ECMWF Forecast: Impact of Combined Negative AO and NAO

The European Centre for Medium-Range Weather Forecasts (ECMWF) indicates that both the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) are expected to remain negative through May 2026, a pattern that could prolong cooler and more unsettled spring conditions across parts of Europe, particularly in Italy and surrounding regions.

When AO and NAO are simultaneously negative, atmospheric pressure patterns tend to favor the southward displacement of cold air from higher latitudes into mid-latitude Europe. This configuration often results in below-average temperatures, increased precipitation and a delayed onset of sustained warmth during the spring months.

The ECMWF’s extended-range forecast, released in mid-April 2026, highlights this dual-negative phase as a dominant signal in its outlook for the coming weeks. According to the model ensemble, the persistence of these oscillations reduces the likelihood of prolonged high-pressure systems that typically bring stable, mild weather to the Mediterranean and Central Europe during late spring. Meteorologists note that while individual variability remains possible, the current signal suggests a spring characterized by frequent frontal passages, cooler daytime highs, and a higher chance of late-season frosts or snow at elevated altitudes in the Alps and Apennines. The NAO, which reflects the pressure difference between the Icelandic Low and the Azores High, tends to suppress the usual west-to-east flow of mild Atlantic air when negative. Similarly, a negative AO indicates weaker polar vortex containment, allowing Arctic air to spill southward more easily. Together, these factors create a circulation pattern less conducive to seasonal warming. Historical comparisons show that prolonged negative AO and NAO phases in spring have previously coincided with below-average temperatures across northern Italy, the Po Valley, and the Adriatic coast, as well as increased rainfall in the Tyrrhenian sectors. However, forecasters caution that subseasonal predictions carry inherent uncertainty, and local outcomes may vary based on shorter-term atmospheric disturbances. As of mid-April 2026, no extreme weather warnings have been issued specifically tied to this oscillation pattern, but national meteorological services in Italy and neighboring countries are monitoring the situation closely for potential impacts on agriculture, energy demand, and outdoor activities. The ECMWF will continue to update its extended-range forecasts weekly, with the next assessment expected to refine the confidence in the persistence of these oscillations into early summer. For now, the signal suggests that spring 2026 may evolve more slowly than recent years, with winter-like conditions lingering longer than typical across much of southern and central Europe.