Green River flood threat low despite predicted heavy rain this weekend
December 9, 2010 · 4:55 PM
Heavy rain is predicted this weekend from a storm front but the Green River is one of the rivers least likely to flood, according to the National Weather Service.
The National Weather Service has perceived a threat and conducted a Hydrologic Outlook Webinar Thursday afternoon with emergency planning officials, according to a letter from Brian Felczak, Kent deputy emergency manager, to city emergency planning committee supervisors.
The threat is that the current moisture plume is taking on an atmospheric river profile which will produce the wettest system we have seen in the last two years, Felczak said in the letter.
The system will move into the area Saturday evening and last through Monday.
The heaviest rains are expected to occur in the Olympics (8 to 10 inches), the North Cascades (6 to 8 inches) and the Central Cascades (3 to 5 inches). The snow level will be around 3,500 feet rising to 7,000 to 8,000 feet on Sunday. Snowmelt typically adds 10 percent to runoff flows.
While the threat to the Lower Green River is considered low, it is not zero but is not expected to reach extreme values.
No cubic feet per second river flow estimates were given but it is anticipated the King County Flood Warning Center could move into a Phase I this weekend meaning the river flow may go over 5,000 cubic feet per second.
Felczak stated the key points include:
• The National Weather Service will issue a Flood Watch Thursday afternoon for several Western Washington counties. This is not specific for the Green River.
• Kent will see some urban and small stream flooding.
• This is a current weather model and changes are possible.
• Landslides on the steeper slopes are possible due to moist soils.
• King County may issue a My State Alert to residents of the Middle Green River due to some local flooding there. This is not a Code Red notification but some residents may have signed on to their system.
• At this time the King County Emergency Coordination Center is not planning on a Level I activation but will be monitoring all future Hydrologic discussions.
• There is a lag time between the rainfall and the river rising (typically a day).
King County expects minor flooding of the Green River at 7,000 cubic feet per second, moderate flooding at 9,000 CFS and extreme flooding at 12,000 CFS. The river had a flow Thursday afternoon of 1,450 CFS.