It’s that time of year once again! As a forecaster, the winter forecast is a rite of passage for Fall for any serious meteorologist. As a meteorologist, even if you are just in school starting to learn, you get the constant question, what will the winter by like? Will we have a lot of snow? Will the winter by cold? So, once again at NY NJ PA Weather, not only will we talk about the winter but we will talk about the factors going into this winter, and what can change this forecast in terms of snow and temperatures.
In case you are new to the winter forecasts from NY NJ PA Weather, let me break down some features in the winter forecast. One, I don’t do forecasts based on other years. I know there are many out there that take some of the factors like La Nina or the state of the QBO and then compare it to other years. The problem with this is no two years are the same because each year is different with various different factors. A great example is La Nina. In 2010/11, there was a moderate La Nina which still featured a healthy amount of snowfall for the northern Mid Atlantic then 2011/12 came with a slightly weaker La Nina but was nothing like a La Nina at all with a warm winter for most. The difference? Different drivers in the weather pattern! That’s two. I will focus and try to pin point what are the driving influences for this up coming winter. I can not nor will pretend to be able to forecast the dates of winter storms. I can give you a basic idea of the themes for this winter along with what can completely throw this winter forecast into a burning fire bin.
DRIVERS AND ENHANCERS:
This year, the process of separating the driving mechanisms of this winter weather pattern and the influences of what will enhance the winter weather pattern are a bit more muddled than what we have seen the past 5 years. This year, we really don’t have a well sustained ENSO phase, but you can make an argument for a weak La Nina being a factor. Meanwhile, there are clearly changes going on in the northern Pacific and Atlantic which puts into question whether the PDO is neutral or positive and what impacts the Atlantic sea surface temperatures will have on the North Atlantic Oscillation. So what are my drivers?
Well this year I am strongly keeping an eye on the stratosphere and how the snow growth will impact the Polar Vortex. However, even in this factor we have the QBO that would be an inhibiting factor in the snow growth influence. Boy, this is a tough one huh? Luckily I have a plan. So first, let’s look at each factor individually and then put all the pieces together.
ENSO: La Nina or La Nada?
The forecasting of ENSO for the Fall and Winter has been rather interesting to watch with the CPC going from a moderate La Nina to neutral and back to a weak La Nina forecast again. Rather than debate with whether this is going to be La Nina or not, I decided to consider the idea of the forcing mechanism of a cold neutral to weak La Nina that is WEST BASED. The orientation here is a huge factor here because this tells me what impacts, if any, these ENSO will have on the location of tropical convection.
When you have a west based La Nina, convection is severely limited around the date line. Now remember, this is a weak La Nina we are dealing with and all the data points to this La Nina peaking in November and weakening thereafter. We also know the coldest water has been and likely will continue to be in NINO 3.4 and NINO 3.0. So when La Nina weakens in December and January, the sea surface temperature anomalies will warm steadily first in NINO 4, which will increase the thermal gradient in these areas.
Still, the bulk of tropical convection in the Pacific will be to the west of the date line this winter. There will be an occasional shift to the date line via MJO phases, but not a continuous development.
These factors tell me that we can expect a weak Sub Tropical jet stream this year and a stronger Pacific/Polar jet stream. The upper level low found around the Aleutians and Gulf of Alaska is typically weaker but a strong ridge is far more likely over Alaska.
The state of ENSO is a weak driver. The enhancer in the influence of tropical convection will be observed in the MJO phases. To learn about the MJO, go here.
To keep this simple, when the MJO is in phases 7 through 2 then convection is near or over the date line which corresponds to colder and stormier weather over the eastern two-thirds of the nation. However, when the MJO is in phases 3 through 6, look for a ridge to be a higher threat over the eastern two-thirds of the nation. I believe we will see both of these influences this winter, the wild card here is how much of each?
NORTHERN PACIFIC SEA SURFACE TEMPERATURE ANOMALIES: PDO
The past few years we have clearly been in a positive PDO phase with very warm waters along the West coast of North America into the Gulf of Alaska with very cold waters over the northwestern Pacific near Russia. To learn more about the PDO, please visit here.
