The first heavy rainfall event on 28 June was associated with a stalled upper-level trough west of Japan and an amplifying and back-building upper-level ridge east of Japan. The interaction of the upstream upper-level ridge and downstream upper-level trough enabled a stalled surface frontal boundary to drift northwestward toward southwestern Japan beneath confluent flow aloft in the equatorward entrance region of an upper level jet. Precipitable water (PW) values > 60 mm and integrated water vapor transport (IVT) values > 1000 kg m-1 s-1 fueled the heavy rainfall. The second heavy rainfall event occurred on 29–30 June under similar upper-level flow conditions, but with the benefit of additional deep tropical moisture from TC Prapiroon. A third heavy rainfall event on 2–3 July occurred when TC Prapiroon brushed past the southwestern tip of Japan. A persistent low-level southeasterly flow resulted in orographic enhancement of the heavy rainfall in all three events.
The antecedent for the fourth, and final, heavy rainfall event was the formation of a high-amplitude anomalously deep trough over central Russia on 4–5 July that facilitated extensive upper-level ridging downstream over China on 5–6 July and the subsequent formation of a very slow-moving weak trough across Korea and the southwestern Sea of Japan. Deep tropical moisture (PW values > 70 mm and IVT values > 1200 kg m-1 s-1 reached southwestern Japan on 6–7 July along a weak frontal boundary behind TC Prapiroon in the aftermath of the storm’s extratropical transition (ET). These heavy rains were associated with warm-air advection along an elongated northeast–southwest inverted trough axis located near the equatorward entrance region of an unseasonably strong 60+ m s-1 jet stream. This jet stream, located east of northern Japan and south of the Kamchatka Peninsula, was a direct result of the ET of TC Prapiroon and the associated enhanced baroclinicity in the middle and upper troposphere.
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