Scientific models forecasting the upcoming winter season are driven mostly by observations of such factors as Pacific Ocean temperature, since weather moves from west to east across the continental United States. It has been known for more than a century that worldwide seasonal phenomena were correlated with trade winds and ocean water temperature, controlling the energy available in the atmosphere: decreased trade winds are associated with waters cooler in the Western Pacific than the Eastern Pacific, a condition named “El Niño” by seafarers; increased trade winds are associated with waters warmer in the Western Pacific than the Eastern Pacific, a condition named “La Niña.” The swing between these two states is called the El Niño Southern Oscillation (ENSO), with the normal state between them called “ENSO-neutral.”
In October, the National Weather Service (NWS) Climate Prediction Center (CPC) in College Park, Maryland, issued a notice that the world had entered La Niña: “The expectations are for a relatively weak and short lived event through winter 2025–2026 before easing back to ENSO-neutral by early Spring.” Combined with other factors, that puts RI at about a 35% probability of a warmer than average winter with average precipitation.
Although there is considerable uncertainty and debate among meteorological researchers, the wider variation in the jet stream over North America associated with La Niña is generally understood to subject southern New England to less frequent but stronger storms. The extent of this effect, and even whether it really exists at all, is unclear. Because the most severe winter storms affecting Southern New England tend to track and redevelop offshore along the coast, and because the overall trend of the resulting jet stream variation from La Niña is to pull such storms toward the northwest, they may drop more rain than snow.
The increased variability in the jet stream from La Niña can lead to significant excursions from normal, allowing the polar vortex, which is a normal circulation of extremely cold air near the North Pole, to expand briefly southward, exposing regions to air temperatures tens of degrees below normal for days at a time. When this happens, interaction with a storm system can result in very heavy snow.
On Nov 21, NWS CPC updated an advisory predicting a Sudden Stratospheric Warming (SSW) event likely to allow polar vortex air to dip southward into the continental US, making colder than normal temperatures 50 – 60% likely in RI during Nov 27 – Dec 1 and 33 – 40% likely the following week. Temperatures lowered by 10 – 15°F for several days due to an SSW can take a few weeks to revert back to normal.
Climatological predictions for as long as an entire season are probabilistic with reasonably high confidence, but are necessarily focused on averages. ENSO excursions toward either extreme will induce wide variation from these averages, and the actual effect can be a warmer winter that still has severe cold snaps and a rainy winter that still has a few big snowstorms. Put more simply, El Niña introduces more forecasting uncertainty than usual.