Linking storm track activity and subseasonal forecast uncertainty in the North Pacific and North Atlantic

Midlatitude synoptic storms are a major source of forecast uncertainty, yet the mechanisms linking storm track variability, ensemble spread and extreme event predictability remain insufficiently understood. Differences in storm track intensity between the Pacific and the Atlantic sectors span multiple timescales, from daily to seasonal and decadal, and have been linked to basin characteristics, strength of the subtropical jet, the latitudinal position of the jet stream, and to the changes in the cyclone life-cycles. Yet, the relevance of these dynamical processes for forecast uncertainty remains unclear.

This study investigates how the occurrence and intensity of synoptic storms modulate ensemble forecast spread related to the storm track in the North Pacific and North Atlantic, with the goal of identifying sources of forecast uncertainty for midlatitude weather at subseasonal lead times (2–6 weeks lead time). We use ECMWF reforecasts from the Subseasonal to Seasonal (S2S) Prediction Project database, verified against EAR5 reanalysis,  to investigate ensemble forecast distributions of the upper-troposphere westerly jet and various storm activity metrics based on eddy kinetic energy (EKE). Our analysis involves a systematic assessment of spread-mean relationships for EKE and other dynamical variables for different midlatitude regions and lead times. 

A key result is a robust linear, positive relationship between ensemble mean and spread of EKE in the Atlantic sector, suggesting the contribution of synoptic-scale storms for reliable forecasts. Furthermore, this spread-mean coupling implies that periods of enhanced storm activity, such as wintertime storminess over the North Atlantic, are associated with systematically larger EKE ensemble spread. In contrast, in the Pacific the relationship seems inherently different, indicating a non-trivial role of synoptic storms in forming forecast uncertainty. Large-scale variation of the storm track - such as those associated with teleconnections - are expected to modulate ensemble spread and thereby induce flow-dependent variations in predictability.

The findings highlight the relevance of storm track diagnostics and EKE-based spread metrics as promising tools to improve forecast accuracy and enhance early warning capabilities for high-impact midlatitude storms.