk comparing this El Nino to the one in 1997/98. Is that a fair comparison? Well, let’s look at October 1997.
Well, I think everyone will agree that the El Nino in October of 1997 was very strong. There is one huge difference though in the Sea Surface Temperature Anomalies (SSTA). That would be in the Gulf of Alaska and down the West coast where temperatures are well below normal to near normal. What you have is called a negative PDO which has far different influences on the atmosphere than a positive PDO.
As you can see, there are fundamental differences between this El Nino configuration and the one in 1997/98. In fact, this El Nino is different from any of the other El Nino’s we have seen over the past several decades due to the unique combination of how this El Nino is developing and the state of the PDO right now. So taking old solutions from previous strong El Nino events simply is not a valid approach. So now that we got that out of the way, let’s move to what we do have.
El Nino and Convection:
The key, in my opinion, to understand the influence of ENSO on a 500 MB pattern, you have to focus on two factors. Where is the strongest thermal gradient in the SSTA and where is the convection developing in the tropical Pacific. Typically, both the thermal gradient and location of the strong convection are one in the same.
I have been watching how this El Nino has been developing all Summer and Fall. This El Nino is rather unique in that while the warmest temperature anomalies have bounced around from NINO 1+2 (east) to NINO 3 and 3.4 (central), the strongest thermal gradient has been continuously in NINO 4 (west). The reason for this is due to the SSTA being so warm from NINO 3.4 to NINO 1+2 that the difference in temperatures from 3.4 to 1 is very small. Meanwhile, in the wester region, specifically from 160W to 160E, the temperature gradient is very strong from 2.25 Celsius above normal to -2.25 Celsius below normal. The result of this gradient is strong low level and mid level convergence.
This leads us to convection that has been in place all Fall. The chart to the left shows the development of convection or thunderstorms in the Tropical Pacific. The areas in blue are where thunderstorms are constantly developing due to rising air while areas in red are where sinking air is found.
Note that the blue areas focus at the date line and then travel up towards North America, specifically around northern Mexico and the southern United States. What we are seeing is the Sub Tropical jet stream.
These observations are extremely important. When convection is constantly developing around the date line (180 degrees Longitude), the Polar and Sub Tropical jet streams are significantly altered. This observation was well documented by Kocin and Uccellini in Northeast Snowstorms, Volume 1 to show that for every major winter storm that impacted the East coast and each winter with active weather patterns, this convection anomaly was present in the Fall months and continued into the Winter months.
The convection in this case is starting to alter the evolution of the 500 MB pattern in the Sub Tropical and Polar jet streams. This influence has started to produce below normal 500 MB heights to the south of the Aleutian Islands in the Polar jet stream and to the east of Hawaii in the Sub Tropical jet stream.
Future of El Nino:
There is a lot of debate on what to make of El Nino moving forward. There is no doubt that El Nino will be in place through this Winter, but to what degree?
It is my opinion based on the temperature trends and sub surface sea temperature anomalies, we are approaching the peak of this El Nino over the next three weeks. The chart to the left is the Equatorial Upper-Ocean Heat Anomalies, basically in all the El Nino regions.
Strong El Ninos typically have two major peaks of above normal temperature anomalies. Following the second peak, every strong El Nino including the historic 1997/98 event, weakened steadily from that point now. As you can see we had one major peak back in April as El Nino ramped up. The second peak is coming to an end now. So now I just showed you the observations, how about the models?
As you can see from above, almost every individual model and all the Ensemble mean guidance in all sections of El Nino suggest a steady weakening of El Nino starting in late October to mid November. There are some guidance that suggest later, some earlier. A moderate El Nino is a far different animal than a strong El Nino in terms of allowing the Polar jet stream to drop south over North America rather than be forced over Canada. Note also that the decline in Nino 3, 3.4, and 4 are gradual and keeps the thermal gradient discussed above in place through the winter.
Meanwhile in the North Pacific:
Meanwhile, in the northern Pacific we continue to feature a strong signal for a positive PDO. Note the above normal temperature anomalies throughout the northern Pacific, the below normal SSTA over the central Pacific and then the above normal SSTA between 10N and 40N and 150W and 100W. These features are certainly rather unique this Fall and have a significant influence on how the Pacific Polar and Sub Tropical jet streams influence the North American patterns.
Much like around the Tropical Pacific, thermal gradients provide us with an important clue on the future of the 500 MB pattern. Remember, the atmosphere is driven by one key rule, the conservation of mass and energy. Mass and energy are in the form of temperatures. That’s why we have weather as the Earth attempts to balance out the cold and warm anomalies.
Now that you understand this factor, we have a powerful gradient surrounding that cold pool in the central Pacific. The thermal gradient along and east of the date line to the north of 30N supports the development of a trough to the south of the Aleutian islands and a ridge to develop along the West coast and over western North America. This is a text book signature of a positive PDO.
As you can see above, when a positive PDO is in place, below normal heights are found to the south of the Aleutian Islands, above normal anomalies around found over western North America and around the Arctic Circle, and finally a trough is found over the eastern two-thirds of the nation. This pattern, in theory, would support a cross Polar wind pattern at 500 MB from the Arctic circle toward the Gulf Coast and East coast of North America.
