Meadows Blog

Thursday, October 20: Floods and Flash Floods

The week of October 16-22 is Winter Weather Awareness Week in the Pacific Northwest, including the states of Idaho, Oregon and Washington. Each day, a new topic will be discussed, along with new informational links. This is republished with permission from the National Weather Forecast Office in Portland, OR.

During most winters, storms bring long periods of heavy rain and snow to the Pacific Northwest. In addition to extended rain, flooding can also occur due to ice jams, where large flows of ice pile up. This pile-up of ice can act like a dam, causing water to back up and flood. Warm weather can also cause snow on the middle and higher mountains to melt, putting more water into already rain-swollen streams.

Flooding causes more deaths and property damage in the U.S. than any other severe weather related event. The majority of flood related deaths occur when people become trapped in automobiles while attempting to drive through flooded areas. Flowing water can be deceptively strong, and pack a powerful punch. As little as six inches of water is enough to float a small car and carry it away. There have been many floods in the history of the Northwest, which include the devastating floods of December 1964 and February 1996. Most recently, in Dec 2007 and Jan 2009, significant flooding struck the Pacific Northwest, closing a twenty mile stretch of interstate 5 near Chehalis Washington under 10 feet of water. Coastal flooding can also occur during the winter months, and poses a threat to life and property. Winds generated from very strong Pacific storms can drive ocean water inland, much like a storm surge, and can cause significant flooding along the immediate coastal areas and estuaries.

A Flood refers to a gradual rise in the water along a stream or river over an extended period of time. Floods result from heavy rainfall, river ice jams, and snowmelt. They can erode an entire mountainside, roll boulders the size of trucks, tear out trees, destroy buildings, wash out roads and bridges, and cause the loss of lives. Rain weakened soils can also result in mudslides capable of closing major highways.

Flood Watch...
This means that flooding is possible within the watch area. You should remain alert and be ready to evacuate on a moment's notice.

Flood Warning...
This means that flooding has been reported, or is imminent. When a flood warning is issued for your area, act quickly to save yourself. If advised to evacuate, do so immediately. Go to higher ground, or climb to safetly. Move to a safe area before access is cut off by rising flood waters.

Nearly half of all flood fatalities are auto-related. Water that is two feet deep will carry away most automobiles. Never attempt to drive through a flooded roadway. The road bed may be washed out beneath the water, and you could be stranded or trapped. If the vehicle stalls, leave it immediately and seek higher ground.

The best advice if you are in a vehicle: TURN AROUND�Don't DROWN.

From the past...
Record Floods of December 1964 [some historical photos]
The December 1964 rainstorm was undoubtedly the most severe rainstorm to ever occur over central Oregon, and among the most severe over western Oregon since the late 1870s. Several observing stations across central Oregon recorded two-thirds of their normal annual rainfall in just 5 days. Scores of stations set new records for both 24-hour totals and December monthly rainfall totals. Widespread severe flooding occurred, with at least 30 major highway bridges receiving such damage as to make them unuseable! The new John Day multi-million dollar bridge was destroyed as were scores of bridges on county and secondary roads. Hundreds of miles of roads and highways were washed out or badly damaged. Thousands of people had to be evacuated due to ensuing floods.

The Willamette River at downtown Portland had a stage of 29.8 feet. This was a record high for the winter season, and was within inches of the peak stage during the Columbia River spring flood of 1948. Hundreds of homes and other buildings were destroyed and an even greater number were badly damaged. .Heavy snow followed by persistent heavy rains lead to record flooding in Oregon during the later half of December 1964 and January 1965. In all, 17 people died. Virtually every river in the state was far above flood stage and mudslides, bridge failures, and inundation closed the state's roads, airports, and railways. Reservoirs were overwhelmed early on in the storm and many proved unable to release water fast enough to prevent overtopping. Dorena Dam, south of Eugene had water flowing over the top more than 8 feet deep.

Read more information on this and other historic storms in Oregon.

    Additional Links of Interest...
  1. 1996 Flood Summary of Northwest Oregon/SW Washington
  2. Severe Emergency Plan for Inland Pacific NW Schools
  3. Each local office may have photographs online (see office links below)

Remember, in times of severe weather, you can get all these vital NOAA/National Weather Service messages via NOAA Weather Radio, your favorite local media, or through NOAA's National Weather Service websites.

For questions about local Severe Weather Preparedness, contact your local NOAA National Weather Service Office:

local office contact by email contact by phone
Medford Ryan Sandler 541-773-1067
Seattle Ted Buehner 206-526-6087
Spokane Anthony Cavallucci 509-244-6395
Pendleton Dennis Hull 541-276-4493
Portland Tyree Wilde 503-261-9246
Boise Jay Breidenbach 208-334-9861
Pocatello Vern Preston 208-233-0834
Missoula Marty Whitmore 406-329-4840

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