Atmospheric Rivers of the Pacific Northwest

One feature of climate that the Pacific Northwest is known for is it's precipitation and moisture. This is part in due by the influence of atmospheric rivers over this region. Now categorized like hurricanes, atmospheric rivers range on a scale from 1-5, 1 being a weak atmospheric river that is beneficial for the environment, and a 5 being an exceptional atmospheric river with primarily hazardous outcomes like flooding and/or landslides.

These extreme precipitation events often cause flooding when surface conditions are already saturated. Flooding can negatively impact humans by affecting agriculture, structures including roads and households, as well as negatively impact wastewater runoff.

Atmospheric rivers, occur ahead of a cold front, when a landfalling extratropical cyclone with enhanced water vapor content gets concentrated into long (~2000km) and narrow (~1000km) plumes (Neiman et al. 2011).

Recent multi-year droughts have left reservoirs under average, putting strain on communities within the Pacific Northwest. This winter has observed atmospheric river after atmospheric river which is helping to reach the 10-year average annual rainfall, filling reservoirs across the region, yet this precipitation has been occurring in such extreme events that flooding across the Pacific Northwest has been prevalent this winter. Record rainfall amounts in Vancouver BC caused record floods and shut many roads and highways down temporarily.

Last weeks atmospheric river broke records that haven't been surpassed since 1977 in areas of Washington:

The Pacific Northwest has characteristic precipitation events that drench the area and cumulatively supply about half of the annual precipitation in the western United States (NASA, 2021). Making these events an important contributor to regional water supplies.

Research has also shown that more than 80% of the western U.S. flood damage over the past 40 years has been due to atmospheric rivers. Shown in the NASA figure above is a historical atmospheric river event during January of 2021 when the Pacific Northwest was drenched in several episodes of rainfall. Between January 11-13th of 2021 the northwest corner of Oregon received over 25 cm of rain. These conditions ranked this atmospheric river as a category 5, the strongest that this type of phenomena can rank.

These atmospheric river events can carry as much water vapor as the Mississippi River and can cause intense rainfall as these meandering plumes come in contact with a topographic feature such as the Oregon Coast Range or the Cascade Mountain Range. This orographic forcing of these systems causes the majority of the moisture out of these systems creating extreme weather events during winter months all along the PNW from California to Canada. Satellite imagery has helped scientists understand these systems more and has allowed them to predict their landfall and impact days in advance.

The majority (~66%) of the precipitation in this region occurs during only half of the year (October-March) (U.S. Fish and Wildlife). The majority of precipitation during these winter months is partly due to the influence of atmospheric rivers

Not all atmospheric rivers cause damage. The majority are weak systems that supply beneficial rain or snow that is important to the water supply. Atmospheric rivers are a substantial feature in the global water cycle and in the Western US these systems are closely tied to both flood risks and water supply.

Overall these extreme events are an important feature of the Pacific Northwest's climate and varies on scales from beneficial to hazardous depending on the amount of rainfall that falls during a short period of time. This winter the Pacific Northwest has seen an abundance of rain due to atmospheric rivers. Atmospheric rivers are beneficial in creating snowpack and replenishing water reserves but can quickly become devastating if the ground is already saturated, leading to flooding and landslides. Atmospheric rivers influence the majority of precipitation received during the winter months in this region and contribute to the climate classification of this region.

Sources:

Kathryn Hansen (2021). Potent Atmospheric Rivers Douse the Pacific Northwest. Nasa Earth Observatory. Retrieved October 28, 2021, from: earthobservatory.nasa.gov/images/147822/potent-atmospheric-rivers-douse-the-pacific-northwest

Neiman et al. (2011). Flooding in Western Washington: The Connection to Atmospheric Rivers. The Journal of Hydrometeorology. Retrieved October 14, 2021, from  Flooding in Western Washington: The Connection to Atmospheric Rivers**.: EBSCOhost (oregonstate.edu) 

Neiman et al. (2008b) Meteorological Characteristics and Overland Precipitation Impacts of Atmospheric Rivers Affecting the West Coast of North America Based on Eight Years of SSM/I Satellite Observations. Journal of Hydrometeorology. Retrieved October 14, 2021, from  Meteorological characteristics and Overland Precipitation Impacts of Atmospheric Rivers Characteristics and Overland Precipitation Impacts of Atmospheric Rivers Affecting the West Coast of North America Based on Eight Years of SSM/I Satellite Observations in: Journal of Hydrometeorology Volume 9 Issue 1 (2008) (ametsoc.org) 

U.S. Fish and Wildlife Service (2011). Climate Change in the Pacific Northwest. Climate Change in the Pacific Region. Retrieved October 14, 2021, from  Climate Change in the Pacific Northwest (fws.gov)