The PDO, based on JISAO data which I’ve used for years, is still in a positive state, however clearly the PDO phase is starting to change from positive to negative. I’ve personally been waiting for this change to occur at some point this year and looks like I was right. What are some of these changes? Note that we are now starting to see near to below normal Sea Surface Temperature Anomalies (SSTA) along the coastal waters of the western United States while SSTA are going above normal near Japan. However, we still have very strong above normal SSTA in the Gulf of Alaska. So I see this configuration is at the early stages to trending towards a negative PDO but that transition likely won’t be realized until Spring or Summer of next year.
As such, for the winter forecast I look at the PDO as weakly positive trending towards neutral by the end of the winter. A positive PDO in general supports a trough around the Aleutian Islands, a ridge over the western United States, and a mean trough over the eastern two-thirds of the United States.
The state of the PDO and the state of ENSO will have competing influences on the 500 MB pattern. However, competing does not exactly mean canceling each other out. The PDO will help establish the 500 MB pattern in the Pacific but La Nina will adjust the orientation of the trough/ridge positions. Of course, there is another factor, the stratosphere, which we will touch on later.
The Atlantic SSTA once again has the western Atlantic very warm. A warm western Atlantic means two very important characteristics. The first is that cold air masses, when available, are likely to drive to the East coast. Second, when low pressure development along the East coast is possible, the low pressure development can be explosive. The warmer western Atlantic leads to enhancement of the thermal gradient along the East coast and also can introduce a stronger moisture feed in East coast cyclogenesis.
The rest of the Atlantic Ocean SSTA strongly supports a potential negative NAO pattern. For years, there has been a debate as to whether the SSTA drives the NAO phases and orientation. In my opinion, the SSTA help to support and feed back the NAO phases which are dictated by stratospheric influences. If the stratospheric environment is favorable for high latitude blocking in higher latitudes, than the SSTA in the Atlantic will help to drive the feed back machine in the atmosphere to keep high latitude blocking in place. Currently the SSTA in the Atlantic would support a NAO with a negative phase.
Eurasian Snow Cover:
The snow cover in Eurasia and specifically around Siberia is heading to above normal levels. Both the snow cover extent and depth are above normal, more so than last year in Siberia. The chart below from AER.com and pioneered by Dr. Judah Cohen explains why Siberian snow growth is so important.
So to keep this simple, the faster the snow advance in Siberia in October, the more likely the Arctic Oscillation will be negative because the Polar Vortex is disrupted by above normal heights at 500 MB.
Now, over the years there has been some misunderstanding of this rule so I want to address that right now. This process does NOT promise snow and cold for the Eastern United States. Many have tried to do a one to one connection with this atmospheric process. This is not a good idea. What this process tells us is that the rapid snow growth in Siberia weakens the Polar Vortex that’s in place in November. If the Polar Vortex is very strong in November, the process to get from a positive Arctic Oscillation (AO) to a negative AO can be delayed. We saw this last year due to the very strong Polar Vortex, at record levels, and also the strong influences from El Nino. This year is very much different. The Polar Vortex this year is weak and the stratosphere overall is warmer. With that said, the influences of the rapid snow advance, which will be positive, will have a faster influence than what was seen last year.
So in this factor, these observations point to a high likelihood of a negative AO and negative NAO due to a weak and likely split Polar Vortex.
SNOW COVER IN NORTH AMERICA:
Unlike last year, snow growth is rather healthy over northern, western, and central Canada. The snow growth here is important because the rapidly building snow cover in October will help make Polar and Arctic air masses stronger.
Think of this factor like a cooler you take to the football game. Now, if you just put a few ice cubes in the cooler with your supreme subs and some sodas, well those subs and soda will not be as cold in an hour or so. Now, add more ice, let’s say the whole bag of ice. Well, then your sodas and subs will be much colder because the cooler overall will be much colder. So the snow growth in Canada is our Polar air mass cooler. As Polar and Arctic air masses build over Siberia and the Arctic, those air masses will not moderate or warm much while dropping south into the United States because of the increased snow cover in Canada this Fall. This factor leads to a feed back loop leading to Polar and Arctic air masses to remain strong and build further south than in years where snow growth is below normal like last year.
