WINTER FORECAST FOR 2014/2015. METEOROLOGIST STEVEN DiMARTINO

Transcription

1 WINTER FORECAST FOR 2014/2015 BY METEOROLOGIST STEVEN DiMARTINO

2 PREFACE It s that time of year again when everyone starts to wonder what to expect for the upcoming winter. The internet is full of folklore from following the size of tails on squirrels to the size of worms in the ground. None of which has any real scientific backing. You can t really blame people though. An active winter either promotes joy or dread. Either way, unlike any season, winter has the strongest emotional connection, even if you hate the winter. So here we are again as we push into mid to late October, ready to issue another NY NJ PA Weather Winter Forecast! I know. I know. Everyone is excited as we dive into winter. The snow. The ice. The massive disappointment after some Social Media forecaster puts out a 40 snow map for all of the Eastern United States and nothing happens. If you are reading this discussion, you likely love studying the weather as much as I do.

3 So let s get some hot chocolate and pumpkin pie and dive into this forecast!

4 Winter Weather Forecast Methods of Forecasting First, as with any scientific discussion or process, let me explain my method of forecast and what I will be studying. In this forecast we will be making a clear distinction between what drives a weather pattern and what enhances a weather pattern. You will understand the distinction between drivers and enhancers of a weather pattern and why the differences are very important. The sea surface temperature anomalies in the Northern Pacific will be studied and explained in relation to their influence on the 500 MB pattern.

5 The latest observations in Siberia and their influences will be discussed as well with a focus on the impacts on the Polar Vortex and high latitude blocking. The use of mathematical calculations will be used to break down the influences of the QBO and MJO. You will learn what the QBO and MJO is and what influences they have on the atmosphere. The state of the stratosphere in terms of where it is now and where the stratosphere is going and what impacts will develop on the troposphere. You will learn why studying the stratosphere is so important in relation to the 500 MB pattern and the potential for high latitude blocking and storm development. The forecaster will focus on known observations and their known impacts on the atmosphere. This process is best as this takes the

6 forecast out of the limitations of long range/climate models that tend to have significant error, typically on the warm side.

7 ENHANCERS VS DRIVERS A very important distinction in a forecast is to determine what is driving the weather pattern and what is enhancing a weather pattern. A Driver of the weather pattern is a feature that fundamentally alters and forces a response in the 500 MB pattern. Examples this year of drivers on the winter pattern is the stratosphere, sea surface temperature anomalies in the north Pacific and Atlantic Oceans, and snow growth in Siberia. An Enhancer of the weather pattern is a feature that enhances a feature of the established weather pattern. Examples this year of enhancers to the upcoming winter weather pattern are the MJO and ENSO. Both of these features will have a growing influence on the Sub Tropical jet stream and the potential for major winter storms.

8 High Volatility in Seasonal Forecast As always with these forecasts, volatility is always a concern. There are several factors that can significantly alter this forecast and the ability in the science to forecast accurately is limited with our knowledge. Stating that, the following factors could significantly alter the winter forecast. The placement of stratospheric warming could have a significant impact on the winter forecast. If the strongest warming takes place and focuses on Asia, that would lead to a dramatically warmer winter than I ll be forecasting for. Two, if ENSO flips and goes La Nina. This factor is not expected, but the way ENSO is going I wouldn t ignore the threat. Three, snow growth rapidly falls off from current growth for the last week of October. Four, the North Atlantic Oscillation never goes negative. Again, not likely but have to consider the threat.

9 These factors would dramatically change the forecast. However, at this time, those factors have not developed to be a credible influence in this forecast. The feature above that most likely to influence the weather pattern would be if the strongest stratospheric warming set up over China rather than over the North Pacific or North America.

10 Sea Surface Temperature Influences

11 Pacific Influences One of the most important factors in any seasonal forecast is the evolution of the Pacific Ocean atmospheric and sea surface temperature patterns. Changes in the Pacific Ocean, the largest body of water on our planet, have a direct and significant impact on the 500 MB pattern over North America. As one fellow meteorologist told me a long time ago when I was starting to study long range forecasting, if the Pacific pattern is hostile towards your going forecast, you better change your going forecast. So in this section we will talk about the Pacific Ocean and the expected influences on the winter weather pattern for North America this year.

12 Northern Pacific Sea Surface Temperature Anomalies: A very important factor in forecasting for a 500 MB pattern in the winter is understanding the nature of Sea Surface Temperatures in the northern Pacific. There are two phases in the Northern Pacific Sea Surface Temperature anomalies that have a significant influence on the 500 MB pattern. These two phases are the negative PDO and the positive PDO. So what is the PDO? The Pacific Decadal Oscillation the following: a long-lived El Niño-like pattern of Pacific climate variability. While the two climate oscillations have similar spatial climate fingerprints, they have very different behavior in time. Fisheries scientist Steven Hare coined the term "Pacific Decadal Oscillation" (PDO) in 1996 while researching connections between Alaska salmon production cycles and Pacific climate (his dissertation topic with advisor Robert Francis). Two main characteristics distinguish PDO from El Niño/Southern Oscillation (ENSO): first, 20th century PDO "events" persisted for 20-to-30 years, while typical ENSO events persisted for 6 to 18 months; second, the climatic fingerprints of the PDO are most visible in the North Pacific/ North American sector, while secondary signatures exist in the tropics - the opposite is true for ENSO. (jisao.washington.edu/pdo) So basically the PDO is like a long term ENSO phase. When the PDO is in a warm phase, above normal sea surface temperature anomalies can be found in the Gulf of Alaska and along the west coast of North

