Some cold spells can be predicted better than others even three weeks ahead

Forecast models sometimes predict cold spells across Northern Eurasia three to four weeks ahead, however, some of these forecasts can be inaccurate. A recent study revealed that forecasts are more likely to be correct when high pressure systems are already present over Scandinavia or the Northern Atlantic, and the weather in Northern Eurasia is already cold. On the other hand, cold spell forecasts are more likely to be incorrect, when strong convective clouds develop over the tropical Indian ocean. 

Tropical and stratospheric phenomena affect European winter forecasts 

While forecast models often do not predict any clear signal for three to four weeks ahead, windows of forecast opportunity occasionally emerge. These opportunities arise when local weather is influenced by remote regions with more predictable weather conditions. During winter, forecast opportunities are mostly associated with certain tropical and stratospheric phenomena. The stratosphere, a part of the atmosphere located 15–50 kilometres above the ground, plays a significant role in these predictions. 

The researchers know that when the stratospheric polar vortex is weakening, there is a higher chance for cold spells to develop. However, when they divided all the cold spell forecasts made during several previous winters into those that were correct and those that failed, they found out that the models predict cold spells after a weak polar vortex too often, resulting in incorrect forecasts.  

The tropical phenomenon called Madden-Jullian oscillation (MJO) can also affect weather in Europe and sometimes lead to cold spells. MJO is a phenomenon characterized by an eastward propagation of convective cloud and rain systems from the equatorial Indian ocean towards the equatorial Pacific. The researchers found that while the development of convective clouds over the eastern Indian Ocean can often result in failed European cold spell forecasts, the forecasts made when these clouds develop over the western tropical Pacific are more likely to be correct.  

The findings will help to improve subseasonal forecasts and they can also be used to improve forecast models. The research is based on forecasts by five different forecast models and reanalysis data for the winter seasons 1993-2020. 

Additional information:  

Researcher Irina Statnaia, Finnish Meteorological Institute, irina.statnaya@fmi.fi 

The research article is available from: Quarterly Journal of Royal Meteorological Society.  

Statnaia, I., Karpechko, A. (2024). Factors influencing subseasonal predictability of northern Eurasian cold spells, Quarterly Journal of the Royal Meteorological Society, https://doi.org/10.1002/qj.4744