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By using the data of conventional observation, NCEP/NCAR 1°×1° reanalysis, and radar echo, a persistent heavy rain occurred on the southern coast of the Shandong Peninsula on 21 September 2012 is diagnosed and analyzed. The results show: (1) The heavy rainfall was affected by the westerly trough at 500 hPa when the first westerly trough passed and the second westerly trough was moving toward the area. Beneath 850 hPa was the southern winds toward the shore, and the wind speed increased with time so that south wind jet was formed below 925 hPa, which transported water vapor and energy and caused water vapor convergence, increased humidity and convective instability energy. The moisture and instability condition of producing heavy rainfall were smaller than those in inland. (2) The mesoscale vortex and convergence ascending motion were formed on the left side of the shoreward wind ultralowlevel jet. The effect of seaboard topographic lifting impelled ascending movement to increase, which triggered the instability energy releasing, leading to strong rainfall. (3) In radar echoes, small scale convective cells grew along the coastline toward southwest, and bowshaped echoes formed along the coast near Rizhao in the later period, and moved toward the southeast sea area.