13 America while colder water is found over the central and western Pacific. The opposite is true when the PDO is in a cold phase. Currently the PDO is in a warm phase as you can see in the observations above. Why is this important? The important here is where thermal gradients are setting up and how this influences the 500 MB pattern over the northern Pacific. We have set up two distinct areas of intense thermal gradients at the surface up through the mid levels of the atmosphere. The first thermal gradient is over the central Pacific, near the date line. You may not have noticed, but going back to last October, we have had a constant presence of a trough around the central Pacific from the Aleutian Islands down through the date line or 180 degrees longitude. This trough has been in place in response to the constant thermal gradient in this region of the ocean, which is attempting to balance out the thermal imbalance in place.

14 The second thermal gradient is between North America, including Alaska of course, and the Pacific Ocean where you have Polar and Arctic air masses interacting with very warm Pacific air masses. This thermal gradient leads to an imbalance in temperatures and pressure from the continent to the Pacific Ocean. So what is the result? Well, what you end up with is a prolonged period of a trough over the western Gulf of Alaska to just south of the Aleutians and an upper level ridge over the rest of the Gulf of Alaska to the west coast of North America, especially for western Canada and the Pacific Northwest of the United States. At times these influences extend down to California and the Southwestern United States, however ENSO factors can override the PDO influences. I will get to ENSO later in this chapter. The influences seen over western North America with a large ridge likely in place, must translate to below normal 500 MB heights or a trough over southeastern North America in order to balance out conservation of mass considerations.

15 As such, in winter seasons when the PDO is positive, you are more likely to have what is called negative Eastern Pacific Oscillation phases or basically a trough to the south of the Aleutians and a ridge over northwestern North America. As a result, these weather pattern typically force Polar and Arctic air masses developing in northwestern Canada into the central and eastern United States.

16 El Nino or La Nada? The El Nino Southern Oscillation (ENSO) has been driving forecasters rather batty for the past few months. If you remember, there were some social media forecasters going about warning of a super El Nino as some models went off the deep end with Sea Surface Temperature Anomalies (SSTA) in NINO 3.4 of over 2.0 degrees Celsius above normal. As always, there are models and then there is reality. The reality is that while there were and still are clearly hints of an El Nino trying to organize, the circulation simply has not been established to really drive a sustained moderate or strong El Nino. Even a weak El Nino can be put into question. This all again comes down to the balance of thermal gradients and air mass considerations.

17 I am going to show you two maps. One from September 1st and one from October 13th of this year. September 1st: Take a look at the SSTA for the Tropical Pacific. Now look to the west towards Indonesia. See how those SSTA are rather warm. In fact almost all of the Tropical Pacific is rather warm. This is an important observation and explains why the models have been horribly bad the past six months. You see El Nino and La Nina are simply processes to

18 balance out thermal imbalances, which comes in the form of pressure differences, wind circulation, and SSTA formations. As you can see above, there is barely any thermal imbalance over the Tropical Pacific, as such you end up with a neutral ENSO. That s the simple reason. I can go more in depth, but I m not looking to write a novel here. So now, let s jump to October 13th. October 13th:

19 Now we are starting to see a slight imbalance! You can see how the SSTA are starting to cool slowly around Indonesia and slightly warmer water is observed from 160E to 100W longitude. So what type of an El Nino are we talk about here? Well, there are two factors to consider, strength and location. In terms of strength, I don t care what model you want to lean on, if you are thinking anything other than very weak, you are going to end up very wrong. There is simply no support from any observations of anything more than a weak El Nino with NINO 3.4 range from 0.4 degrees Celsius to 0.7 degrees Celsius above normal from November through March. So the next consideration is location. There is a difference between a west based and an east based El Nino. The differences once again comes down to where thermal gradients are located and influences on atmospheric pressure. This factor is a bit more complicated because you can not look at ENSO in a vacuum. The atmosphere just does not work in that respect. Further, as discussed

20 above, I think we can clearly state that influences from the PDO out weigh influences from ENSO for this winter. As such, we know that atmospheric forcing from the Polar jet stream will influence where forcing and low/mid level wind convergence is developing towards the Tropical Pacific. The eastern regions of ENSO have been very volatile, swinging from 1.4 degrees Celsius above normal to near neutral in the span of a few weeks. This is due to the very shallow ocean depths in NINO 1, 1+2, and 2 compared to NINO 3, 3.4, and 4. As such, when determining orientation of ENSO phases, I typically throw this area out because the rapid changes lead to minimal influences on the 500 MB pattern. So what I study is ENSO 4, 3.4, and 3. What I have been observing is that as SSTA to the east near Indonesia have cooler, SSTA in NINO 4 and 3.4 has slowly and steadily warmed. We have also observed Sub SSTA pooling in western areas of ENSO as well.