Earlier we talked about drivers and enhancers in the weather pattern. For the Pacific influences we defined two key drivers in the weather pattern, El Nino and the positive PDO. Now, let me touch on the enhancer of the weather pattern at least in terms of tropical forcing which is the Madden-Julian Oscillation (MJO).
The MJO in El Nino years typically is locked into phases 7 through 2, which basically means thunderstorm development is located in the central Pacific for most of the time. The MJO will not be active all the time and can not be used to forecast seasonal patterns due to the high volatility of the MJO wind components. However, we do know that when the MJO does start to amplify and become stronger that a far more active Polar and Sub Tropical jet stream is far more likely. Further, in phases 7 through 2, below normal heights are likely on the East coast which basically supports an increase threat for winter storms.
THE ATLANTIC OCEAN
While the Pacific is a very important factor in seasonal forecasting, the Atlantic can not be ignored either. One of the most important factors is the development of the North Atlantic Oscillation. The North Atlantic Oscillation is a measure of pressure anomalies in the northern Atlantic. When there are above normal heights over Greenland and below normal heights to the south of Greenland to around Iceland, this is called a negative NAO. The pattern leads to enhancement of a trough over the eastern United States and typically supports a storm track over the coastal waters of the northern Mid Atlantic. A positive phase of the North Atlantic Oscillation (NAO) features the exact opposite features and a jet stream that quickly moves west to east. The Eastern United States can still become cold, however the weather pattern is what we call progressive which means low pressure development can exit out into the Atlantic very quickly.
So now, lets look at these Sea Surface Temperature Anomalies! Clearly what stands out is the cold pool in the northern Atlantic and the very warm water over the western Atlantic. Both of these features are extremely important for this forecast.
First, let’s tackle this cold pool of water. There was a lot of unnecessary hysteria about this cold pool due to a study by Dr. Mann whom claimed the Atlantic Multi-decadal Oscillation Current was starting to collapse. Rest assured there is no actual observations to support this claim. But what of this cold pool?
This area of cold water is nothing new. We saw this signature in 2009, 2006, 2002, 1997, and 1995; you get the point. What do all of these years have in common? All of these year features a cold weather pattern in western Europe and the Eastern United States and all of these years features a moderate to strong west based negative NAO. A west based negative NAO is when the below normal heights are found around 50N/50W while above normal heights build from Greenland into northeastern Canada.
Second, lets focus on the very warm waters over the western Atlantic, specifically off the East coast. There are several factors that are influenced by this very warm water anomaly over the coastal waters from the East coast to Bermuda. With very warm water, air at the lower levels of the atmosphere will rise. We have seen this all Fall with constant areas of low pressure developing off the East coast. This process of rising air also means that there is an imbalance in the conservation of mass and energy. This void or lower pressure MUST be filled. This means that dense Polar/Arctic air must be drawn towards the East coast. This environment strongly supports a high threat for cyclogenesis (low pressure development) on the East coast.
The influences of the stratosphere can be complicated to break down on what can create stratospheric warming, where high latitude blocking may set up, and how all these factors can influence the 500 MB pattern over North America. There are several influences to consider with the stratosphere which include snow growth in Siberia, the activity of the sun, and the evolution of stratospheric winds measured via the Quasi-biennial Oscillation. I have pin pointed two drivers and one enhancer in the stratospheric evolution. The two drivers are solar activity and snow growth in Siberia while the enhancer, although poorly, is the QBO.
There has been a lot of talk the past few years about the declining activity of the sun. Yes, Solar Cycle 24 is one of the weakest seen in over a thousand years. There is no doubt about that. Does that mean an ice age is upon us? Right now, there is no data to suggest that extreme of a climate event. However, the weakening solar influences DO matter.
As you can see from the chart, we are on the decline and the sun’s activity is declining. We went through the same period in the mid to late 2000’s as well with an apex of no activity and a completely quiet sun in 2009.
Why is this important?
The process is a bit complicated to I will make this simple. A decline in solar activity leads to a gradual increase in ozone in the atmosphere which allows for warming in the stratosphere. The stratosphere expands down, but must press down onto the troposphere thus leading to cooling of the troposphere along with lower 500 MB heights.
As you can see, since the sun has become rather quiet since about mid March, stratospheric temperatures at 70 MB have been averaging above normal. A recent spike in activity matches a brief cooling of late, however with the sun activity declining again, stratospheric temperatures are already rising at 1 MB and increasing. So we can see right now how solar activity has a real and significant impact on stratospheric temperatures via the build up of zone in the stratosphere and ionosphere.
Given this data and the trending declining activity of the sun, I expect the stratosphere overall to remain warm which should lead to an increase in high latitude blocking in the troposphere and help to weaken the Polar Vortex this winter.
Snow Growth In Siberia:
The correlation between snow growth in Siberia and a weak Polar Vortex is pretty strong. The connection is not at 100% but pretty close. In fact, it is one of the strongest connections we have in meteorology. Below is a chart from Atmosphere and Environmental Research or AER.COM.
