Seasonal forecasting is quite the challenge, but one we always look forward to. It seems every year we learn something new about the atmosphere. It is critical to have a fundamental understanding of how the atmosphere works, and the implications that changes from its base state can have on the weather pattern.
Instead of focusing on individual numerical indexes and their verbatim values, we are going to try to paint a picture of the atmosphere and what it will be doing over the next few months based on these phenomena:
- ENSO (El Nino Southern Oscillation) and tropical forcing
- High latitude behavior and blocking
- Analog years based on global pattern evolution
El Nino Southern Oscillation (ENSO) and Tropical Forcing
Prediction: Weak to Borderline Moderate East-Based La Nina
After one of the strongest El Ninos on record in the 2015-2016 winter, this will be our second consecutive winter with a weak La Nina. Not all La Ninas are alike, and over the past few years we have begun to realize that for the purposes of making a forecast and evaluating the atmosphere, it is not necessarily important to match up previous years based on numerical values. Instead, it important to look for analogous years based on atmospheric behavior and global circulation effects.
The Pacific Jet stream is a dominant force in the weather pattern both globally and across the United States. La Nina’s tend to feature a weaker, Pacific Jet stream that dips and oscillates. Because the Pacific Jet is weaker, its forward momentum tends to stop at a much further west position than it does during El Nino years (you may recall that El Ninos are famous for their frequent intrusions of Pacific air into the United States). With the forward momentum of the Pacific Jet stream stopped further west, a ridge tends to crest and form in the Pacific Ocean, often near the Aleutian Islands in the Pacific Ocean.
Its exact positioning and amplitude can often be determined by what is going on in the high latitudes and in the upper troposphere and stratosphere, which will be discussed later. But with a ridge developing near the Aleutians, Arctic air is often times displaced towards Alaska and British Columbia, oftentimes sliding southeastward into Canada and the Northern ⅓ of the United States, particularly the northwestern third of the US.
While this year’s La Nina does not necessarily qualify (by CPC standards) as one that is very strong, we have already seen evidence of a La Nina circulation in the atmosphere, via a strong Aleutian ridge displacing Arctic cold in Alaska and Western Canada.
This La Nina has intensified over the past month or so, with colder anomalies in the Eastern Equatorial Pacific towards Peru, defined more appropriately as an “East-Based La Nina.” Often times, ENSO events are characterized by the focus of their most anomalous sea surface temperatures. In other words, a La Nina event with the deepest, most anomalous colder waters and most impressive upwelling in the Central equatorial Pacific Ocean would be characterized as a “Central Based La Nina”. This is important, because the location of the most anomalous waters has a great impact on how the atmospheric circulations are affected across the globe.
East Based La Nina’s have typically favored more amplified ridging in the Pacific near the Aleutian Islands, and a propensity for high latitude blocking to develop in the Arctic regions. There are many studies available which research this topic – particularly the below piece by Zhang which discusses the composite differences between the positioning of ridging in the Pacific Ocean when comparing Central and East based La Ninas. Zhang refers to the events as “CP (Central Pacific)” and “EP (Eastern Pacific).
The above diagram of SLP and 300mb height anomalies (from the aforementioned Zhang paper) with “EP” La Ninas vs “CP” La Ninas aids in the visualization – Eastern Pacific La Nina’s tend to favor a much more poleward reaching Pacific ridge when compared to a Central Pacific La Nina. This is a critical piece of the forecast, as a more poleward reaching ridge in the Pacific Ocean will disrupt the arctic circulations and dislodge colder air further south into Alaska, Western Canada, and the Northern ⅓ of the United States. Ridges that are located in these regions are much more likely to be displaced into the arctic via upstream latent heat release and jet stream wave breaking.
Interestingly, and probably not coincidentally, this is very much in line with what has been observed over the past several weeks, and is forecasted by ensemble guidance to occur into the middle and end of November.
(This image is the European Ensemble forecast valid for November 15th, showing a strong Aleutian ridge building northward into the Western Arctic, and displacing plenty of cold into Western Canada. Credit: wx.graphics)
Checkpoint: As we move through the forecast, we are able to paint the first part of our picture. We have established a base state of an east-based La Nina with a strong ridge in the Pacific building northward towards the Aleutians and into the Western Arctic. This will send plenty of cold into Western Canada and the Northwestern third of the US. At times, this will result in a Southeast US ridge anomaly. However, with this Arctic air being more consistently displaced, it is readily available to be sent anywhere in the US depending on the configuration of high-latitude blocking in the rest of the Arctic and in the North Atlantic.