21 Considering these observations and how the rest of the Pacific to the north is behaving, I am expecting a very weak, west based El Nino to eventually take hold. I want to be clear in stating that El Nino will NOT be a primary driving factor to the winter weather pattern. Instead, this year El Nino will act as an enhancer to the pattern overall, specifically showing influences in the Sub Tropical jet stream over the southern tier of the United States and up the East coast. A west based weak El Nino typically leads to growing convection around the date line or 180 degrees Longitude and 10 degree Latitude. The increased convection in this area of the Pacific Ocean helps to enhance the base of the Polar jet stream to the north thus enhancing the negative EPO pattern as discussed previously. In fact, we have seen over the past month how convection has started to focus around the date line as the weak El Nino starts to slowly evolve. Note the blue areas (below) is where air is rising, thus thunderstorms and orange areas is where air is sinking, thus no thunderstorms. So again,

22 we are focusing on observations, not models, to determine what is happening in the atmosphere. The trends are clear and the factors that support these observations clearly point to a west based weak El Nino influencing the winter weather pattern.

23 MJO Influences: Finally, let s discuss the MJO or the Madden Julian Oscillation. Many meteorologist and weather fanatics tend to focus on the MJO which is the following: an intraseasonal fluctuation or wave occurring in the global tropics. The MJO is responsible for the majority of weather variability in these regions and results in variations in several important atmospheric and oceanic parameters which include both lower- and upper-level wind speed and direction, cloudiness, rainfall, sea surface temperature (SST), and ocean surface evaporation. The MJO is a naturally occurring component of our coupled ocean-atmosphere system and the typical length of the MJO cycle or wave is approximately days (Madden and Julian, 1971, 1972; Madden and Julian, 1994; Zhang, 2005). The best way to understand the MJO is to take into study where convection is developing in the tropics. Where air is rising, that is where thunderstorms will develop. Where air is sinking, that is where the thunderstorms are suppressed. The MJO phases are separated into 8 separate phases that each produce a unique type of weather conditions

24 depending on the time of year and intensity or magnitude of the MJO wave. For this discussion we will focus on the months of November to March. I like to divide the MJO phases into El Nino-like impacts, Pattern Change, and La Nina-like impacts. As you can see with the chart on the left, there are 8 phases of the MJO and then a circle in the middle. The circle in the middle represents a neutral phase. The further away the MJO is from this circle, the stronger the influence on the atmosphere. The two phases I consider pattern changing phases are phase 2 and 6. These phases we start to see fundamental changes in the trough/ridge alignment in the atmosphere. Phases 7, 8, and 1 are typically El Nino like phases while Phases 3, 4, and 5 are typically La Nina like phases.

25 The MJO though is NOT a driving mechanism in forcing a weather pattern but simply an indicator and enhancer to the weather pattern. The MJO is influenced by multiple base factors as the definition explains, each having a significant influence on where the MJO is and much influence these waves will have on the 500 MB pattern in the troposphere. The basic way to look at the MJO is this, where are the thunderstorms forming in the Tropics? That s it. Simple as that to gain an understanding. So can we use the MJO to forecast a whole winter? I know some try to use this feature to justify a seasonal forecast, but you really can not because convection has a high degree of volatility. However, you can use the overall trends of the MJO to observe how the atmosphere is reacting to various impacts and then gain a better understanding of how low and mid level forcing is influenced by various factors from sea surface temperature anomalies to stratospheric considerations.

26 From watching the MJO the past 6 months, the MJO has started to focus on specific areas again and again. Those areas are neutral, phase 7, phase 8, and phase 1. The MJO also has been rather weak so that means the influence from the MJO is rather weak and limited. As such, the correlation between MJO phases and impacts on North America are a bit weaker. Considering the growing influences of ENSO moving forward, the MJO being in a neutral phase a great deal of time should be expected. If this El Nino was going to be moderate or strong, the MJO would be completely dead. I do not expect this though. The MJO will likely see a lot of time in a weak phase 7, 8, and 1 states from November through March. These phases typically support an active Sub Tropical jet stream and below normal temperatures over the

27 eastern two-thirds of the United States. You can find these composites from the CPC, here.

28 Atlantic Influences While the Pacific Ocean has significant influences on the Winter Weather Pattern for North America, one should not ignore the Atlantic Ocean SSTA, especially for the Mid Atlantic and Northeast. A warm Atlantic Ocean can spell trouble for the Eastern United States for several reasons. In this section, we will discuss the state of the Atlantic Ocean and what impacts the Atlantic Ocean will have on the Eastern United States and the northern Mid Atlantic.

