We are at that time of year again where everyone is wondering what the winter will hold for us. The winter forecast is the most anticipated seasonal forecast of the year. Television stations and weather sources issue them yearly with a lot of excitement and advertisement. In the years past I would issue a public and premium winter discussion with the paid version including detailed technical analysis.
However, over the past few years, the public has lost some confidence in the winter forecast mostly because most forecasts are issued with a map with descriptions and that’s about it. So I would like to address this issue but issuing both to the public. Premium Members are getting an upgrade with a video chat on the winter forecast in detail next Wednesday.
Many people look for specifics in the winter forecast as in how much snow will fall in their hometown. I wish I could give you an exact number for your street, but the science simply isn’t there for an exact snowfall prediction, although many will try. The objective of this forecast is to provide an overview of the national and regional themes of this upcoming winter. Below the general forecast, there will be a detailed analysis of the forcing parameters on this winter pattern and what factors I will be watching that could produce a warmer and colder outcome for this season.
The overall forecast has a strong influence from La Nina this year which tends to support a strong Polar jet stream and a colder winter over the northern tier while the southern tier is drier and warmer, especially over the Desert Southwest and the Southeastern United States.
Northwest United States: In this region, storms will invade from the Gulf of Alaska with significant snowfall for the mountains and above normal rainfall along the coast. At times, arctic air masses will drop south into the region from central and western Canada, especially over the northern Rockies.
Southwestern United States: Most of the storm tracks will be north of this region with a straight Pacific influence with above normal temperatures and below-normal precipitation. Most of the snowfall will be found northwest and north of this region, but an occasional arctic air mass over the southern Rockies will be possible.
Southern Plains To Tennessee River Valley To Southern Mid Atlantic: This region will feature an occasional Polar or Arctic air mass to invade, especially in December through the middle of January. However, the storm track will be to the north of these locations normally for much of the winter. Waves of above and below normal temperatures are expected which will average above normal while near to below normal precipitation can be expected.
Southeastern United States: A ridge over this region will be a persistent theme through the winter with the storm track northwest of the region. Overall, warm and dry weather conditions can be expected through the winter.
Central Plains To Upper Mid West To Ohio River Valley To Northern Mid Atlantic To Southern New England: An active storm track and very volatile temperature pattern with weeks of well above followed by well below normal temperatures can be expected through the winter. Precipitation will be above normal. Snow anomalies will vary significantly from coast to interior locations. A high threat for winter storms, especially snow to ice/rain storms towards southern sections of the zone.
Northern New England: Near to below normal temperatures and above-normal precipitation can be expected. Above-normal snowfall is likely in the Green and White Mountains. Sky season should be pretty healthy in New England.
Great Lakes: Waves of Arctic/Polar air masses will produce enhanced Lake Effect Snow this season. Look for below normal temperatures of 1 to 2 degrees Fahrenheit below normal and near to above-normal precipitation. When the storm track shifts northward, widespread snow events are likely.
Northern Plains: This region will feature constant invasions of Polar and Arctic air masses which will keep temperatures below normal for the season by 1 to 3 degrees Fahrenheit. Precipitation will average near to below normal.
NORTHERN MID ATLANTIC FORECAST
The winter for the northern Mid Atlantic will come down to simply how many times high latitude blocking aligns to force a storm track over the coastal waters rather than over the Delaware River Valley. This year will be colder and snowier than in previous years, but the overall amounts along the coast are likely to range near to below normal unless we experience a robust sustained high-latitude blocking regime. If that happens, all bets are off in terms of how much snow and ice as the season will feature above-normal snow and ice amounts for sure.
Maryland and Delaware: Most of the storm tracks will be to the north of these locations. When low-pressure systems redevelop from the Ohio River Valley, most of the storms will develop north of these locations to produce limited precipitation. Polar and arctic air masses are likely to stall and retreat faster in these locations.
I-95 Corridor and Coast: While waves of cold conditions can be expected, the storm track is likely to feature much snow to ice to rain events. The closer to the coast, the faster the transition to rain. Temperatures will average below normal in December, near normal in January, and above normal in February. The best potential for snowstorms will be in December to the middle of January, although winter storms will be possible through early March. Precipitation will be above normal but snow near to slightly below normal. If a sustained negative NAO pattern becomes established, this weather pattern will produce significant snowfall for these locations.
Interior: Waves of cold conditions can be expected. The storm track is likely to feature several snowstorms and ice events. Snowfall will be near to above normal, but ice concerns will be a constant factor. Temperatures will average below normal in December, near normal in January, and above normal in February. The best potential for snowstorms will be in December to the middle of January, although winter storms will be possible through late March. Precipitation will be above normal and snow near to slightly above normal. These locations will have the potential for significant snowfall this season regardless of the NAO regime.
