Propagating Atmospheric Patterns Associated with Midwest Winter Precipitation

For winters over eastern North America, complex Hilbert empirical orthogonal function (HEOF) analysis was used to objectively identify propagating patterns in four atmospheric fields that have potential relevance to precipitation: jet stream–level wind speed, 850-hPa moisture transport (qv), temperature advection (TA), and vorticity advection (VA). A novel phase shift method was used to show the location where each propagating pattern was most correlated with Midwest precipitation, and each of the four phase-shifted HEOF patterns was compared to its respective high-precipitation composite view. The leading HEOFs of the three transport fields (qv, TA, and VA), which collectively represented the dynamics associated with a midlatitude cyclone, accounted for almost half of Midwest precipitation variability and were associated with lake effect snow when propagating downstream from the Midwest. Correlation and spectral analyses revealed how the propagating transport patterns were related to the Pacific–North American pattern and other teleconnections. The leading HEOF of jet stream–level wind speed, which represented the tendency for the jet stream to migrate equatorward over the study region during winter, accounted for only about 4% of Midwest daily precipitation variability. In contrast, the second HEOF of jet stream–level wind speed, which represented an eastward propagating trough dynamically consistent with a midlatitude cyclone, accounted for 16% of Midwest daily precipitation variability.