29 Atlantic Ocean Sea Surfact Temperature Anomalies: The Atlantic Ocean is in a state of change from going from a warm phase to a cold phase. This change in SSTA is called the Atlantic Multidecadal Oscillation or AMO. Much like the PDO in the Pacific Ocean, this oscillation describes the changes in the transport of warm and cold water in the Atlantic and the resulting impacts on the SSTA. This evolution leads to a variety of weather impacts, however the influence of the AMO is a bit more tricky this year as there are clearly changes

30 developing in the Atlantic SSTA. A perfect example of these changing anomalies can be found in the Tropical Atlantic where SSTA were below normal for much of this Summer, limiting Tropical low pressure development. Stating this, my focus for this forecast is on the northern Atlantic. First, let s study this cold pool in the northern Atlantic. This cold pool to the south of Greenland is a direct result of constant upwelling from storms developing to the south of Greenland and Iceland. This anomaly is EXTREMELY important because what we are seeing is text book support for sustained high latitude blocking in the Northern Atlantic Ocean. This type of configuration leads to what is called a negative North Atlantic Oscillation. I ll discuss this high latitude blocking feature later. The other factor that stands out to me is the very warm waters over the western Atlantic, specifically around the Gulf Stream. The Gulf Stream

31 is basically a very warm river of water in the Atlantic currents that helps transports warm tropical waters from the Gulf of Mexico and Caribbean Sea into the northern Atlantic. Without this important Ocean Current, New York City and London would be ice cold and Ice Land would truly become an ice land because of the influences of the Arctic. This year, the western Atlantic is running 1.7 degrees Celsius above normal. Some areas off the East coast from Florida to the Canadian Maritimes and 300 NM out range from 2 degrees to an impressive 10 degrees Celsius above normal for this time of year. You might be wondering why this observation is so important. Remember what I said about the conservation of mass and the associated thermal gradients. What we have off the East coast is a powder keg of energy with warm, moist rising air along and just off the East coast. With rising air, you have a void in mass which means another air mass must fill that mass to achieve a balance. This means that cold, dense air

32 (you know, like that Polar and Arctic air mass in northwestern Canada) will race south and southeast to achieve a balance in the atmosphere. This leads to the potential for powerful storms along the East coast. When you combine the factors in the western Atlantic with the favorable SSTA environment in the northern Atlantic, we clearly have a very favorable environment for significant East coast storms or cyclogenesis. In short, Nor easters.

33 Stratospheric Influences

34 When I started to do Long Range and Seasonal forecasting about a decade ago, one topic that had barely if ever mentioned in school were stratospheric temperature conditions and influences on the troposphere. The troposphere is where we live and where most weather occurs, however the stratospheric temperatures and pressures just above the troposphere can have a major influence on the weather patterns over the Northern Hemisphere. Stepping back further, we also must recognize that the activity of the sun has a direct impact on the upper portions of the atmosphere and this includes the stratosphere. So in order to understand where the stratosphere is going we have to look at a variety of signals including solar activity, stratospheric winds, and where ozone is building up in the stratosphere. By understanding these factors we can start to pin point where and how intense high latitude blocking can become and what influences they will have on the 500 MB pattern.

35 Solar Influences We will start at the sun, where some very interesting observations are starting to take shape and will have a profound impact on the Earth s climate as a whole and some rather impressive impacts for the winter this year and beyond. First, I want to talk to you about Sun Spots. Sun spots are areas of intense activity and explosions from the surface of the sun. A significant amount of energy is released from the sun via sun spots. Why are sun spots important to us? Well, this all goes back to how everything is connected. Ozone in our atmosphere naturally builds up in the stratosphere. The more ozone in the stratosphere, the more the stratosphere expands and presses down on

36 the troposphere. This leads to lower 500 MB heights and increased support for a colder atmosphere overall. UVA radiation from the sun is strongest when sun spots are more active. When sun spot activity starts to decrease, the environment is more favorable for ozone build up which in turn leads to a warmer stratosphere and thus a colder troposphere. This brings us to the Solar Cycle with the chart below detailing the Sun Spot Number Progression as of September.

37 As you can see with the chart, we are exiting the peak of Solar Cycle 24, which is one of the weakest cycles observed in over 100 years. Remember what I said above, if Sun Spot activity decreases, the support for more ozone in the stratosphere and thus a warmer stratosphere overall increases. So the Sun is about to go into an increasingly more quiet period of solar activity over the next several months and years. Now that we have past the peak of the cycle, the potential for a warmer stratosphere on average will start to increase significantly.

38 Quasi-biennial Oscillation While studying the influences of the sun is very important, we would be in error to look at the stratosphere from just a top-down approach as atmospheric circulations just are not that linear nor simplistic. To be blunt, there is a lot we still don t understand about our atmosphere and the variety of circulations involved in the development of our weather systems. No, science is never settled. Just in case you are wondering. That takes me to the Quasi-biennial Oscillation or QBO. The QBO is a quasi-periodic oscillation of the equatorial zonal wind between easterlies and westerlies in the tropical stratosphere with a mean period of 28 to 29 months. Basically the QBO is a transportation system to take warmer

39 stratospheric air from the tropical regions to the polar regions. The analysis of wind components is important because easterly winds tend to support more high latitude blocking and strong 500 MB disturbances than western winds. There is some debate also about the magnitude of strength of the QBO and high latitude blocking. Some meteorologist below the if the QBO is between +8 and -8 then high latitude blocking is far more likely. Others disagree in stating that a negative QBO (easterly wind component) supports high latitude blocking and a positive does not. So who is right? I think both are. First, let me explain why the QBO phases are important. When the QBO is positive, the Polar Vortex in the stratosphere is stronger. The stronger the Polar Vortex, the more progressive the Polar jet stream becomes and the more likely that the Polar Vortex stays near the North Pole and keeps the cold air in the

