**3. Study area and its characteristics**

*Rainfall - Extremes, Distribution and Properties*

Sea. In early winter the Mediterranean-borne maritime air increasingly disturbs the monsoonal air movement and displaces it in the low altitudes. These maritime depressions draw the tropical continental air masses into the warm sectors, and extreme weather conditions occur that are associated with the passage of a hot sector. Both (2) and (3) move toward the east and prevail in the winter season. During this season, the western region, particularly the coastal area, is characterized by its relatively low rate of rainfall [38], whereas, due to the topographic effects, the highlands receive a considerable amount of rainfall. In spring, the impact of the Mediterranean air movement diminishes, whereas the monsoon from the south takes its place, penetrating the southern part of the Kingdom. During summer, the cyclonic flow sweeps along the Mediterranean Sea from the west toward the east and continues moving over the northern and central regions of the country preventing the maritime air masses of the northeasterly monsoon from penetrating the north regions of the Kingdom. Due to

this, the summer season will be somewhat dry in the area considered.

and in the west, the valleys are steep and flowing towards the Red Sea.

The mean annual rainfall (from 1960 to 2018) over the KSA can be shown in **Figure 2**. The rainfall rate in the KSA (except for the Empty Quarter desert) is 109 mm per year. The southwestern region is characterized by a heavy rainfall compared with the rest of the Kingdom. Rainfall is more than 500 mm per year in some locations. The peaks of the Sarawat Mountains, which extend from the northern part of the Kingdom to the south, parallel to the Red Sea, are the dividing line for the distribution of surface water. The rains that fall in the east of this line take place in the valleys heading east,

Recently, there is a definite increasing trend in rainfall in frequency and intensity in the KSA. Many areas in the KSA have shown an increasing trend in the annual rainfall and flood events. Most of the rainfall in the KSA occurs during the monsoon. Monsoon is a term derived from the Arabic word "mausim," meaning season. It was used to describe the seasonal winds of the Arabian Sea. The Jeddah area is characterized by wet and dry seasons that generally occur from November to May and from June to October, respectively. One example is Jeddah city which has shown an increasing trend in the rainfall events. These events cause disasters that result in human, property, and economic loss.

*Average annual rainfall values for 270 rainfall stations distributed across the KSA area (from 1960 to 2018).*

**20**

**Figure 2.**

The study area, the Jeddah area, is located in the western region of the KSA covering an area of ~1731 km2 and lies between latitudes 21°15′ and 21°57′N and

**Figure 3.** *Study area and its meteorology station locations.*

longitudes 39°06′ and 39°31′E (**Figure 3**). From the latitude point of view, the Jeddah area is considered to be in the arid zone (Köppen's climate classification). The Jeddah area has different geomorphological features. It represents a part of the Red Sea coastal zone. Jeddah drainage system comes from the east (a series of hills) toward the city which is located to the west. The catchment areas have a variety of landforms such as low- and midsize hills and flattened foothills in some places followed to the west by the floodplain areas. The monthly average relative humidity ranges between ~85% from September to October and ~34% from April to June. The Jeddah area is characterized by scarce rainfalls with high variability, which occur most often in November–December–January (the winter season). Additionally, monsoonal rainstorms take place in March and April due to the moist air currents from the Indian Ocean and the Arabian Sea. The average annual precipitation is ~52.5 mm/year. The maximum rainfall was recorded in 1996, with ~284 mm/year. Recently these thunderstorms increased in their frequencies and intensities causing flash floods and inundation events within Jeddah city.
