An observational diagnostic analysis is performed to study variations in the behavior of upper tropospheric Rossby waves prior to intraseasonal variability in the North Pacific storm track. The research is motivated from considerations of the midwinter suppression of the North Pacific storm track. During midwinter strong upper tropospheric cyclone events in the North Pacific storm track are preceded 7 days by a large-scale cyclonic circulation anomaly located over Siberia. The Siberian precursor anomaly is found to be strongly linked to one of the primary modes of upper tropospheric intraseasonal variability over Asia. More generally, this Asian mode is determined to provide a substantial modulation in the structure and amplitude of the North Pacific storm track at lead times of +5 to +8 days. This downstream influence is concentrated during midwinter and is likely related to intraseasonal variations in the East Asian winter monsoon.