40 Arctic. However, when the QBO is negative, the Polar Vortex weakens and is more likely to break up and head towards the Mid Latitudes. So this takes us back to the debate. My theory is that the QBO does not have a profound influence on the stratosphere until values exceed +/-8. So in cases where the QBO is positive but below +8, high latitude blocking is still possible. However, when the QBO is negative, magnitude of the winds becomes a non factor in terms of potential high latitude blocking. When the QBO is negative, the Polar Vortex weakens and pieces of the Polar Vortex break off and drop south. Below you can see a chart of influences of a positive and negative QBO.

41 So where are we now? As of September, the QBO at 30 MB has fallen to , which is rather strong. This strong easterly QBO strongly favors a weak Polar Vortex that will likely feature a Polar Vortex that will break up and drop south towards the mid latitudes this winter. Currently, the QBO has been steadily strengthening and will continue to lead to an environment where high latitude blocking is far more likely than not.

42 Location Is Key! Much like in real estate, location is key when we discuss impacts of the stratosphere on the troposphere. As I discussed at the start of this winter forecast analysis, one of the wild cards that we simply do not know yet is where the stratosphere will be the warmest. A perfect example is the winter of 2006/2007, when like this year, many factors where coming together to support a stormy winter. Well, there was a stormy winter, but not in North America, China ended up with one of the coldest and stormiest winters in recorded history. Meanwhile, the northern Mid Atlantic had a rather boring winter with only a winter storm or two to speak of. The reason why this happened was because the strongest stratospheric warming and thus lowest 500 MB heights focused over China where the

43 Polar Vortex dropped south and the active winter weather pattern kicked in. So what do we have this year? Well, let s look at a few factors. E-P Flux Analysis: The Eliassen-Palm (E-P) flux (Eliassen and Palm 1961) is widely used to characterize the wave activity. The direction of the E-P flux is proportional to the group velocity and indicates the direction of the propagation of the waves. Approximately, the vertical and horizontal components of the E-P flux are proportional to the eddy heat and momentum flux, respectively. The divergence of E-P flux is proportional to the northward flux of quasi-geostrophic potential vorticity, so that it is a direct measure of the total forcing of the zonal mean flow by eddies. Therefore, the E-P flux and its divergence are important and useful to diagnose planetary waves propagation as well as the effective mean zonal force induced by the waves.

44 As you can see here, the E-P flux clearly shows there is an establishment of warm air at the upper tropospheric lower and mid latitudes being transported to the upper stratosphere, specifically around 1 to 30 MB. This transport of warmer air leads to an increase in mass at the stratospheric level and thus a warmer stratosphere overall. What I find interesting is the magnitude of the E-P Flux already which would explain why there is a warming of the stratosphere already developing over parts of northeastern China, the northern Pacific, and now bleeding into North America.

45 So now let s go back to the ozone values. The latest ozone analysis shows some interesting trends as of the end of September. As you can see, we started to see a build up of ozone over the northern Pacific and northwestern Atlantic. What we are observing here is the gradual shift and support for the warmest stratospheric conditions over the northern Pacific and eastern North America. Remember, a build up

46 of ozone means there is more mass and since that mass can not expand upward into space, the stratosphere presses down in these locations leading to below normal 500 MB anomalies over the central and northern Pacific and eastern North America.

47 We can even start to see those trends develop with steady warming being observed over the northern Pacific and North America. This warming in the stratosphere appears to be strengthening moving forward. Taking a step further, let s look at the model guidance from the ECMWF 10 days from now.

48 As you can clearly see, the ECMWF continues the shift of the warmest stratospheric temperatures into the northern Pacific and North America, exactly where we find the build up of ozone as of late September. The lesson learned here is that stratospheric warm anomalies are more likely to set up over northern Pacific and North America than over Eurasia, which would point to lower 500 MB heights and the set up of the Polar Vortex towards eastern North America. The equation below is basically what drives the stratospheric and tropospheric interaction. The basic way of understanding the process is the look at a compressor. When the stratosphere warms (cools), that layer in the atmosphere expands(contracts). Remember, that the atmosphere of the Earth can not extend out into Space, thus pressure is exerted on the lower layers of the atmosphere, specifically the troposphere, where we live. When the stratosphere expands (contracts) the heights at 500 MB decreases (expands) to balance out the mass in the atmosphere. The warmer (colder) the stratosphere, the lower (higher)

49 the overall 500 MB heights are. Now, taking this basic yet important influence on the atmosphere, we know that when the stratosphere is warm, the troposphere as a whole tends to feature below normal 500 MB heights.

