Seasonal forecasts are always a challenge, but this year’s winter forecast is especially difficult as there are several factors that in some cases contradict each other. La Nina is part of the factor for this winter forecast and would suggest cold air will have trouble heading south or at least remaining at a given location. Meanwhile, the development of a warm pool in the central and western Pacific would suggest the potential for a ridge in the West and a trough in the East to counter La Nina’s influence. Also, there are hints that high latitude blocking will have an increased potential for forming, especially over North America and the northwestern Atlantic.
This winter is expected to be highly volatile and rather active. The cold air masses expected to invade the United States will be strong but transient. This means that while cold air masses will pack a punch from the Rockies to the Mid Atlantic and New England coasts, those same air masses will struggle to push south and will retreat quickly. When the Arctic Oscillation and North Atlantic Oscillation are in a negative phase, these cold air masses will be able to remain sustained further south and an active storm track will result from the southern and central Rockies to the Southern Plains to the Tennessee River Valley to the Mid Atlantic coast. Powerful winter storms would be the result. However, if the Arctic Oscillation and North Atlantic Oscillation are positive, the storm track would shift north from the northern and central Rockies to the central Plains to the Ohio River Valley and then up into the central or eastern Great Lakes with powerful winter storms resulting in the Central Plains, Upper Mid West, and western/central Great Lakes.
Traditional winter expected with near normal temperatures and precipitation through the season. Temperatures will range from -1 to +1 F with near-normal snowfall.
California/Desert Southwest/Southern Rockies:
Drier than normal winter is expected with above normal temperatures. However, one or two major winter storms can not be ruled out. Temperatures will be +2 to +1 F with below normal snowfall by 10 to 15 %.
Northern and Central Rockies:
Temperatures will be near to slightly below normal with above normal snowfall. One of the best ski seasons in years. Temperatures will be -2 to -1 degrees below normal and +10% to +15% above normal.
Cold winter expected with temperatures well below normal. Snowfall will average near normal for the season. Temperatures will be -3 to -1 degrees below normal.
Upper Mid West, Great Lakes, New England:
Below normal temperatures are expected with above normal snowfall. The Lake Effect Snow season will be very active, especially in December. A high threat for major winter storms via storm tracks along the East Coast and also tracking north into the St. Lawrence River Valley. Temperatures will be -2 to -1 below normal and snowfall +10% to +15% above normal.
Central Plains, Mid Mississippi River Valley, Tennessee River Valley, Ohio River Valley, and Mid Atlantic:
Temperatures will average near normal between +1 and -1 through the season. These areas will be at the heart of the storm track and will have a volatile temperature region. One week in the 60’s, the next in the 20’s for highs. Several snow, ice, and rain events can be expected depending on the storm track. There is a threat for at least one or two major winter storms on the East coast from the Mid Atlantic into New England. Temperatures will be -1 to +1 and snowfall will be +5% to +15% above normal depending on prevailing storm track.
Southern Plains, Lower Mississippi River Valley, Gulf Coast, Southeast:
A warm winter is expected overall with temperatures +1 to +3 through the seasons. An active severe season is expected in these areas as strong low-pressure system pass to the north with sweeping cold fronts following. Precipitation will average near to slightly below normal. Temperatures will be +1 to +3 and snowfall will be near -5% below normal.
There are several factors that will help develop the weather pattern for this winter. We’ll go over a few of these factors and how they will impact the weather pattern overall.
We’ll start with La Nina. La Nina is a cooling of the waters in the Tropical Pacific, which can be clearly seen above. This La Nina is basin wide with the coldest anomalies over the eastern portions of the ENSO region. The fact that the coldest anomalies are towards the eastern side of the ENSO regime is an important observation, as the potential for convection to develop around the dateline or just to the west is greatly increased. A wild card in the forecast is how far west and how strong will La Nina get for areas between 170E and 150 W, as this factor will influence the location and strength of convection in the Tropical Pacific.
This takes us to the actual sea surface temperatures! Notice how the sea surface temperatures are over 28 degrees Celsius between 175W and 120E Longitude. The location of this very warm water in the Tropical Pacific means that thunderstorms can form in these locations.
The development of convection or thunderstorms in this location of the Tropical Pacific tends to help enhance the Polar jet stream in the central Pacific and tends to support high-pressure ridges to form along the West coast and broad troughs from the Rockies to the East coast. The La Nina influence tends to support a ridge to build over the Southeast but centered over the Bahamas rather than the eastern Gulf of Mexico which was observed last year. The location of tropical thunderstorm development is extremely important to this forecast. If the convection is between 140 East and 180 Longitude along or just north of the Equator, then a deep trough is likely to the south of the Aleutian, a ridge on the West coast, and a broad trough over the eastern two-thirds of the United States. However, if the convection is further west, then a milder pattern will take hold from coast to coast with more influences from Pacific air masses rather than Polar and Arctic air masses.
The atmospheric influence of La Nina typically means cold air masses have a more difficult time to push south into the mid-latitudes as the Polar jet stream is more likely to be centered further north. However, this influence can be overwhelmed by the development of high latitude blocking.
Other impacts from sea surface temperature considerations are what is developing in the northern Atlantic. Last year, there was below normal sea surface temperatures in the northwestern Atlantic. This year the sea surface temperatures are well above normal in the northwestern Atlantic and to the south of Greenland. The warm sea surface temperatures increase the potential for the low-pressure system to develop and thus leads to the potential for high latitude blocking, which is called a negative North Atlantic Oscillation.
There are several other factors to consider for this winter, but two factor stands out to keep an eye on. One of those factors is the influence of the QBO and solar influences. This factor is still a highly debatable consideration, but one that needs to be considered.
The QBO this year is east based, which is a significant change from last year. Meanwhile, solar activity is falling into a minimum for the first time since the winter of 2010/11. When the QBO is east based and solar activity is at a minimum, the Polar Vortex tends to be warm and disturbed. This observation can be further studied in Gray et al (2001 a/b), Gray et al. (2003), and by Palmer and Gray, 2005 among others. What this means is when the QBO is east based and solar activity is at a minimum, the Polar Vortex tends to be weak. When the Polar Vortex is weak, high latitude blocking is more likely and cold Polar/Arctic air masses are able to invade the mid-latitudes more frequently.
Another factor that has been talked about a lot recently but is still hotly debated and still needs more study, is the connection between snow growth in Siberia and the influence on the Polar Vortex. There is still uncertainty as to whether the above normal snowfall causes the Polar Vortex to weaken or if the above normal snowfall is a symptom of a weak Polar Vortex.
Overall though, the fact that snow growth is once again above normal does point to a weak Polar Vortex is a theme this year. A weak Polar vortex would be additional support for high latitude blocking to develop this winter.
The above normal snowfall in Siberia and in Canada also would suggest that Polar and Arctic air masses will have supportive surface conditions to become strong, which may act as a feedback mechanism to create stronger Polar and Arctic air masses.
The primary driving influence on the winter weather pattern will be the potential development of convection between 130 East and 175 West longitude. If warm waters over 29 degrees Celcius is focused around the dateline, the potential for strong high latitude blocking increases. The Polar jet stream will be able to drive south and with it, Polar and Arctic air masses into the Rockies and Plains than towards the Mid Atlantic and New England coastlines. However, if convection develops further west towards 120 East, then the Polar jet stream will be forced northward and a mild winter will be expected from coast to coast. With the degree of uncertainty on convective development rather high, the seasonal forecast is naturally highly volatile. However, given the latest observational data, the forecast is for a colder solution of the two with an increase in high latitude blocking expected.