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Floods and Flash Floods



The week of May 4 - May 10 is Severe Weather Awareness Week in the Pacific Northwest, including the states of Idaho, Oregon and Washington.
This is an excellent time for all individuals, families, businesses, schools, radio and television stations to review their spring and summer storm preparedness plans. It is especially important for new arrivals to the Pacific Northwest to become familiar with NOAA's National Weather Service Watch and Warning definitions, and their safety procedures.

Each day, a new topic will be discussed, along with new informational links:
Intro May 5 May 6 May 7 May 8 May 9 May 10

Would you stop, or go on through this flooded roadway?
flash flood image
Looks not too bad...
(normally dry stream bed)
flash flood image
Hmmm, then again...
Turn Around...Don't Drown!

Flooding and flash flooding are a significant threat to life and property across the Pacific Northwest.

"Flash floods are a significant concern in the region because of our rugged terrain," said Mike Vescio (Meteorologist in Charge - Weather Forecast Office Pendleton, OR). "On June 14, 1903 , the worst natural disaster in Oregon history occurred when a flash flood swept through Heppner destroying the town and claiming 247 lives during the second worst flash flood fatality event in American history."

What is the difference between a flash flood and river flooding? Flash flooding occurs as a result of heavy rains over a short period of time. Typically the flooding takes place on smaller streams and creeks, and has a duration of only a few hours. Conversely, river flooding takes half a day or longer to develop. The duration of this type of flooding is also much longer, and rivers will frequently remain above flood stage for days at a time.

A flash flood refers to a dangerous sudden rise in water along a stream, river, wash, or over a normally dry land area. Flash floods result from heavy rainfall, river ice jams, snowmelt, and dam or levee failures. Flash floods can occur within a few minutes or hours, and can move at surprisingly high speeds, striking with little warning. They can erode an entire mountain side...roll boulders the size of trucks, tear out trees, destroy buildings, wash out roads and bridges and cause loss of lives. Rain weakened soils can also result in mud slides capable of closing interstates.

Here in the Pacific Northwest, river flooding is more commonly a problem during the wet season from November through March. Flash flooding usually occurs more during the Spring and Summer, however it can (and does) occur during any month of the year. For instance, rain falling on heavy snow pack has the potential to cause rapid snowmelt, flooding small streams and even larger rivers. This can occur during both the Winter, or Spring while mountain snow pack is still high.

An additional flooding threat across the Pacific Northwest is flash flooding due to heavy downpours from thunderstorms. FLASH FLOODING from thunderstorms is caused by the rain falling faster than it can be carried away by normal drainage channels. The result is rapid rises in small streams as the large volume of water races downstream.

Flooding and flash flooding occur every year at some location across the Pacific Northwest. Since 1986, the states of Idaho, Oregon, or Washington have averaged over 21 flash flood events per year. Some years there are only a few events, but in a bad year more than 75 such events can occur.

To stay informed about a flash flood listen for:

A Flash Flood Watch...

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

A Flash Flood Warning...

This tells you that flash flooding has been reported or is imminent. When a flash 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 safety. Move to a safe area before access is cut off by flood water.

An Urban and Small Stream Flood Advisory...

This tells you that flooding of small streams...streets and low lying areas such as railroad underpasses and urban storm drains, is occurring, but rainfall is not expected to produce a flash flood situation.

The following basic flash flood safety rules should be observed when you see flooding or hear about a flash flood warning...

Do not camp or park your vehicle along streams and washes, particularly during threatening conditions.

If you are near a river be aware of water levels and be prepared to take action to move to higher ground if river levels rise.

Do not enter areas that are already flooded.

Do not try to cross a flowing stream on foot when the water is at or above your knees.

If walking or fishing along a river, be aware that erosion from swift running water can cause river banks to collapse.

Never let your children play around high water, storm drains, viaducts or arroyos.

Be especially cautious at night when it is harder to recognize flood dangers.

Nearly half of all flash flood fatalities are auto related.  While driving your automobile look out for flooding at highway dips, bridges and low areas.  Two feet of water will carry away most automobiles.  Never attempt to drive over a flooded road.  The road bed may be washed out under the water and you could be stranded or trapped.  If the vehicle stalls, leave it immediately and seek higher ground.  Rising water may engulf the vehicle and sweep it away.

THE BEST ADVICE IF YOU ARE IN A VEHICLE...TURN AROUND, DON'T DROWN.

During flash flood season, and year round, stay abreast of the latest weather information.  You can receive the latest forecasts and immediate notification of warnings on NOAA weather radio.


From the past...
Deadly Flash Flood on June 14, 1903 at Heppner, Oregon [ historical photo ]
This was surely the most deadly natural disaster in Oregon's recorded history. A strong thunderstorm, accompanied by extremely heavy rain and hail, moved near Heppner, Oregon. The storm covered a very small area, probably no more than 50 square miles. Heavy rain fell in a very short time, creating severe flash flooding along Willow Creek, normally a peaceful stream flowing through the town center. The entire town was swept away in just a few short minutes, drowning 247 people. Eyewitnesses say thunderstorm rains arrived as a 40-foot wall of water and the ensuing flood raged through town for over an hour. In all, one-third of the towns' structures were wiped out. The massive runoff of water was a result of heavy rain falling on the barren rocky hills, then flowing into the Willow Creek watershed. Only fifteen minutes separated the first rainwater in Willow Creek at Heppner and the flood crest! There are no rainfall records available for this storm because the weather observing station was completely destroyed, drowning the observer and his entire family.

A similar fate would have been in store for the citizens of Ione, just 20 miles downstream. However, telephoned warnings prompted an immediate evacuation and residents escaped to high ground. At least 150 homes were destroyed at Ione and bodies were washed more than 40 miles downstream to the Columbia River.


    Additional Links of Interest...
  1. Historic Floods of Oregon
  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 Andrew Brown 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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Tel: (503) 261-9246

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