50 Cryosphere analysis One of the factors that can make or break a winter forecast is what is happening at the surface. There have been many times where a winter forecast looked pretty good but ended up failing horribly. When I was younger, I couldn t figure out what I did wrong. We had high latitude blocking. We had warm water off the East coast and the Pacific Ocean looked pretty good too. Where is the cold air? I would ponder this question when I was in my teens and didn t have access to the internet. Yes, the stone ages when the internet was some weird gate way via AOL that screamed you got mail. Ahh, the 1990 s.

51 The missing ingredient I learned when I got to college was that I was not taking into account the low level atmospheric environment of where our Polar and Arctic air masses came from. This of course brings us to the development of snow cover and depth in northern Canada and Siberia. Before we look at the observations, I think I should explain why studying the snow cover in these locations is so important. Now, I know many will simply point to the study by J Cohen and another study by RJ Allen which both pointed to snow cover in Eurasia and the Arctic Oscillation. I will attempt to make this whole connection very simple. Let s say you have a cooler you are getting ready for the big game. You have your subs, sodas, beer, and some hot dogs that you need to keep cold. Now, while the cooler is excellent at keeping these items cool, you need to prepare the environment to support a cold/cool environment. So what do you add? ICE! That s right, you prepare the environment to

52 support and sustain cold air. So the more ice you add, the more the cooler can sustain the cold environment. In a very basic way, that s what we are studying here. The more snow we have develop, the stronger the Polar and Arctic air masses can get. So here is the basic break down. The more snowfall that develops in Siberia, the cooler the environment becomes. As the cold air mass builds, higher pressure is established and strong high pressure systems develop in Siberia. The building heights in the atmosphere eventually builds up to the point of amplifying the Tropospheric jet stream, which means warmer air is transported towards the upper Troposphere and lower Stratosphere. The next step leads to the Polar Vortex weakening and breaking down. The broken pieces of the Polar Vortex than propagates down through the troposphere and eventually from the higher latitudes to the mid latitudes. This finally leads to a sustained negative Arctic Oscillation where higher 500 MB heights are found near the

53 North Pole and lower 500 MB heights are found in the Mid Latitudes like the Eastern two-thirds of the United States. So now that we understand why studying the snow cover is so important, let s look at the data! Well now, this is what I call impressive! As you can see from the chart above, we have set up an environment where Snow Advance Index (SAI) in October is thus far well above any record levels. The SAI measures the rate of snow growth in October. The faster the rate of

54 snow growth, the stronger the influence on the atmosphere and the more likely you ll see high latitude blocking and cold, stormy weather conditions on the East coast. What we have here in the observations basically are levels well above the average nothing we have seen in over 10 years. In fact, at the rate snow is building in Siberia, we are going to blow away levels seen in 2009/10 and 2010/11 when the Arctic Oscillation was strongly negative. If you remember those winters, your back might start complaining already. So we clearly have a very strong cold and stormy signal from Siberia for this up coming winter. The following map showing the snow coverage so far.

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56 The Forecast All the factors that go into a seasonal forecast has been examined and broken down. Now comes the difficult part in terms of putting all the pieces together. I say difficult because how all the factors above interact is what leads to each year having unique attributes for the winter season. This is an important factor to consider and is why one can not simple use one factor or another to forecast for an entire season. A perfect example of this factor is how some organizations, like say the CPC, focuses far too much on ENSO states and not at the whole big picture. This factor again comes down to the question of Drivers of the weather pattern versus Enhancers of the weather pattern.

57 So first we will look at what I expect in terms of the high latitude indices and then we will break down the national and then northern Mid Atlantic winter forecasts.

58 Teleconnections Teleconnections are statistical analysis that describes the nature of the weather pattern. The positive and negative phases of each teleconnection tells us that the atmosphere is a specific 500 MB pattern. In fact, typically these teleconnections act like a domino effect from Pacific to the Atlantic. Typically a negative East Pacific Oscillation is followed by a negative Arctic Oscillation and negative North Atlantic Oscillations. Conversely, a positive East Pacific Oscillation is followed by a positive Arctic Oscillation and positive North Atlantic Oscillation. So what can we expect this year? Well, let s start to put together all the observations we have discussed!

59 Eastern Pacific Oscillation (EPO): The EPO is an important aspect in the 500 MB pattern over the Pacific. To keep this simple, let me explain the key difference between a positive and negative EPO. Overall, when the EPO is positive, a deep upper level trough or upper level low is located in the Gulf of Alaska. The position of the upper level low in this area leads to Pacific air flooding into the western United State and western Canada. As a result, this mild air mass drives through the rest of the United States leading to typically a zonal 500 MB pattern (upper levels winds going west to east) and a quiet, warm pattern for much of the nation. A negative EPO though is just the opposite! In a negative EPO, a trough or upper level low is found between the date line and south of the Aleutian Islands with a powerful ridge over the Gulf of Alaska and along the west coast of Canada. This powerful ridge can at times build into the Arctic Circle, leading to some of the coldest weather patterns of the

60 season. This process is how you end up with Arctic invasions over North America. Down stream, a deep trough develops over the central and eastern United States. Given the development and sustained nature of a moderate positive PDO pattern and the increasing nature of convection around the date line as discussed in the ENSO section, the predominant nature of the EPO this season will be negative, which sets us up for the next teleconnection. Pacific North American Index (PNA): Considering what we know about the EPO, we now go to the PNA index. The PNA index is not going to be strongly positive because of one interesting factor, the developing weak west based El Nino. We have already established the fact that the best forcing and therefore convection (thunderstorms) have been and continue to be focused along

