Cumulative Extreme Weather Events and Subsequent High-Impact Flooding in Southern Japan in Early July 2018

Devastating flooding in southwestern Japan on 7–8 July claimed more than 200 lives and was the deadliest flood-related disaster in Japan since 1982. This flooding was the cumulative effect of four heavy rainfall events across southwestern Japan between 28 June and 8 July 2018. The accumulated rainfall from the first three heavy rainfall events saturated soils across much of southwestern Japan. Rainfall rates during the fourth heavy rainfall event on 6–8 July reached 50–100 mm and triggered massive flooding as the saturated soils were unable to absorb the extensive runoff. The purpose of this presentation is to document the meteorological events responsible for the four heavy rainfall episodes, and to provide an overview of the resulting widespread impacts that collectively devastated southwestern Japan.

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.