Using a global space‐time diagram of column water vapor (CWV) at 30° N latitude from daily reanalysis data, we find two quasi‐stationary parts of atmospheric rivers (QSARs) that feature locally enhanced CWV and evolve from the Eastern Pacific/Atlantic basins in the winter to the Western Pacific/Atlantic in the summer. East Asian Summer Monsoon onset coincides with the time CWV in the Pacific QSAR first exceeds 40 mm, which also typically occurs just before it makes landfall. QSARs exist in 39‐year (1979–2017) daily climatological CWV, demonstrating the seasonal cycles of these features are quasi‐stationary and potentially useful for monsoon onset prediction. East Asian Summer Monsoon onset is particularly predictable following El Niño‐Southern Oscillation, consistently occurring 25–40 days after the QSAR crosses the dateline. Analysis of local wave activity reveals QSARs as fronts of wave breakings, thus opening a new window into dynamics of subtropical monsoon extensions.