NEW YORK - Coming up with a long-term winter forecast provides many challenges based on various weather and climate phenomena that occur around the globe. Some are short-term patterns while others can last many months. Understanding and predicting these events is the key to an accurate forecast.
So get your parkas and snow boots ready as we dive into the ingredients that make up a long-range winter forecast.
The more common ones we look at each year are El Niño, La Niña, the PDO, the AO, and the NAO.
El Niño and La Niña are probably the most commonly discussed phenomena which play a role in how the jet stream moves across North America. An El Niño occurs when sea surface temperatures across the equatorial latitude in the Pacific Ocean are warmer than average. This then tends to result in a stronger southern jet stream. Conversely, a La Niña pattern results from colder than average temperatures fostering a stronger northern or polar jet stream.
For this upcoming winter season it appears like we'll have neither, what I call "La Nada," which nothing or neutral conditions. More on that later.
Traveling north across the Pacific, we discover the PDO, or Pacific decadal oscillation, which also deals with sea surface temperature. In the positive or warm phase, temperatures are warmer than average in the northeastern Pacific with the opposite happening during negative or cold phase. Each phase can last more than 20 years.
Currently, the PDO is in the warm phase, which may result in a stronger and more northerly jet stream pattern for the West Coast with the jet stream moving more towards the south for the rest of the United States.
We now head up to the arctic and the AO, or arctic oscillation, and its relationship to the polar vortex. Over recent years, the polar vortex has received a lot of hype from the media. But in truth, it is present 365 days a year over the polar regions. How far south it moves is a determining factor as to how much cold air reaches our part of the country.
When the arctic oscillation is in the positive phase, polar circulation is stronger which forces cold air and storms to stay to the north. The negative phase results in a weaker jet stream and weaker polar vortex which can become fractured. This brings a colder and stormier pattern to much of the central and eastern U.S. The arctic oscillation is more of a short-term pattern compared to the PDO, El Niño, or La Niña, and this winter is likely to be in more of a negative phase.
Closer to home, we look at what's taking place over the Atlantic Ocean where the North Atlantic oscillation, or NAO, takes place. Here we analyze pressure difference from north to south across the ocean. In the negative phase, you have weaker low and high pressure in the eastern Atlantic, which can lead to a blocking pattern further west. This then allows colder air southward and can result in more coastal storms.
The positive phase brings stronger low and high pressure which allows for a more northern and west to east jet stream pattern resulting in a warmer conditions. Another short-term pattern, the NAO can only be predicted accurately a few weeks in advance. But you can see that a collision of a negative AO and negative NAO can lead to a cold and snowy pattern.
Another outlier in winter weather prediction is the extent of Siberian snow cover that builds during the fall months. With higher than average snow, colder air may ultimately seep across the North Pole and eventually invade the U.S. So far, snow cover is slightly above average.
These factors will lead to a more active northern jet stream, which will carry storms down from south-central Canada towards the Ohio Valley. These storms will then have the tendency to redevelop along the East Coast and then intensify as they move northward. While we may still get storms that originate in the Gulf of Mexico, I feel this pattern will dominate.
While we'll begin to see an occasional threat in December, the first half of January will bring more action with February featuring multiple coastal storm threats.
March is a wild card at this point but could also end up being an active month. It's hard to say at this point how many of these forecast ingredients will apply then.
For the tristate area, this should bring average to slightly above-average temperatures but be prepared for a few severe cold outbreaks that may last a few days.
Higher snowfall amounts will occur to the north and west of New York City, where there will be less of an influence of the warmer Atlantic Ocean leading to an above-average snowfall season.
In New York City and the nearby areas, due to the uncertainties with coastal storm tracks, storms that threaten may bring a combination of snow and rain.
I'm predicting 30 to 35 inches of snow for New York City and nearby suburbs with 40 to 60 inches further north and west. (The average snowfall for NYC is around 27 inches.)