This is why when we have La Ninas with the Aleutian ridge building towards the Arctic (like this year), it is more likely that the entire northern third of the US will experience cold air rather than cold remaining confined to the Pacific Northwest/Intermountain West. The poleward extent of the ridge allows deeper, full latitude-type troughs to dig and expand cold air into the USA.
High Latitude Blocking (AO, NAO, QBO, Stratosphere)
The state of the weather pattern in the higher latitudes can have tremendous impacts on the weather across the entire United States. Often times, airmass intensity and duration is heavily modulated by what is occurring in the high latitudes of Canada, Greenland, and the Arctic. Understanding how these processes work is a key factor in forecasting seasonally, but especially in winter as wavelengths change and the high latitude weather pattern becomes even more volatile and critical.
What occurs in the stratosphere is a good place to start. While it may seem irrelevant to the naked eye, the weather pattern in the stratosphere has huge effects on what occurs in regards to our sensible weather. How vortexes and winds behave in the stratosphere give us critical clues as to how the weather pattern as a whole in behaving. For instance, when the stratospheric polar vortex is stronger and more resistant to any disruption, cold air often has a propensity to remain bottled up in the higher latitudes. When it is weaker, more mobile, or broken apart, cold air can shift and maneuver around the globe with ease, and often times even the polar vortex in the troposphere (that’s where most of our weather occurs) can respond.
The past several years have featured a stronger than normal stratospheric polar vortex from late Autumn going into winter. The stronger than normal stratospheric vortex has aided in a more positive Arctic Oscillation and North Atlantic Oscillation on average, with less high latitude blocking especially early in the winter months. The timing and magnitude of stratospheric warming and polar vortex weakening events, along with their impacts on the tropospheric pattern, have been met with much forecast difficulty. However, a number of a events occurring this Autumn appear to strongly indicate that a weaker than normal stratospheric polar vortex is likely for much of the winter ahead.
Multiple mid and upper level atmospheric ridges have disrupted the flow of air in the arctic and polar regions over the last week. Additionally, medium and long term forecast models continue to indicate that ridges will build over the high latitudes in the troposphere during the next 10 to 15 days, some of which could be extremely anomalous, especially from the Kara Sea through the Davis Strait and possibly even over Greenland. This will result in a weakened and/or disrupted polar vortex from 100 to 50mb.
Analogs from weak La Nina years featuring similar global pattern configurations also support weaker than normal stratospheric polar vortex conditions moving forward. While periods of fluctuation are likely, it does not appear that a strong stratospheric polar vortex will exist during this upcoming winter. In fact, this year’s stratospheric polar vortex could average weaker than normal, during the winter.
Another important factor in the state of the stratospheric vortex is the presence, intensity, and modality of zonal winds at multiple levels. Zonal winds are typically running westerly in the stratosphere. A sudden stratospheric warming event occurs when temperatures warm and zonal winds reverse to easterly at 10mb. At this time, zonal winds remain westerly in the upper-stratosphere, but have been running weaker than average at 10mb and other levels since late October.
The Quasi-biennial Oscillation, or QBO, is a stratospheric phenomenon observed over the tropics. A positive QBO features westerly wind anomalies in the stratosphere over the tropics, while a negative QBO features easterly winds over the same areas. The QBO historically has an oscillation period of 25 to 30 months. The QBO is now negative at 30mb — with easterly winds expected to continually descend this winter and promote a weakened and or perturbed stratospheric polar vortex.
Overall, confidence is high that the stratospheric Polar Vortex will remain weaker than normal and disrupted at least early in the winter. This could lead to the development of some high latitude blocking events. What happens from the mid-winter onward is a very low confidence forecast.
Periods of higher solar and geomagnetic activity have been linked to a stronger stratospheric polar vortex and a positive NAO modality. Currently, we are moving toward the end of solar cycle 24. This solar cycle has lower amount of sunspots then past solar cycles recorded over the past 30 years. While a spike in solar activity occurred in September, related to a sunspot group, the overall activity has continued to drop off rapidly as a solar minimum approaches.
Lower solar radio flux values are occurring and solar minimum is forecast near 2018-20. Solar values are currently predicted to be in the upper 70s for monthly averages this December, January and February. This is higher than the 70-80 monthly values observed in the 2009-10 winter — which was the last “solar minimum” winter and featured a strong -AO and strong -NAO. The values this year are also projected to be only slightly higher than December of 2010, which also featured high latitude blocking and a -NAO/AO.
