Reliable subseasonal prediction of extratropical intraseasonal variability remains a major challenge, despite its importance for Eurasian high-impact weather. Here we evaluate real-time forecasts of three dominant zonally propagating extratropical intraseasonal oscillation (EISO) modes in the upgraded IAP-CAS subseasonal-to-seasonal (S2S) prediction system from 21 August 2024 to 30 April 2026. Compared with major operational S2S models, the upgraded IAP-CAS system reaches prediction day limits of 12, 14, and 19 days for EISO-SJE, EISO-PJE, and EISO-PJW, respectively, within 0–1 day of the leading system, and achieves mean real-time prediction scores of 74.8, 76.9, and 78.6. Process diagnostics show that this improvement is associated with reduced upper-tropospheric wind and geopotential-height errors along the subtropical and polar-front jet waveguides, particularly near the jet entrance and exit sectors, indicating a more accurate large-scale circulation environment for EISO propagation. The revised second-order exact sampling strategy also alleviates ensemble underdispersion and improves spread-error consistency in EISO phase space. Composite analyses further show that the upgraded system better captures the propagation properties and spatial structures of EISO-SJE, EISO-PJE, and EISO-PJW, although amplitude damping persists at subseasonal leads. These results identify a physically grounded pathway for improving real-time extratropical S2S prediction through targeted initialization and ensemble-design upgrades.