As you can see, snow growth this year is above normal but not as impressive as the past two years. So what we can learn from this is that based on the chart above, the process is starting to unfold with snow expansion in Siberia which will lead to surface cooling and a strong Siberian High.
The QBO is a rather interesting influence on the stratosphere. The eastern phase leads to overall a warm stratosphere but less intense stratospheric warming events. Last year the QBO was strongly negative or easterly but was supposed to weaken. The QBO though did not weaken and thus disrupted the development of a negative Arctic Oscillation.
This year the QBO has shifted to a western phase which strongly supports sudden stratospheric warming events, a weaker Atlantic Polar jet stream, and typically colder winters in northern Europe and eastern North Americas, specifically in the United States (Ebdon 1975). The QBO is NOT a driver in the atmosphere but does help to enhance the environment that is already in place.
YOUR WINTER FORECAST!
Okay so you can see what I’ve been tracking for months and breaking down all the factors. So let me put this all together for you.
We start with El Nino which is strong right now but expected to weaken. A typical climatology strong El Nino would suggest the Polar jet stream gets blocked to the north and a mild winter ensues, however this is no typical strong El Nino as we have seen above. The location of convection all Fall is a major warning sign against a warm winter from coast to coast. There is also the matter of a positive PDO which points to below normal heights below the Aleutian Islands and above normal heights over western North America. The positive PDO also points to below normal heights over the Gulf Coast and Southeast. So these factors suggest a rather stormy weather pattern along the Gulf Coast, Tennessee River Valley, Southeast, and Mid Atlantic.
Next, we have the Atlantic with a very cold area of water in the northwestern Atlantic and well above normal water temperatures along the East coast. These factors have supported in the past and likely will this Winter a negative NAO pattern in the northern Atlantic along with a high threat potential for low pressure development from the Southeast coast to the Mid Atlantic coast. This would suggest a high threat for Nor’Easters via Miller A and B type varieties.
Finally, the stratospheric influences from solar activity, Siberia snow growth, and QBO influences all point to a high threat for strong high latitude blocking which will show up as a negative Eastern Pacific Oscillation, negative Arctic Oscillation, and negative North Atlantic Oscillation.
Expected Storm Tracks:
Red Storm Track: This storm track will be seen frequently from late December on through March and will have the potential to have major winter impacts from the southern Plains to New England. This storm track will be driven by the Sub Tropical jet stream and influenced by interaction by the Polar jet stream.
Pink Storm Track: If the North Atlantic Oscillation happens to be positive, then the storm from the red storm track would end up tracking inland. Best potential for this evolution in early December.
Green Storm Track: This storm track will be frequent in early December and will bring significant rainfall to southern and central California. This would be a storm track that will bring significant snowfall to the central and northern Plains and Great Lakes.
Blue Storm Track: This storm track is driven by the Polar jet stream and is called an Alberta Clipper. Alberta Clippers are associated when strong Polar and Arctic air masses invade the United States. Most of these clippers will produce only light snowfall. However, at times these clippers can become Miller B type storms which redevelop off the Mid Atlantic coast and produce significant winter weather over the Mid Atlantic and southern New England. These clippers will need to be watched with a storm track red feature from the Sub Tropical jet stream in place.
Purple Storm Track: This type of storm track is rare but is capable of producing a lot of trouble this year. These lows develop in the Gulf of Mexico and can intensify with the Polar and Sub Tropical jet streams can phase. This type of storm track can lead to very heavy winter precipitation from the Gulf Coast to New England.
General Winter Forecast:
MONTHLY BREAK DOWN
December will be the warmest month of the winter. The pattern for the first 15 to 20 days of the month is expected to be driven by a strong upper level low in the Gulf of Alaska which will drive a Pacific air mass through much of the United States. Look for significant rainfall in California and the Southwestern United States. The pattern over the northern tier will be warm as the Polar jet stream will be focused along the Canadian Boarder.
By the end of the month, the Pacific pattern starts to shift with the upper level low/trough developing towards the Aleutian Islands. Look for the Polar jet stream to amplify and drive the first Polar/Arctic air mass into the eastern third of the United States towards the end of the month. Best potential for a winter storm along the East coast is towards the end of the month.
The first 15 to 20 days will feature a rather active period of weather over the Southwest, southern Plains, Gulf Coast, Southeast, Mid Atlantic, and potentially coastal New England. The Polar and Sub Tropical jet stream will interact in this period and will have the potential to produce significant cold snaps and winter storms.
The pattern should relax towards the end of January briefly leading to a moderation in the weather pattern.
This month is expected to be the stormiest and the coldest with another period of an amplified Polar jet stream and an active Sub Tropical jet stream. A major Arctic air mass invasion is likely.
March: Storm threat is likely through March 15th with a cold start to Spring likely.
What Could Go Wrong
There are two factors that could change this winter forecast. The first factor is if El Nino continues to strengthen into late December and shift completely East based while the SSTA in the northern Pacific cools. This would force the Polar jet stream northward and thus a mild winter.
The other factor is if the snow growth in Siberia ends up failing, then the development of a negative Arctic Oscillation becomes less likely.