SEASONAL STORM TRACKS
The following two maps are the seasonal storm tracks that I am expecting this winter. The two maps are split on the type of high latitude blocking regime we can see this winter. They are split between when the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) are positive (warm phase) and negative (cold phase). I expect both phases to show up this winter. My expectation is to see the positive phases more so than the negative. However, if the negative phases dominate which is possible, we’ll have a far snowier winter for the coastal plain than I currently expect.
When the NAO/AO is in a positive phase, the storm tracks are typically more north and west of the region. In this regime, storm tracks can cut up towards the Eastern Great Lakes, the Hudson River Valley, or move straight through the northern Mid Atlantic. These storm tracks tend to produce mostly rainfall along the coast, although a brief initial snowfall or period of icing is possible. Interior locations remain at threat for significant snow and ice event, but clearly, the focus for snow and ice is towards the Ohio River Valley, Great Lakes, and central/northern New England. Storms exiting from the Ohio River Valley will likely not redevelop along the coast and support widespread change over from snow to ice and then rain. These storms would be minor snow producers for the region but can produce significant ice accumulation for locations like the Poconos and Catskills. Meanwhile, storms that exit out of the Tennesee River Valley can produce significant snow and ice accumulation for the Poconos, Catskills, and central New England while coastal areas will quickly transition to rainfall.
When the AO/NAO is negative, the storm track is suppressed further south, and redevelopment off of the Delmarva Peninsula is far more likely. In this colder regime, the focus for snowfall shifts towards the Tennesee and Ohio River Valleys on through the Mid Atlantic and southern New England. Major winter storms are possible in this phase with storm tracks from the Tennesse River Valley (dark blue and light blue) more like to redevelop off the Mid Atlantic coast (red track) and produce significant snowfall. With La Nina a significant influence, storm redevelopment is more likely off of the Delmarva Peninsula rather than the North Carolina coast, which would mean there will be many times when Washington D.C. and Baltimore will miss out on snow events while further north will be significantly impacted. Alberta clippers (purple) also would be more frequent and be capable of slowing down to produce significant snowfall for the region at times, however most of the time a cold light to moderate snowfall event is usually the result.
FORCING FACTORS FOR THE WINTER OF 2020/21
This part of the discussion focuses on the scientific thought behind the forecast. By breaking down these factors, we can also pick up on features that may create havoc with the seasonal forecast. In this section, we’ll look at various tropical based parameters, stratospheric influences, cryosphere state, and more!
There used to be a time when you can look at the state of the ENSO and roll out a reliable forecast. El Nino provides one type of influence and La Nina another, neutral can be more of a wild card which then the MJO dominates. I don’t think that rule really applies any more due to the warming we are seeing in the Indian Ocean and western Pacific. Still, we are clearly dealing with a La Nina this year and one with a focus towards NINO 3 rather than NINO 4, which would suggest more of an east based orientation in this year’s La Nina. We’ll get to La Nina in a second, but let’s look at all of the Pacific.
When I look at the sea surface temperature anomalies in the Pacific Ocean, we do not have a clear PDO signal, but I can state that we clearly aren’t anywhere near a negative PDO at this point. A positive PDO regime with a moderate La Nina can set up a pretty tricky combination of forcing parameters. When the PDO and ENSO state counters each other, they tend to weaken the influence of ENSO. So in this case, while ENSO is in a moderate east based La Nina state, the overall warm Pacific anomalies would suggest the atmospheric response would more likely be a weak east based La Nina 500 MB pattern.
Back to the La Nina pattern itself, I studied the various winters with La Ninas and compared their orientation. In this year’s orientation, this La Nina is closest to 2010/11 and 1995/96 (note, there was an error in which 09/10 and 94/95 were typed, but that was incorrect). Now, that doesn’t mean we’ll end up with the results for you snow lovers, but the atmospheric themes will be the same in the Pacific weather pattern.
I am noting the orientation of this La Nina event because we know that the closer the warm water is to the dateline (180° Longitude), the more likely the 500 MB pattern in the Pacific will feature a strong upper-level low-pressure system or trough around the Aleutian Islands and a ridge in the Gulf of Alaska through western Canada. This pattern is called a negative EPO, which tends to support cold weather over the eastern United States.
As far as the strength of this event, you can make a case for a designation for a strong La Nina coming on, especially when you look at the sub-surface temperature anomalies, however, I would caution against that approach. What matters is the atmospheric response and in this case, the SOI has been and continues to be in the range of around +8 SOI for the seasonal and monthly, which would correspond to a weak La Nina to a moderate La Nina. I am expecting that type of atmospheric response to continuing given the warmer anomalies outside of ENSO.