Before we talk about the stratosphere, I think it is important to acknowledge the back ground story here on the sun’s influence on the Earth’s atmosphere as that sun spots are rapidly declining. Clearly a solar minimum is on the way. I want to stress here that this meteorologist is NOT advocating an ice age on the way as some have tried. The purpose of me bring up this factor is that the more spotless days on the sun, up to 23 according to NASA for 2016, the less solar radiation impacts the Earth’s atmosphere and the more that ozone in the stratosphere can build. The more stratospheric ozone that is present, the warmer the stratosphere overall can become and thus the weaker the overall Polar Vortex at the stratosphere and upper troposphere. Unlike the past 6 years, the state of sun spot development and the frequency in which the sun is becoming quiet is starting to influence the state of the stratosphere which will be a back ground influence to consider moving forward.
There is a lot of debate on the influences of the quasi biennial oscillation (QBO) impacts on the winter forecast. The general theory, which you can read about here and here, which basically is when the QBO is negative or easterly than the Polar Vortex is weaker, the NAO is negative, and winters over the Eastern United States are cold and snowy. When the QBO is positive or westerly, the opposite is the case. The QBO this year at 30 MB has been positive and trending towards positive at 50 MB. See the chart below.
However, as you can see in this chart, we can see several cases where the QBO values were not a factor in what happened in the winter weather patterns. For example, look at the time period from late 2009 to early 2011 where both winters had been very snowy winters for the Mid Atlantic yet the QBO was both negative and positive. So the QBO is not a make or break factor in the forecasting for the winter.
What we do know is that in a positive QBO regime, that major stratospheric warming is less likely but not impossible. As such, overall, a positive QBO regime would suggest a stronger stratospheric Polar Vortex. However, we know via observations thus far this Fall that the stratospheric and tropospheric Polar Vortex is actually weak and the QBO has been positive for some time. As such, one has to question just how much of an influence the QBO is having on the stratospheric environment. I should also note that the Arctic Oscillation has gone strongly negative since October 1st.
Beyond that point, I want to point out that the QBO at 30 MB may be peaking in the Fall. A falling QBO to below -8 m/s does suggest a reduction in influence on the stratosphere. There has been some indications that when the QBO is between -8 m/s to +8 m/s at 30 MB that high latitude blocking increases significantly.
So I am putting the QBO as an enhancer and not a driver in the winter forecast, which if the 30 MB QBO falls into the -8 m/s territory or lower, the high latitude blocking over North America would become significantly amplified.
Current Polar Vortex:
To follow up on the overall structure of the Polar Vortex (PV), we start at 30 MB. Unlike last year, the PV is already clearly warped with the heart of the PV shifting towards North America. Note how heights are building over the northern Atlantic and also over the Barring Sea.
These observations were also noted at 50 MB and below with a significantly weaker PV than in previous years all the way down to 500 MB.
We can also see that the warmest stratospheric anomalies are focusing over the northern Pacific, North America, and the northern Atlantic. These warm stratospheric anomalies would suggest the lowest 500 MB heights are more likely to develop around northwestern Asia, eastern North America, and the northwestern Atlantic.
THE WINTER FORECAST:
Below is the combination of the factors discussed above. I will break down the 500 MB pattern, the expected storm tracks, month to month evolution, national winter forecast, and finally what could completely make this winter forecast wrong.
500 MB Pattern:
The map above illustrates the base 500 MB pattern that I’m expecting this winter. The 500 MB pattern is based on a hybrid influence of a weakening positive PDO in the northern Pacific with weak La Nina influences like a weak 500 MB ridge over the Bahamas and Southeast.
BLUE LINE: This is the base 500 MB pattern where a ridge is found over the Gulf of Alaska but pushed to the east due to enhancement of the upper level low around the Aleutian Islands. A broad trough is found from the Rockies to the East coast with a trough axis between the Plains and Mississippi River Valley. The base jet stream configuration suggest a weak to moderate negative NAO pattern this winter, just enough to suppress the storm track and Polar jet stream south over the Mid Atlantic states.
