An afternoon heavy rainfall event in northern Taiwan, observed on June 20, 2000 during the monsoon break, is investigated using surface observation and Doppler radar data and a nonhydrostatic model with a horizontal grid spacing of 1.33 km. Heavy rainfall was brought majorly by two precipitation systems, namely A and B. System A was initiated and developed in Taipei Basin, associated with a local wind convergence line. System B was formed on the western slopes south of Taipei Basin, extended northward of the Taipei Basin, and lasted for 4 h. The formation and maintenance mechanisms of the two systems are examined using sensitivity experiments in which the terrain of Central Mountain Range (CMR) or Yangmin Mountain (YM) located north of the Taipei Basin is removed or reduced. In addition, the effect of cold outflow from System A on System B is examined. The wind convergence line responsible for forming and maintaining System A is brought from the interaction between the prevailing southwesterly winds and the easterly/northeasterly flow caused by the terrain effect of CMR. Furthermore, the northeasterly onshore flow and the low-level northwesterly/northerly flow resulting from the prevailing southwesterly winds turning southward over southern YM, enhance System A inside Taipei Basin. System B is produced over relatively high sloped areas (elevations exceeding 200 m) in southwestern Taipei Basin by the prevailing southwesterly winds. Over relatively low lands (elevations less than 200 m), the cold outflow from System A inside Taipei Basin converges with the prevailing southwesterly winds and helps the maintenance of System B.