**3. Synoptic Environment during This Epic Rainfall Event**

From 27 to 31 August 2018, a monsoon depression evolved, moving west from the East China Sea. The resultant precipitation system was moving from west to east along the coast, causing heavy precipitation nearly over the whole Guangdong province, especially in the south and southeast parts of Guangdong on 29 and 30 August. The synoptic evolution of this precipitation system was analyzed using the NCEP-FNL data at 12:00 UTC 29 August and 12:00 UTC 30 August (Figure 2). Additionally, Figure 3 shows the stratification curve and convective available potential energy (CAPE) based on the sounding data at QY and ST stations.

**Figure 2.** Geopotential height (gpm, solid blue lines at intervals of 10 gpm), winds (m s<sup>−</sup>1, black vector arrows, vector scale at upper right), and vorticity (shaded, <sup>×</sup>10−<sup>4</sup> s<sup>−</sup>1) at 500 hPa (**a**) at 12:00 UTC 29 August, and (**b**) at 12:00 UTC 30 August. The red (blue) shadings in panels (**a**,**b**) indicate positive (negative) vorticity. Geopotential height (gpm, solid blue lines at intervals of 10 gpm), winds (m s<sup>−</sup>1, black vector arrows, vector scale at upper right), and the temperature advection (shaded, <sup>×</sup>10−<sup>4</sup> ◦C s<sup>−</sup>1) at 850 hPa (**c**) at 12:00 UTC 29 August, and (**d**) at 12:00 UTC 30 August. The red (blue) shadings in panels (**c**) and (**d**) indicate warm (cold) temperature advection. The 1000–300 hPa vertically integrated water vapor transport (IVT, shaded, kg m<sup>−</sup>1s−1, also in black vector arrows, vector scale at upper right) (**e**) at 12:00 UTC 29 August, and (**f**) at 12:00 UTC 30 August. The dashed red rectangle indicates the area of the southeast of Guangdong province.

**Figure 3.** The skew T-log10P sounding profiles at (**a**,**b**) QY station and (**c**,**d**) ST station at 1200 UTC 29 August (left) and 1200 UTC 30 August (right). The red dashed line and black and blue solid lines represent the stratification curve, temperature profile, and dew point profile, respectively.

It can be seen that the entire region of southern China was under the influence of a monsoon depression across the mid- and lower-troposphere on 29 August. The depression moved west, bringing strong moistures transported to Guangdong province driven by the persistent south and southwest wind (Figure 2a,b). In the coastal areas of Guangdong, warm advection from the ground and lower troposphere continued to transport warm air to inland areas (Figure 2c,d). Due to the combined evolution of depression and topography lifting effect, the precipitation system was further strengthened on 30 August. The wind and temperature changes between Guangdong and the adjacent South China Sea were more violent (Figure 2c,d). Meanwhile, the vertically integrated vapor transport (IVT) over the coastal zone was much stronger on 30 August than that on 29 August (Figure 2e,f).

Figure 3a illustrates the sounding curves at 12:00 UTC 29 August at QY station. The large convective available potential energy (CAPE, <sup>≈</sup>860 J kg−1) and low level of free convection (LFC, <sup>≈</sup>980 hPa) suggested that the atmospheric environment was favorable for the development of convection [37]. At 20:00 UTC 30 August, the CAPE became larger and LFC became lower, indicating that the convection had further developed. The soundings at ST station also showed a conducive environment for convection development though the CAPE decreased slightly from the 29th to 30th.