61 the Date Line. This leads to a stronger trough to the south of the Aleutian Islands and a more robust Sub Tropical Jet stream. As a result, I do expect an increase in the amount of disturbances moving through the Southwestern United States before developing and interacting with the Polar and Arctic jet streams in the Plains and East coast. As such, there will likely be times where a split flow will develop where a ridge dominates western Canada and the Pacific Northwest while disturbances are moving into California and the Desert Southwest. As such, there will be times when the PNA will be a weak signal while the EPO will be strongly negative. Overall, I expect the PNA to be in a positive phase for the most part. Arctic Oscillation (AO): The Arctic Oscillation and the Polar Vortex are interlinked in that the Arctic Oscillation describes the location and strength of the Polar Vortex.

62 When the AO is positive, the Polar Vortex is strong and tends to stay around the higher latitudes. This means all the coldest air typically stays over central Canada and has a hard time coming south. Basically the lowest 500 MB heights are located towards the North Pole and this leads to above normal 500 MB heights over the United States. When the Arctic Oscillation is negative, the exact opposite happens in the atmosphere. Above normal 500 MB heights are focus around the North Pole and below normal heights are focused over the Mid Latitudes, typically around the eastern two-thirds of the United States. As a result, cold air is driven south from the Arctic and northern Canada towards the United States, typically the eastern two-thirds of the United States. This year I am expecting a moderate to strongly negative Arctic Oscillation and this cold air should be focused towards the eastern twothirds of the United States. First, we know the impacts of a rapidly

63 growing snow pack over Siberia which is at record levels. This factor is a known and well tested influence on the atmosphere and this alone would lead to a negative Arctic Oscillation. However, we have more factors. We know the QBO is also in an easterly or negative phase as well which disrupts and weakens the Polar Vortex, leading to the Polar Vortex breaking apart. These two factors both point to a negative Arctic Oscillation. To put the nail in the argument, remember when we talked about the very warm western Atlantic. This factor points to rapidly rising air along the East coast. We have already seen an increase development of low pressure systems along the East coast the past several weeks and the first real Nor Easter this coming week. The reason why is the increasing thermal gradient. With the warm western Atlantic, air is rising constantly over the coastal waters of the East coast. So this means, cold dense air is going to be drawn towards the East coast. As a result, I expect most Arctic and Polar air masses to dive towards the Northern Plains and then south towards the Gulf Coast and southeast towards the East coast this winter.

64 North Atlantic Oscillation (NAO): The North Atlantic Oscillation or NAO is a legendary teleconnection. This teleconnection has a significant impact on weather patterns along the East coast. The North Atlantic Oscillation is the difference between pressure systems from the Icelandic low to the Azores high. When the NAO is positive, a near zonal pattern sets up from the East coast to the eastern Atlantic with below normal heights typically around Greenland and above normal heights around Iceland. The storm track is rather progressive and typically leads to a rather quiet weather pattern along the East coast. When the NAO is negative, above normal 500 MB heights are found over Greenland and northeastern Canada while below normal heights are

65 found from the Canadian Maritimes to Iceland. There are two separate phases within the negative phase, west and east. An east based negative NAO has above normal heights generally over central and eastern Greenland and below normal heights around Iceland. Although this type of negative NAO does feature height latitude blocking, the position of the block is typically too far east to impact the storm tracks along the East coast. Most storms develop over the coastal waters and not along the Eastern seaboard. A west based negative NAO has the above normal heights extend into northeastern Canada and the below normal heights or upper level low is located to the east of the Canadian Maritimes or around 50N/50W. The west based negative NAO is associated with most major East Coast winter storms because of the nature of the 500 MB block. The configuration of the 500 MB pattern leads to a strong high pressure

66 system over New England, which keeps cold air locked in along the coastal plain. With fresh cold air in place, a strong thermal gradient remains in place along the East coast, helping to generate surface low pressure system. Meanwhile, the position of the 50N/50W upper level low forces the whole weather pattern to slow down and keeps a storm track focused along the East coast to 200 miles off the east coast. Classic snow producing storm tracks typically take low pressure system from the coastal waters of North Carolina to 70N/40W bench mark or southeast of New England. There is a misconception that a storm only happens when the NAO is negative. This is an incorrect assumption. In fact, based on research by Kocin and Uccellini via Northeast Storms Volume 1 and 2, most major winter storms occur when the NAO is in a state of change from positive to negative or negative to position or when the NAO is weakly negative. A strongly negative NAO leads to a suppressed storm track over the

67 Southeast leading to rare winter storms over the Southeast states like Georgia and South Carolina. This year I do expect a predominantly negative NAO for several reasons. First, let s go back again to the Siberia snow growth rate. When there are above normal Siberia snowfall in October, there is an enhanced potential for a negative NAO in the winter months. Then let s look at the Sea Surface Temperature Anomalies in the Atlantic.

