King County and cities in the Green River Valley continue to prepare for possible flooding due to the weakened condition of Howard Hanson Dam on the upper Green River.

Thousands of supersized sandbags that stand 4 feet tall and weigh about 5,000 are being placed along the Green River the much of the length of the Valley.

Once in place, the bags are pushed together tightly, then covered with black plastic to keep them from weathering.

Crews were placing the big bags Friday near Costco in Tukwila.

To prepare for the worst, King County has created a Valley-wide map that shows evacuation routes in case of major flooding.

Installation of signs designating the routes will be installed starting on Monday. County crews will install 27 blue and white signs with directional arrows along 11 roads in unincorporated areas.

King County, along with the cities of Tukwila and Renton, will use the same directional sign, while motorists in Auburn will be instructed to follow evacuation routes previously established to flee eruptions from Mount Rainier.

The city of Kent will use slightly different signage that can be used for all evacuations.

A map of roads that have been identified as evacuation routes in the Green River Valley will be posted online beginning Monday.

In addition to identifying evacuation routes, contingency planning has also been underway to identify Metro Transit impacts in the Green River Valley, according to county officials.