Predicting weather conditions several weeks in advance poses considerable challenges due to the inherent chaos of weather. The skill of such long-range forecasts is generally quite low. Therefore, even moderate improvements beyond this low skill prove valuable for various sectors, from agriculture to energy supply and medical preparedness. For instance, farmers rely on these forecasts to determine optimal sowing and harvesting times, energy providers use them to anticipate fluctuations in renewable energy production, and public health officials use them to prepare for outbreaks of diseases such as Malaria or Dengue Fever, which are correlated with specific weather conditions.

Previous work has shown that during Northern winter the state of the circulation in the polar stratosphere (at ~15-50 km altitude) may provide useful information for improved long-range forecasts, especially for weather over the North Atlantic and Eurasia. In particular, when the polar vortex (a band of strong eastward circumpolar flow at stratospheric levels) strongly weakens or breaks down the North Atlantic jetstream tends to shift southward and the likelihood of cold spells over Eurasia increases. Such vortex breakdowns are relatively rare events that only happen approximately every other winter. One such event is currently unfolding with corresponding expected impacts on Eurasian weather in the coming weeks.

In our study we highlight an additional aspect of stratospheric influence on long-range weather forecasts: weak polar vortex states are typically followed by reduced uncertainty of 3-5 week forecasts over Northern Europe. We find that ensembles of forecasts show a reduced range of possible weather conditions (by about 25%) and therefore enhanced predictability over this sector.

We attribute this reduced forecast uncertainty to the southward shift of the North Atlantic jetstream. The associated southward shift of the tracks of winter storms, which are the main source of forecast uncertainty during this season, causes less storm activity and thereby reduced forecast uncertainty over Northern Europe. Conversely, forecast uncertainty increases over Southern Europe.

Overall, our study sheds light on situations where uncertainty of weather forecasts several weeks in advance systematically reduces or increases, underscoring how the practical use of such forecasts can benefit from a deeper understanding of the remote coupling across different atmospheric regions.