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Using observational data and model sensitivity test, the present study diagnosed the evolution of an upper-level trough over 2-7 May 1998 during the mei-yu season in East Asia, with a focus on the role of latent heating downstream from the trough on the cutoff process and the formation of the cold-core low (CCL). Compared with conceptual models based on dry dynamics, the formation process of this cutoff low (COL) was inconsistent with the model of migratory jet streak-trough interaction, but the evolution of the upper-level trough (including its narrowing and rotation of axis) and the scale of the resultant COL generally agreed with the theory of interaction between trough and mean westerly flow with anticyclonic background shear. East/southeast of the trough, active and persistent convection appeared and induced 1) divergent ageostrophic flow near the tropopause, 2) an increase in mid- to upper-level thickness due to effective heating, and 3) amplification of an upper-level ridge downstream. Through a model sensitivity test, the diabatic heating is suggested to play a key role in the formation of the current CCL as the trough moved and extended southeastward to approach the amplifying ridge. Because of the east/southeast ageostrophic flow, the trough encountered a critical layer and became distorted, and its southern segment eventually detached to form a COL. When latent heating is turned off, the model produced a trough evolution anticipated by the dry theory but without cutoff. Such a role played by the moist process downstream of the trough during cutoff in the mei-yu season has not been previously noted in the literature. [ABSTRACT FROM AUTHOR]