Now this all might be screaming for a cold and snowy winter, but I am also noticing some other factors as well. For one, the MJO is not responding to these influences this month. The MJO or Madden Jullian Oscillation is an oscillation of rising air around the tropics and thus the location of heavy precipitation. I am of course watering down this definition as the MJO is a bit more complicated but that’s the overall idea. The MJO is measured in 8 phases. When the MJO is in phases 3 through 6, the eastern United States tends to have warmer weather patterns and when the MJO is in phases 7 through 2, the eastern United States tends to have colder weather patterns. Typically, the MJO is less of a factor in ENSO active years, however, this year I think given the warmer western Pacific, the MJO phases can give us a clue on where the pattern is going over the next 10 to 15 days. When the MJO is in the warm phases, you can expect a storm track cutting from the central Plains to the eastern Great Lakes and when the MJO is in a cold phase the storm track will likely be suppressed through the southern Plains into the Mid Atlantic. So one of the wild cards this winter is how often the MJO will be in either one of these phase regimes.
Of course, when it comes to tropical forcing, we shouldn’t forget the Atlantic Ocean. The extremely active hurricane season has caused sea surface temperatures to run below normal in the Gulf of Mexico heading into the winter, especially in the coastal waters while the western Atlantic remains warm with widespread above normal sea surface temperatures. This environment would suggest more support for rising air along the East coast this year which would be a signal for cyclogenesis. The warm SSTA over the northwestern Atlantic would be favorable for a sustained feedback loop for a negative NAO pattern should the stratospheric and upper tropospheric environment become favorable for a negative Arctic Oscillation and North Atlantic Oscillation regime.
To wrap up tropical forcing, this year La Nina is the dominant tropical forcing mechanism. This year’s La Nina is starting out moderate and east based, which would suggest a higher potential for a robust upper-level low around the Aleutian Islands and a strong ridge around the Gulf of Alaska and western Canada. The movement of the MJO will help us understand how La Nina will influence the 500 MB pattern from week to week with warm phases to producing a storm track from the central Plains to the Great Lakes and cold phases producing a storm track from the southern Plains to the Mid Atlantic.
CRYOSPHERE AND SEA ICE FACTORS
One factor that typically many people do not consider in a winter forecast is the influence of the cryosphere or the surface snow and ice coverage over the higher latitudes and the North Pole. The state of coverage and depth of snow and ice can have a profound influence on how the weather pattern develops. As with all features, the influence of the cryosphere is not a stand-alone feature and must be considered in combination with all the other factors. In this part of the discussion, we’ll look at where we stand with these features.
This year’s snow advance has become aggressive after a slow start to the season. Snow growth has pushed from a negative anomaly to a positive anomaly over the past three weeks. According to Dr. Cohen, (you can follow his work here) when we see this signal of snow growth as we have seen this year, typically the Polar Vortex can be disrupted and displaced towards the lower latitudes in the troposphere and stratosphere. We must remember though that while the Polar Vortex can be disrupted, the orientation of the Polar Vortex and location of the displacement are key factors that can turn a potentially cold winter into a warm winter very easily. Of course, we also have to look at the state of sea ice in the Arctic, especially around Siberia.
The Arctic sea ice this year is at a historic low value that I frankly have never seen before. I have to be honest that this factor is very much a huge wild card. A significant reduction in Arctic Sea Ice in the late Fall can produce a significant thermal gradient between the landmasses and the Arctic Ocean, which in turn can disrupt the Polar Vortex and cause a significant displacement in the Polar Vortex towards the Mid Latitudes. In fact, several studies have shown that a season with low Arctic sea ice tends to support a displaced Polar Vortex. I must stress again that just because the Polar Vortex splits or displaces, that the orientation will be favorable for a cold winter in the Eastern United States. However, the potential for high latitude blocking is far greater than with a strong Polar Vortex over the North Pole.
Finally, I think it is very important to consider the growth of snowfall over North America. The snow growth in northwestern Canada has been impressive and would help to build strong Polar and Arctic air masses. This type of environment can produce strong Polar/Arctic air masses that can drive into the United States, first over the Northern Plains and then into the Upper Mid West, Great Lakes, and eventually the Northeast. We are already seeing an expansion of the snowfall into the Northern Plains. As a result, future Polar and Arctic air masses are less likely to moderate quickly while moving through the Eastern two-thirds of the United States.