With solar activity continuing to fall and predicted values lower throughout the winter months ahead, the effects of solar activity on the stratosphere should be trivial at best. Periods of enhanced high latitude blocking have been linked with years where a -QBO was observed and near-solar minimum was occurring.
For a lack of better terminology, we are in a bit of uncharted territory regarding analog years this winter. The QBO oscillation during last winter was as unexpected as it was unprecedented, which immediately throws a wrench into analogs based off of stratospheric progression. With that being said, one of the reasons why we wait until November to issue a winter forecast is to analyze how the weather pattern throughout the planet is progressing during that month,particularly in the 2nd half of November.
This time frame is crucial in understanding the overall flavor of the winter — or at least weeding out some ideas moving forward. If some previous analog years matched well numerically in September and October, but were heading in a completely different direction as we approached winter, those analogs lose validity. There is validity to the phrase “What happens in November, winter remembers.”
Our analogs below were generally vetted based on the following criteria
- La Nina
- Similar 500mb pattern progression during November
- Weighted by 500mb, QBO intensity and downwelling forecast match
To begin, the pattern being projected for the end of November continues to suggest a poleward -WPO ridge breaking into the Arctic, helping to drop the AO negative, helping start a chain reaction to further support Atlantic wave breaking events actually resulting in a big -NAO. The Greenland blocking signal is thus tremendous and may be evidence of the downwelling QBO having an effect on the pattern.
The closest analog years we found were 1950, 1955, 1973, 1995, 2005, and 2010. The two years that we thought were the closest matches as well as also having a downwelling QBO were 1995 and 2005, so they were included twice.
We then rolled these over into December, January, February, and March, as well as a composite of the pattern as a whole for the December through February period, and also the December through March period.
The analog years tell us a story that begins to come together, sensibly. The main driver OF the weather pattern is still the consistent presence of an Aleutian/-WPO ridge that goes poleward into the Arctic, and sends plenty of cold into Western Canada and the Northwestern third of the US. Depending on the exact orientation of this trough, it is a -PNA pattern that can often lead to downstream ridging in the East. And while this will certainly be the case at times, this main driver still does provide plenty of cold in and near parts of the US, so when there is high latitude blocking in the Atlantic side, this cold can get sent eastward into the entire northern third of the US, and even at times in the entire Eastern US.
Analogs also suggest the strongest blocking occurs in December, with plentiful cold and snow opportunities in the East. High latitude blocking weakens and becomes more transient in January, resulting in a large-scale moderation of temperatures across the Central and Eastern USA, with cold focused in the West — classic La Nina. High latitude blocking makes a return during the latter half of the winter.
While the pattern will almost certainly not shift in conveniently monthly timetables to completely satisfy this breakdown, it does illustrate our ideas of general progression. The blocking, cold, and potentially snow will last into a good chunk of December, but the Stratospheric Polar Vortex will not completely break down, and thus is expected to recover during the middle to later part of December. This suggests the potential for a “January thaw” (similar to what was observed in 1996 and 2006 — remember 1995-1996 was not necessarily wall-to-wall cold and snow!), especially in the East where temperatures really warm, with snow totals in the Intermountain West and Northern Plains really piling up.
Then, later in the winter, there could be a bit of a reload, depending on how much La Nina influence is still present. We thought it was important to include an analog year like 1973-1974, because although 1995 and 2005 were good matches, we must take into account the stronger La Nina flavor 2017 has HAD compared to those two years, and also compared to their respective late November patterns. This allows us to work in risks of a Southeast ridge in February, where the Ohio Valley and Northeast are battlegrounds and thus interior locations could observe above normal snowfall, with colder air and snow chances reaching the coastal plain when blocking is stronger.
Another analog that has been mentioned in the weather community (but we did not include) was 2012. 2012-2013 was not a La Nina, and it had more of a Pacific Jet influence in December. Thus, while November, 2012 did have a poleward -WPO ridge, it was not displacing as much true Arctic cold, as the Pacific Jet was modifying it in some areas. It also did not have a true -NAO block that was displacing real cold from Western Canada into the entire Eastern US — it had a more bootleg chilly airmass for just the Southeast.
Monthly Highlights and Overview
The Winter of 2017-2018 is expected to be highly changeable and volatile, especially compared to recent years. Its most notable characteristic may be the opportunity for an early start, which is much different than years past.
