spawn_flood_in_region_effect

spawn_flood_in_region_effect = {
	$REGION$ = { save_scope_as = river_region }

	random_county_in_region = {
		region = $REGION$
		limit = { development_level >= 10 }
		alternative_limit = { development_level >= 5 }
		alternative_limit = { development_level >= 2 }
		weight = {
			base = 1
			modifier = {
				any_county_province = { terrain = farmlands }
				add = 1
			}
			modifier = {
				any_county_province = { terrain = plains }
				add = 1
			}
			# Distribution
			modifier = {
				save_temporary_scope_as = county_temp
				any_in_global_list = {
					variable = ongoing_floods
					var:epicenter_county.kingdom ?= scope:county_temp.kingdom
				}
				factor = 0.5
			}
			modifier = {
				save_temporary_scope_as = county_temp
				any_in_global_list = {
					variable = ongoing_floods
					var:epicenter_county.duchy ?= scope:county_temp.duchy
				}
				factor = 0.5
			}
			modifier = {
				save_temporary_scope_as = county_temp
				any_in_global_list = {
					variable = ongoing_floods
					var:epicenter_county ?= scope:county_temp
				}
				factor = 0.1
			}
		}
		spawn_flood_in_county_effect = { COUNTY = this }
	}
}

As written in game/common/scripted_effects/10_dlc_tgp_natural_disaster_scripted_effects.txt.