LOS ANGELES, Aug. 1 (Xinhua) -- The U.S. National Weather Service (NWS) issued weather alerts to Hawaii on Thursday, as two tropical cyclones are passing south of the archipelago back-to-back over the next few days.

The NWS issued a Tropical Storm Warning at 5:39 p.m. local time (0339 GMT), downgraded from a Hurricane Warning posted four hours earlier.

Tropical Storm Erick moved westward into the southeastern waters of Hawaii late Thursday. It gradually weakened from a hurricane before passing within about 200 miles (about 321 km) south of the Big Island, Hawaii.

Tropical Storm Flossie, also on a westward itinerary, will enter the east Hawaiian waters Sunday, the NWS reported.

The agency forecasted that Flossie would weaken too. However, both storms would enhance rainfall and churn up high surf in the Hawaii archipelago.

A flash flood watch was issued for the Big Island from Thursday to Saturday. A high surf warning is also in effect for east-facing shores of the island, some of which could see waves up to 20 feet high (about 6 meters), making "entering the water very hazardous," said the NWS.

Parts of Maui island and Big Island could encounter up to 30 mph (48 kph) of winds and 45 mph (72 kph) of gusts.

According to the NWS, tropical cyclone activity in the eastern part of the North Pacific Ocean has been below average so far in 2019.