YELLOW LINE: This jet stream configuration is when the NAO relaxes to a neutral to positive state or in an east based negative NAO state. This configuration allows for the storm track to lift from the Plains into the Great Lakes and is generally a warm pattern for all of the East coast.
RED LINE: This is a 500 MB pattern possible if major stratospheric warming happens and leads to an amplification of high latitude blocking and the negative NAO becoming moderate to strongly negative. Such a pattern would drive an Arctic air mass into the East coast.
ARROW, BLUE AREA, RED AREA: The arrow suggest that this year, considering boundary layer conditions already in Canada and Siberia, that Arctic air masses will be able to drop south and a cross polar jet stream dynamic is highly likely. The blue area is where the Arctic air masses are most likely to be focused and where cold anomalies will be strongest. The red areas signify where there is a high threat for constant winter storms. Note, I did NOT state above normal snowfall. The red areas is where I believe we will see a constant clash between warm air masses over the Atlantic and the Arctic air masses attempting to press south. Naturally, when the NAO/AO is negative the push south of the Polar/Arctic air masses will be stronger.
IMPORTANT NOTE: Important factor here to point out is my lack of a true Sub Tropical jet stream. Based on the evolution of convection in the Pacific and the nature of the La Nina orientation, I do not believe the Sub Tropical jet stream will not be strong and that the dominant jet stream will be the Polar jet stream. We will have to watch with each storm threat the potential for Sub Tropical jet stream short waves being a factor, but any influence would only be possible if the Polar jet stream short waves do not overwhelm any Sub Tropical jet stream influences. In short, the Polar jet stream is king.
Given the 500 MB pattern above, the following is what I believe to be the storm tracks for this year.
YELLOW: This is when high latitude blocking is weak. Storms via a strong Pacific jet stream coming into the West coast and moving through the Rockies. The low pressure systems then cut from the Central Plains or Southern Plains with a Polar/Arctic air mass invading the Rockies and Plains as the low pressure system lifts out. The low pressure system cuts up through the Great Lakes with a trailing cold front reaching the East coast a few days after with temperatures falling from well above normal to near normal along the East coast. This storm track brings the potential for major snow and ice storms into the Rockies, Plains, Upper Mid West, and Great Lakes.
GREEN: Slightly more high latitude blocking developing with this storm track with the mean trough axis pushed east into the Mississippi River Valley. However, the NAO in this storm track is either positive or an east based negative NAO, which allows surface low pressure systems to track into the St. Lawrence River Valley. This storm track leads to a deep invasion of Polar/Arctic air masses into the Plains, Mississippi River Valley, Great Lakes, and Ohio River Valley. This storm track would produce the potential for significant snow and ice in the Pacific Northwest, northern Rockies, Central Plains, Southern Plains, Mississippi River Valley, Upper Mid West, Great Lakes, interior Mid Atlantic (via warm front initially before change over), and interior New England.
RED: This storm track is similar to Green initially but a negative NAO is in place to block the progression of the low pressure system into the St. Lawrence River Valley. As a result, the low pressure system is forced to redevelop off the Mid Atlantic coast and then track northward towards the New England coastal waters. This storm track leads to a deep invasion of Polar/Arctic air masses into the Plains, Mississippi River Valley, Great Lakes, Ohio River Valley, Tennessee River Valley, New England, and Mid Atlantic. This storm track would produce the potential for significant snow and ice in the Pacific Northwest, northern Rockies, Central Plains, Southern Plains, Mississippi River Valley, Upper Mid West, Great Lakes, coastal Mid Atlantic, and coastal New England. This type of storm development tends to produce significant snowfall along the coast while interior sections tend to miss out on even light snowfall in the northern Mid Atlantic.
BLUE: This is a storm track that is featured in a strongly negative AO/NAO pattern where an arctic air mass is pressing south. Called Alberta Clippers, these low pressure systems typically produce light to moderate snow from the Northern Plains, Upper Mid West, Great Lakes, and Mid Atlantic. If high latitude blocking is just right, there are times where these clipper lows can redevelop off the Delaware or New Jersey coast and produce significant snowfall for the coastal northern Mid Atlantic and coastal New England. Typically though these low pressure systems produce 2 to 4 or 3 to 6 inch snowfalls.
