1. science
2. weather

WINTER
FORECAST
FOR
2014/2015
BY
METEOROLOGIST
STEVEN DiMARTINO
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.
So let’s get some hot chocolate and pumpkin pie and dive into
this forecast!
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.
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
forecast out of the limitations of long range/climate models that
tend to have significant error, typically on the warm side.
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.
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.
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.
Sea Surface
Temperature
Influences
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.
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
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.
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.
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.
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.
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
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:
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
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.
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,
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.
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 30-60 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
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.
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.
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
eastern two-thirds of the United States. You can find these composites
from the CPC, here.
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.
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
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
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
(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.
Stratospheric
Influences
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.
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
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.
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.
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
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
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.
So where are we now? As of September, the QBO at 30 MB has fallen
to -23.24, 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.
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
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.
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.
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
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.
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.
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)
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.
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.
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
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
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
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 2011/12 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.
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.
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.
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!
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
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
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.
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
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.
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
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
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
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.
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.
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.
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
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
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
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