A powerful atmospheric river is set to make landfall in the Pacific Northwest and British Columbia late Thursday, Aug. 14, bringing heavy rain — and even light snow — to Washington and surrounding areas.
While the storm is not expected to produce record-breaking totals like some past atmospheric river events, it is still notable for its impressive early-season timing, as these systems normally peak in late fall to early winter, according to Fox Weather meteorologist Scott Sistek.
Combined with the substantial amount of moisture carried by atmospheric rivers, this raises the risk of localized flooding, particularly in areas near recent burn scars that are more vulnerable to runoff.
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What is an atmospheric river?
On the West Coast, atmospheric rivers approach from the Pacific Ocean, carrying tremendous amounts of water. They occur most often during certain weather patterns, including El Niño, which can increase their frequency or intensity, though they can occur in other years as well.
These long, narrow corridors of air have been responsible for as much as 65% of the West’s extreme rain and snow events, USA TODAY formerly reported.
According to the National Oceanic and Atmospheric Administration, an atmospheric river — described as a “river in the sky” — transports water vapor outside of the tropics. While they vary in size and strength, the average system carries an amount of water vapor equivalent to the average flow of the Mississippi River at its mouth.
Past atmospheric rivers have caused widespread impacts. For example, earlier this year, two systems drenched Southern California, triggering severe flooding, tornadoes, avalanches and mudslides. Cities including San Francisco and Redding received up to a month’s worth of rain, prompting evacuations and rescues from southern Santa Barbara to Orange County.
When will the rain hit?
Rain is likely to begin Thursday evening, Aug. 14, with the heaviest rainfall expected late Friday, Aug. 15, into early Saturday, Aug. 16. The storms should start to taper off by midday Saturday, lingering as scattered showers through Sunday afternoon, Aug. 17.
How much rain is in the forecast for Washington?
Over the next five days through Tuesday, Aug. 19, rainfall will vary widely across Washington, with the heaviest totals in the mountains and along the coast.
Cascade Mountains: Significant accumulation of 2-6 inches, with localized higher amounts along western slopes. Higher elevations could also see snow or rain-snow mixes.
Western Washington and Coastal Areas: 1.5–2.5 inches, especially on the western slopes of the Olympic Mountains.
Seattle / Puget Sound: Approximately 0.75–1.25 inches, with northern areas lighter due to the rain shadow effect.
Eastern Washington: Mostly light rain, 0.1–0.5 inches, though higher terrain may reach 0.75 inches.
The atmospheric river will also impact Oregon, though rainfall is lighter there. The greatest totals are centered in southern and coastal British Columbia, near Vancouver and the Lower Mainland — marking the first significant rainfall in weeks for the region, which is experiencing a growing number of wildfires in what is now the second-worst wildfire season on record.
Seattle weather radar
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Brandi D. Addison covers weather across the United States as the Weather Connect Reporter for the USA TODAY Network. She can be reached at [email protected].
This article originally appeared on Kitsap Sun: What is an atmospheric river? Several inches of rain to hit Washington
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