Monsoons are now increasingly considered seasonal and local migrations of the IntertropicalConvergence Zone (ITCZ). However, the East Asian and North American monsoonsextend well into the subtropics and midlatitudes and do not feature a well-defined crossequatorialflow, thus indicating that this well-adopted theory may be inadequate. Becauseof its importance to humanity, predicting monsoon onset remains a pressing goal of modernmeteorology. From a global space-time diagram of column water vapor (CWV) at 30Nlatitude from daily reanalysis data, we found two longitude-origin of atmospheric rivers(LOARs) that feature locally enhanced CWV and evolve from the Eastern Pacific/Atlanticbasins in the winter to the Western Pacific/Atlantic in the summer. East Asian SummerMonsoon (EASM) onset coincides with the time CWV in the Pacific LOAR first exceeds40 mm, which also typically occurs just before it makes landfall. LOARs exist in 39-year(1979-2017) daily climatological mean CWV, demonstrating the seasonal cycles of these featuresare quasi-stationary and potentially useful for monsoon onset prediction. EASM onsetis particularly predictable following El Niño Southern Oscillation (ENSO), consistently occurringone month after the LOAR crosses the dateline. Simplified GCM experiments anda semi-empirical theory reveal a plausible mechanism of LOAR formation, thus opening anew window into seasonal-scale prediction of subtropical monsoon extensions.The impotance of mid-latitude waves on monsoon precipitation distribution is also observedduring Last Glacial Maximum (LGM, 21ka) climate and leads to moistening of theLGM American Southwest. Here, we compare a synthesis of pollen proxies of precipitationduring Last Glacial Maximum (LGM, 21ka) over North America to constrain the PaleoclimateIntercomparison Model Project (PMIP3) ensemble. In model simulations that comparefavorably with proxy synthesis, we find a strong correlation between monsoon precipitationand meridional wind anomaly, suggesting the patterns are mainly governed by stationarywave forcing. The major perturbations of meridional wind anomaly under LGM consist of topographicforcing due to rising elevation from the Laurentide ice sheets and thermodynamicforcing due to alteration of the mean state from albedo change. Weak stationary waves andwesterlies are generated by models with favorable comparison to the proxy synthesis.