The seawater level change, wind and rainfall by the low atmospheric pressure or typhoon are the important extemal forces acting in a brackish lake. In order that they may take various transfer types, the influence on a brackish lake is not uniform Then, we classified the low atmospheric pressure and the typhoon according to the transfer course. And we clarified the relation between their transfer types and the flow in Lake Nakaumi. The following results were obtained; (1) On the transfer course, a low atmospheric pressure can classify into two types and a typhoon can classify into four types. The water level change by Type 2 typhoons is characteristic. (2) Since the water exchange accompanied by typhoon passage is greatly influenced by inflow of river water, the water environment changes suddenly by the Type2 typhoon with much precipitation. (3) The change in the density distribution is recovered in about one week On the other hand, since the turbidity distribution is biochemistry-influenced, its influence remains several weeks. (4) It is necessary to accurately evaluate water exchange ability that the low atmospheric pressure and typhoon have in order to grasp flow and water quality change of Lake Nakaumi.