Long before the first snowflake falls, meteorologists are already piecing together clues from the ocean, the Arctic, and the upper atmosphere. These signals show up months in advance, often while most of us are still thinking about back to school shopping or the last barbecue of the summer. They rarely make headlines on their own, but together they tell a surprisingly detailed story about what a coming winter might look like.
None of these signs work in isolation, and none of them guarantee a specific outcome down to the day or the inch of snow. Still, when several of them line up in the same direction, forecasters start paying much closer attention. Here is what those signs actually are, and why professionals watch them so closely every year.
The ENSO signal: El Niño or La Niña sets the stage
The single biggest driver of a season’s personality is the state of the El Niño Southern Oscillation, or ENSO, a cycle built around the most well known and familiar pattern for the those of us in the United States is the El Niño Southern Oscillation (ENSO) which is based on sea-surface temperatures across the equatorial Pacific Ocean. The positive phase of this pattern is known as El Niño while the negative phase is called La Niña. Winter 2025 to 2026 leaned toward the cooler La Niña phase, and that La Niña, which officially arrived on the scene in October, was favored to last through most of the 2025 to 2026 winter before dissipating sometime in early 2026.
By mid 2026 the picture had flipped entirely. Model agreement grew strong that El Niño conditions would strengthen further during 2026 and persist into early 2027, and NOAA responded accordingly. The Climate Prediction Center moved its alert level to El Niño Advisory, upgraded from El Niño Watch on June 11, 2026, with the probability of El Niño persisting through winter 2026 to 27 sitting at 96 to 98 percent. That kind of swing from one ocean state to another in less than a year is exactly the sort of thing meteorologists track first, because it shapes almost everything that follows.
Rising ocean heat hiding beneath the surface
Surface temperature readings only tell part of the story. Forecasters also study what is happening well below the waves, since heat building under the surface often shows up at the surface months later. In the case of the developing 2026 to 2027 El Niño, a massive downwelling Kelvin wave propagated east across the equatorial Pacific, with subsurface temperature anomalies of up to 8 degrees Celsius between roughly 50 and 250 meters depth.
That deep pool of warm water eventually rises and spreads, reinforcing the surface signal that meteorologists use to classify the strength of an ENSO event. NOAA CPC estimates have put the probability of a very strong Super El Niño peaking between November 2026 and January 2027 at around 63 percent. A stronger event tends to produce a more pronounced shift in storm tracks, which is why forecasters watch subsurface heat almost as closely as the headline sea surface numbers.
Early and heavy snow cover across Siberia
Few signs get more attention from serious winter forecasters than what happens across Siberia in October. Climatologist Judah Cohen, whose work centers on Arctic variability and its link to winter weather in the middle latitudes, has built much of his career around this single indicator, and his forecasting approach places unusual emphasis on Siberian snow cover in October as a predictor of winter conditions in the eastern United States and Europe.
The mechanism behind it is fairly intuitive once explained. When Siberia has less fall snow, the polar vortex tends to stay strong and keeps the frigid Arctic air penned up near the pole, but when there is more snow, the polar vortex weakens and the frigid air escapes toward the United States. A heavy, early snowpack across that vast region acts almost like a warning flag for the months ahead, especially in years when the tropical Pacific signal is weak or unclear.
Shrinking sea ice in the Barents and Kara Seas
Just north of Siberia, the Barents and Kara Seas play their own supporting role in the winter setup. Reduced sea ice there allows more heat to escape from the ocean into the atmosphere, disrupting the usual temperature balance near the pole. Research summarized by scientists studying this connection found that years where Siberia received high levels of early season snow while the nearby Barents and Kara Seas had less sea ice tended to correspond to anomalous cold snaps in the central United States.
This combination, heavy snow on land paired with open water nearby, creates the kind of temperature contrast that can nudge the atmosphere into a more unstable pattern. Forecasters watching the 2025 to 2026 season noted a similar setup, where warm ocean temperatures in the Barents and Kara Sea, combined with an easterly phase of the quasi biennial oscillation, pointed toward a potentially weaker polar vortex in early winter. It is a quieter signal than ENSO, but one that Arctic specialists treat as increasingly important.
Instability in the stratospheric polar vortex
The polar vortex itself is the mechanism that ultimately decides whether cold air stays locked up north or spills southward. This ribbon of air high in the stratosphere twirls around the polar cap, moving steadily east to west, and traps cold air over the polar region while much milder air remains in the lower latitudes. When conditions are right, energy from temperature contrasts between continents and oceans can cause the vortex to meander, wobbling like a top under the influence of rising atmospheric waves.
El Niño winters carry their own added risk here. Historically, an El Niño winter has a high chance of producing a stratospheric warming event, and analysts tracking the 2026 to 2027 season have noted that a strong El Niño can increase pressure on the polar vortex and raise the chance of a mid winter disruption, even though a full collapse is not guaranteed. When that kind of warming event does occur, it tends to send cold air surging southward weeks later, often catching casual weather watchers off guard.
Shifts in the jet stream and blocking patterns
All of the signals above eventually funnel into one final, visible outcome, the shape of the jet stream. During the La Niña winter of 2025 to 2026, forecasters expected the pattern to promote a dip in the jet stream over the Great Plains and Great Lakes regions, which in turn set up predominantly cool weather from the Pacific Northwest through the Northern Plains while warmer weather stayed confined to the Deep South and East Coast.
Meteorologists rarely rely on ENSO alone to read the jet stream, though. As one seasonal outlook put it, other key patterns, the North Atlantic Oscillation, the Arctic Oscillation, and the Western Pacific Oscillation, work together with the dominant ENSO phase to shape the weather over any given stretch of winter. When several of these oscillations align in the same direction, blocking patterns can set up that lock a particular type of weather in place for weeks, which is often what separates a routine cold spell from a genuinely rough winter.
Final thoughts
No single sign on this list can predict a winter by itself, and even meteorologists with decades of experience treat these indicators as probabilities rather than certainties. What makes a forecast credible is the way these pieces, ocean temperatures, Arctic snow and ice, the polar vortex, and the jet stream, reinforce or contradict one another as the season approaches.
With a strong El Niño already taking shape for winter 2026 to 2027 and updates expected from NOAA’s Climate Prediction Center in the coming weeks, the picture will likely sharpen considerably before the first real cold front arrives. Until then, these six signs remain the best early clues available, quietly building toward whatever the season has in store.
