A flood is defined by the National Weather Service as an overflow of water onto normally dry land. The inundation of a normally dry area caused by rising water in an existing waterway, such as a river, stream, or drainage ditch. Ponding of water at or near the point where the rain fell. Flooding is a longer-term event than flash flooding: it may last days or weeks. Flash Floods are a flood caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours. Flash floods are usually characterized by raging torrents after heavy rains that rip through river beds, urban streets, or mountain canyons sweeping everything before them. They can occur within minutes or a few hours of excessive rainfall. They can also occur even if no rain has fallen, for instance after a levee or dam has failed, or after a sudden release of water by a debris jam.
Clatsop County can also experience coastal flooding when powerful storms, usually in the winter time, produce a surge of seawater along beaches and allows for higher wave run up that normal. In these situations, and particularly when coincident with a high tide, low lying areas near the beaches, and streams that drain to the ocean, can fill with water and cause damage to homes and businesses.
With high annual rainfalls, and mountainous topography and a coastline subject to tides Clatsop County is pre-disposed is a likely place to experience flood events. One of the most notable flood events in recent history occurred in February 1996, Widespread major flooding affected the entire Pacific Northwest and was especially notable in western Oregon and Washington. The flooding was the culmination of a series of unusual weather events:
First, the fall and winter had above-normal precipitation, about 125 percent above normal, although snowpack was below normal. Second, in mid and late January, tremendous amounts of snow fell in mid and high elevations of the Cascades and coastal mountains, and significant snow accumulated even at low elevations. Third, the period of snow was followed by a deep freeze, with freezing rain, frost, and frozen ground. Fourth, the weather pattern changed dramatically in early February, with a strong subtropical jet (“pineapple express”) bringing warm, moist air to the region, which resulted in very heavy rain and rapid snowmelt.
Some river basins had 4-day rainfall totals exceeding 15 inches combined with another 10-15 inches of water equivalent in melted snow. Rivers rose rapidly February 6-9, with smaller creeks and rivers cresting on the 7th and 8th and larger rivers cresting on the 9th and 10th. There were 8 fatalities in Oregon. Several of these were the result of people driving their cars into flooded areas and being swept away. Total damages across the Pacific Northwest exceeded $1 billion.
KNOW THESE TERMS:
Flooding is possible. Tune in to NOAA Weather Radio, commercial radio, or television for information.
Flash Flood Watch:
Flash flooding is possible. Be prepared to move to higher ground; listen to NOAA Weather Radio, commercial radio, or television for information.
Flooding is occurring or will occur soon; if advised to evacuate, do so immediately.
Flash Flood Warning:
A flash flood is occurring; seek higher ground on foot immediately.
VIDEOS & PUBLIC SERVICE ANNOUNCEMENTS
"Turn Around Don't Drown" PSA https://youtu.be/eI6mIlHKrVY
ARTICLES & DOCUMENTS
Flood-- Facts & Information (NatGeo) https://www.nationalgeographic.com/environment/natural-disasters/floods/
"Remembering the Big Storm" https://www.oregonlive.com/oregonianextra/2008/11/remembering_the_big_st...
"The Great Coastal Gale of December 1 - 3, 2007" https://climate.washington.edu/stormking/December2007.html
"Estimates of Twenty-First-Century Flood Risk in the Pacific Northwest Based on Regional Climate Model Simulations" https://journals.ametsoc.org/doi/full/10.1175/JHM-D-13-0137.1
"The Great Coastal Gale of 2007 Brought Neighbors Together" https://www.washingtontimes.com/news/2017/dec/9/the-great-coastal-gale-o...
Floods Ready.gov https://www.ready.gov/floods
National Flood Insurance Program (NFIP) https://www.floodsmart.gov
NOAA National Weather Service Program https://water.weather.gov/ahps2/index.php?wfo=pqr
Sandbagging Resources https://www.co.clatsop.or.us/em/page/sandbagging-resources
Traffic Impacts https://www.co.clatsop.or.us/publicworks/page/traffic-impacts