So as we can see here, the cryosphere is in a favorable position to support the development of high latitude blocking, displacement of the Polar Vortex, and for strong Polar and Arctic air masses to push southward into the United States.
STRATOSPHERIC ANALYSIS AND SOLAR INFLUENCES
Last year this forcing mechanism created havoc with the winter forecast as an area of cold air from the ionosphere rapidly intensified the Polar Vortex in the middle of December. As a result, the potential for high latitude blocking was severely cut short. In fact, last winter barely had any type of major winter event for much of the region due to this influence. The potential for a similar event to occur is unheard of but given the quiet sun we have in place heading into the winter, I can’t 100% discount something unusual happening. So let me break down the current environment and what this data means for the winter forecast.
I like to study the stratosphere at around 50 MB to gain a clear understanding of what is happening at this level and how that influence can impact the troposphere. Of course, when forecasting week to week, you want to study the whole column to catch any unusual trends, like I did last December.
The current state of the stratosphere is averaging above normal with a focus of warm anomalies over northern Asia, eastern North Americas, and the northwestern Atlantic Ocean. The overall environment has been supporting a recurring development of the Ural Blocking mechanism, which has been known to cause disruptions of the Polar Vortex.
The evolution of the Quasi-Biennial Oscillation or QBO this Fall has seen the QBO shift from an easterly (negative) to a westerly (positive) state over the past 12 weeks. The evolution of the QBO mimics the 2010/2011 season which also is a healthy match given that the winter season also featured a moderate La Nina. A positive QBO regime supports more frequent warming events that create a reflection response on the Polar Vortex and tends to support below normal temperatures over the Eastern United States. However, a positive QBO phase typically does not feature powerful events, but frequent weak to moderate events. Still, a powerful stratospheric warming event is possible this year.
Solar activity has started to increase this Fall as we move into Solar Cycle 25, but if it wasn’t for the fact that last year was historically quiet, this year would be right in the ballpark of other record-breaking years we have seen in the past. At this point, I would put the influence of this year’s Solar Activity for the Winter as a quiet year. We have above normal ozone in place this Fall and that is not likely to decline rapidly given the most we can muster is one or two sunspots at a time, if that.
Above normal ozone over the higher latitudes allows for the expansion of the stratosphere which supports warming of the overall environment. This influence combined with some additional impacts from solar particles on the ionosphere should support a weakening Polar Vortex.
So given the observational data, I expect the Polar Vortex to have periods of weakening and displacement. I can’t rule out a split of the Polar Vortex. The process of the Polar Vortex disruption and displaced will have a profound impact on the weather pattern, specifically associated with high latitude blocking.
FACTORS THAT COULD CREATE HAVOC ON WINTER FORECAST
So now we have the basic forecast and the forcing mechanisms behind the forecast, now let me list some of the factors that can cause this forecast to change drastically warmer or colder depending on the factors.
- The La Nina reaches the strong category level: A strong La Nina will cause powerful jet streaks to form in the Polar jet stream and limit the ability for high latitude blocking to form. As a result, the weather pattern ends up warm and relatively dry.
- The La Nina shifts to a west-based orientation: Should La Nina shifts the coldest anomalies to the west towards the 180° dateline, the whole 500 MB pattern will shift westward which would put a trough over the Western United States and a very strong, sustained ridge over much of the eastern United States. This scenario would lock in a constant +AO/NAO regime for the whole winter.
- Major Stratospheric Warming Event: A major stratospheric warming event would have the chance to severely disrupt the Polar Vortex and produce powerful high latitude blocking regimes, specifically a sustained -AO/NAO pattern. Such a pattern would support sustained cold and wintery weather from the Upper Mid West through the Mid Atlantic.
- Severe Displacement of the Polar Vortex to China: This type of displacement would keep all of the cold air over China and produce a warm weather pattern over North America with a zonal weather pattern.
- Severe Displacement of the Polar Vortex into Canada near the southern Hudson Bay: This pattern would drive very cold and dry air over the northern tier and may suppress the storm track severely to the south and produce a sustained, cold and dry weather pattern for the Eastern United States.
- Unknown Impacts Of Low Sea Ice: Many of you know that I am no alarmist, but the degree of a lack of sea ice in the Arctic, especially by Siberia is, well, shocking. Given that I have never seen this type of lack of sea ice, the type of influence this could have on the tropospheric and stratospheric Polar Vortex is unknown. I’m going with the idea that the Polar Vortex will be disrupted and displaced due to this factor, but how that process evolves can go several different ways from extreme cold to depressing warmth.
- Aliens, Godzilla, meteors, etc: It’s 2020, so can’t rule it out.