68 As you can see in the SSTA map as of October 16, 2014; the Atlantic Sea Surface Temperature Anomalies have aligned to strongly support a negative NAO in the winter. In most cases when this SSTA alignment developed in October, the following winter featured a prolonged negative NAO pattern. So when considering these factors I do expect a weak to moderate negative NAO this winter that would feature an increased potential for winter storms along the East coast this year.

69 National Forecast The National Forecast is split into eight different regions with unique impacts through the winter. The following analysis breaks down each one of these regions.

70 REGION ONE: This region remains under a dry and warm pattern that has been in place going back through the Summer. These locations will be under the influence of an upper level ridge through the entire winter. December: Temperatures: Above normal Precipitation: Below Normal January: Temperatures: Near Normal Precipitation: Below Normal February: Temperatures: Above Normal Precipitation: Below Normal REGION TWO: This region will experience a slight increase in precipitation due to the development of a weak El Nino and therefore a slightly more active Sub Tropical jet stream. Temperatures through the winter will average near to slightly above normal while precipitation will return to near normal levels, however drought conditions will remain an issue. December: Temperatures: Above normal Precipitation: Below Normal January: Temperatures: Above Normal Precipitation: Near Normal February: Temperatures: Above Normal Precipitation: Near Normal

71 REGION THREE: The majority of the Southwestern United States will see an increase in precipitation as the Sub Tropical jet stream becomes more established. Temperatures through the winter will generally remain above normal. December: Temperatures: Above normal Precipitation: Near Normal January: Temperatures: Above Normal Precipitation: Above Normal February: Temperatures: Above Normal Precipitation: Above Normal REGION FOUR: This region will be under the influence of two types of pressure systems, powerful power/arctic high pressure systems and fast moving Alberta Clippers. As such, I expect below normal temperatures, potentially record breaking at times, and below normal precipitation due to the fast moving nature of the low pressure systems. December: Temperatures: Below normal Precipitation: Below Normal January: Temperatures: Below Normal Precipitation: Below Normal February: Temperatures: Below Normal Precipitation: Below Normal

72 REGION FIVE: This region will also feature a constant onslaught of Arctic high pressure systems and Alberta Clippers, however this region will have the Great Lake to work some magic with Lake Effect Snow. However, the wild card here is the fact that the Great Lakes are averaging below normal, almost at record levels. December: Temperatures: Below normal Precipitation: Above Normal January: Temperatures: Below Normal Precipitation: Near Normal February: Temperatures: Below Normal Precipitation: Below Normal REGION SIX: This region is one of the regions that will feature the interaction of the Sub Tropical jet stream and the Polar/Arctic jet streams this Winter. These regions will feature a volatile temperature pattern and above normal precipitation. This region will feature the potential for snow and ice events with bouts of significant cold out breaks. December: Temperatures: Near normal Precipitation: Above Normal January: Temperatures: Near Normal Precipitation: Above Normal February: Temperatures: Near Normal Precipitation: Above Normal

73 REGION SEVEN: The Gulf Coast and Southeast coast will be under the influence of the Sub Tropical jet stream and thus likely above normal precipitation events. Temperatures will average below normal due to the increased days of precipitation. December: Temperatures: Near normal Precipitation: Above Normal January: Temperatures: Near Normal Precipitation: Above Normal February: Temperatures: Near Normal Precipitation: Above Normal REGION EIGHT: This region will be a high impact area with a significant threat from a variety of winter storms, but especially Nor Easters. These locations will have the potential to feature above normal winter precipitation. December: Temperatures: Near normal Precipitation: Above Normal January: Temperatures: Below Normal Precipitation: Above Normal February: Temperatures: Below Normal Precipitation: Above Normal

74 Northern Mid Atlantic Winter Forecast

75 Much like the National Forecast, I broke down the Northern Mid Atlantic forecast into three regions which I believe will feature unique characteristics this winter. The overall forecast idea this winter is that with a warm western Atlantic and Gulf Stream combined with advancing Polar/Arctic air masses from central Canada, that many of the winter storms will be focused along the coast where the best dynamics and lifting properties will be focused. Region One: This region will be the furthest away from the aforementioned thermal gradient, which means these locations are most likely to miss out on the majority of the precipitation events. That is not to say these locations will not have their fair share of heavy snow, just that most of the steady precipitation events will be focused further south and southeast. Stating that, temperatures are most likely to average below normal in these locations.

76 REGION TWO: This region, based on all available data, will most likely be in the best area to have a combination of ample cold air, moisture, and low to mid level forcing to support significant winter storms this year. These locations are the most likely to have all snow events or predominantly snow/ice events rather than change overs to rain. Temperatures in this area will average near to slightly below normal. Precipitation will average above normal. REGION THREE: This region will share many of the same potential winter storm impacts as region two. However, given my experience of the climatology in this region, there is a greater potential for snow to winter mix type storms. I produced this region because of the warm sea surface temperatures that will in one respect support storm development but also lead to a warmer low level environment, thus the higher potential for snow to mix events rather than all snow events. Stating that, this region is still at threat for high impact snow threats.