December 2017 Highlights
This month is likely to feature a pattern characterized by a ridging in the North-Central Pacific Ocean, often extending poleward into the arctic regions. Arctic cold is likely to be displaced over British Columbia and parts of Western Canada, where below normal temperatures are expected. Periodic ridging near Greenland and the Davis Strait is likely to result in the extension of cold air further south and east at times, into the Great Lakes, Ohio Valley, and Northeast States. This pattern is especially likely during the first half of the month.
As the month goes on, some moderating effects appear possible as the pattern undergoes a bit of retrogression. Ridging should remain in the West and Southwest United States for a large majority of the month, extending eastward at times into the Central Plains states. Storm threats are expected throughout the Great Lakes and Northeast, with wintry weather especially possible in those regions during the months first half.
January 2018 Highlights
The large majority of this month is favored to feature moderating temperatures and less snowfall than average across the Eastern United States. Much of this forecast is driven by the idea that high latitude blocking will be weaker and less prevalent during this 30-day period than it had been during the month of December. As La Nina continues to affect global circulations, ridging is expected to build in the Southeast United States, shifting the mean storm track further west. Arctic cold is expected to be heavily centered in Western Canada and the Northern U.S Plains.
Winter weather events appear possible during this time frame across the United States Northern Plains and parts of the far Northwestern Great Lakes region. Milder air will have a propensity to exist across the Eastern United States, specifically Southeastern United States, while cooler risks will continue to work in to parts of far Northern New England.
February 2018 & Beyond Highlights
While some uncertainty still exists, the pattern is expected to feature more transience during the month of February, with a more stout North to South gradient developing across the United States. Colder air will be more prevalent across the Northern 1/3, from the Northern Plains to New England. Warmer risks will continue across the southern tier from the Four Corners into the Southeast States. This gradient may serve as a focal point for storm development during the month.
Most notably, the weather throughout this month will be heavily modulated by the development of any high latitude blocking, which remains an uncertainty. Blocking in Canada would act to dislodge cold air further south at times, while a lack of blocking would result in temporary moderation. The forecast takes into account a blend with the gradient serving justice to both events which are expected to transpire at times throughout the month.
This pattern may continue into March, when high latitude blocking may again have a tendency to develop and force larger, cutoff storm systems with a risk of high precipitation events from the Ohio Valley into the Northeast.
Snowfall Overview and Highlights
While snowfall forecasting in terms of percentages and numbers is inherently difficult at this range, there are some general areas that can be highlighted for departures at this juncture. Namely, an active storm track during the first half of the Winter is expected to benefit the Interior Northeast and parts of the Great Lakes states. Multiple disturbances and winter storm opportunities should bring snowfall between 125%-150% for these areas for the DJF trimonthly period. Lake Effect Snow is also expected to be fairly significant with multiple shots of cold air.
In addition, a below normal anomaly near 75% of normal snowfall is expected throughout the Southeast. The presence of the Southeastern US ridge is expected to be particularly inhibitive of snowfall as the season draws on throughout these areas. A large positive snowfall anomaly is expected to exist in the Northern Plains this year.
NYC-Focused Winter Highlights
December Temperature Anomalies: -1 to -3 F (Below Normal)
January Temperature Anomalies: +1 to +3 F ( Above Normal)
February Temperature Anomalies: -1 to +1 F (Near Normal)
Snowfall Departure From Normal: 125% to 150% (Slightly Above Normal)
As previously discussed largely in our monthly breakdown, we anticipate winter to get off to early start in NYC metro, with more high-latitude blocking, especially during first half of December. There will be opportunities for winter storms with significant snowfall, especially just northwest of NYC. Then perhaps some moderation during the second half of month, as the high-latitude begins to relax. But ridging out West will likely persist and tend to keep to more troughs over the East with some colder temperatures. January will likely feature warmer temperatures with less high-latitude blocking expected. February will likely see a more north to south gradient over Northern ⅓ of the US, that will support more cooler weather to return in NYC metro. If more high-latitude blocking returns there will be opportunities for sustained cold and significant winter storms.
The 2017-2018 Winter Forecast was a collaborated effort between the forecasters at New York Metro Weather including John Homenuk, Doug Simonian, and Miguel Pierre. This forecast was compiled over a period of several months and features various sourced images. The image credits are included either on the images themselves, or within text above or below the image.
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