PINK: This storm track can produce significant widespread snow and ice. The low pressure system typically begins to develop to the lee of the Rockies and draws moisture north into the Southern Plains from the Gulf of Mexico while Polar/Arctic air invades south. The storm track is suppressed south due to a moderate to strong negative AO/NAO pattern with significant snow and ice for a prolonged period of time to the north of the surface low pressure system track. Severe thunderstorms are possible to the south and southeast of the low pressure system as well. This storm track can either be produced by one strong low pressure system or a series of weaker low pressure systems as Arctic and Tropical air masses clash along a stalled frontal boundary.
NATIONAL WINTER FORECAST:
This is going to be an interesting year in terms of who gets the most snowfall in the Mid Atlantic. Based on everything I am seeing, I can certainly make the case that this winter is more likely to feature near to above normal snowfall over Boston, New York, and Philadelphia while well below normal snowfall is found in Baltimore and Washington, DC. The forecast map I have above is a hybrid of a weak La Nina with stronger high latitude blocking leading to an invasion of Polar/Arctic air masses that become rather shallow on the East coast.
In terms of winter storm forecasting, I’m going to have to buy a lot of coffee. I doubt I will have one winter storm without having to figure out when and where snow will mix with sleet and rain. Change over timing will be a constant theme and will help determine who gets the most snow in each storm. Another uncertainty that will be an issue will be the position of high latitude blocking and thus high pressure systems to the north of these storm tracks.
To say this winter is going to be colder than last year is pretty much an easy slam dunk. The question for the Northern Mid Atlantic is whether or not high latitude blocking will set up correctly to lead to several winter storms or a lot of lake cutters.
MONTHLY BREAK DOWN:
Winter pattern slowly begins to evolve through the month with a step down to Polar air masses advancing further and further south. Lake Effect Snow will become a major focus towards Thanksgiving.
DECEMBER: Winter pattern expected to be established by mid month and will start to feature winter storms from the Rockies on east for the end of the month.
JANUARY: Heart of winter. Winter storm threats high along with invasions of Arctic air masses. There will be the potential for a relaxation in the pattern in mid to late month for about a week or so.
FEBRUARY: Winter storm threat returns for first half of the month followed by a slow relaxation for the end of February. Arctic air masses begin to retreat and high latitude blocking relaxes.
MARCH: Look for an early arrival of Spring.
WHAT COULD GO WRONG:
1. The QBO remains too strong: In this case, the QBO remains too strong and as such the high latitude blocking is shifted east or not able to become established. As a result, the storm track would feature constant lack cutters with a more pronounced La Nina look.
2. La Nina intensifies: The forecast of ENSO the past few months has been questionable at best. As such, if La Nina intensifies into a moderate west based version with very cold waters at the date line to south of Hawaii, then a stronger Southeast ridge would be in play leading to above normal temperatures up the East coast and a storm track into the Great Lakes.
3. Intense Stratospheric Warming leads to Arctic Oscillation tanking: Sometimes too much of a good thing (for snow lovers) is bad. If the Arctic Oscillation happens to crash strongly negative and overwhelms all factors than the storm track is suppressed and only cold, dry weather will be in place over the eastern two-thirds of the United States.
4. Polar Vortex sets up in China: This is a possibility where the high latitude blocking sets up, the QBO weakens, La Nina is very weak to near neutral; but the Polar Vortex drops into China leading to a brutal winter while a zonal pattern sets up over the United States. Given the snow growth in Siberia, I don’t think this will happen but it is possible.
5. Stratosphere becomes very cold in November: A cold stratosphere means the Polar Vortex intensifies, which can negate the influences of the Siberian snow advance. Although not expected, this factor is being watched very carefully.
So to recap this forecast for the Northern Mid Atlantic. This winter is expected to be colder than last year, honestly not a stretch there, and will be focused more in December, January, and early February this year. The Polar jet stream will be the dominant jet stream with storm tracks focused towards the Great Lakes, redeveloping over the Mid Atlantic coastal waters, and from the southern Plains to the Mid Atlantic coast. There is potential for a significant snow and ice gradient through the year via this type of storm tracks.