Strong Atmospheric River Threatens Northwest With Over Six Inches Of Rain, Heightens Flood Risk

Monday’s storm is the latest in a seemingly never-ending train of atmospheric rivers slamming the region, with each increasing the flood risk more than the last. Several inches of rain and feet of snow have fallen since late Friday, but Monday’s event will transport much warmer air and drench even high, snow-covered elevations with rain, increasing the flood threat.

On Monday, flood watches were issued for more than 10 million individuals in the Northwest in anticipation of extended periods of heavy rain affecting areas of Washington, Oregon, Northern California, and Idaho until Wednesday. Northwestern Washington also faced a Level 3 out of 4 moderate risk of excessive rainfall on Monday.

Commencing Monday evening, the most substantial rainfall has been expected along the coastal regions of Washington and Oregon, extending into the Cascade Mountains. Areas of the Cascades that recently experienced snowfall over the weekend could receive over twelve inches of rain from late Monday through Wednesday.

In areas beyond the mountains, precipitation levels in western Washington and western Oregon may reach the range of 3 to 6 inches throughout this week.

The quantity of rainfall expected this week is sufficient to elevate river levels independently. However, the impact of this week's intense precipitation will be heightened due to its occurrence shortly after the region experienced a deluge of rain and snow over the weekend.

During the initial atmospheric river of this multi-day event, parts of Washington, Oregon, and California received over 4 inches of rain from Saturday to Sunday. This significantly diminished the ground's capacity to absorb additional water. Simultaneously, the event brought substantial snowfall to the Cascades and northern Rocky Mountains. Stevens Pass in Washington, for instance, accumulated 40 inches of snow over the weekend, with several other locations in the state recording more than 30 inches.

The rainfall has the effect of melting snow, and the resulting melted snow will subsequently feed into streams and rivers. This process contributes to further increases in river levels and the potential flooding of low-lying areas.

Consequently, certain rivers in western Washington are predicted to reach moderate and major flood stages this week. There is particular concern for the Skagit and Snoqualmie rivers, as both are expected to overflow onto roads, farms, and even some residential areas in certain spots.

With heavy rain pouring on mountains covered in deep snow, another critical hazard raises concerns: avalanches.

Areas of Washington and Oregon had avalanche warnings in place over the weekend. Owing to the expected sustenance of the current weather conditions, there are high chances of more such warnings being issued in the coming days.

Intense rainfall has the potential to induce landslides or debris flows in regions previously affected by wildfires, as the ground's reduced ability to absorb water increases the risk.

As Thursday and Friday approach, the forecast indicates a reduction in storm activity throughout a significant portion of the Northwest. However, there is still a possibility of showers and high-elevation snow lingering.

Forecast computer models are showing preliminary indications of another atmospheric river impacting the region by the early weekend. The precise strength and overall impact of this potential event are expected to become more apparent once the current week's atmospheric river subsides on Wednesday.