After an active weekend, yet another powerful atmospheric river will drench the Pacific Northwest with heavy rainfall, wind and some mountain snow through midweek.
The Pacific storm fueled by the atmospheric river will make its way into the Pacific Northwest in the overnight hours Monday into Tuesday, bringing multiple rounds of heavy rain, gusty winds, and a significant flood risk.
Over 9 million people were under flood alerts Monday, as the region braces for what could be a significant rain event. Rainfall rates across the region of 2 to 3 inches per hour combined with excessive runoff from melting snow will pose a risk for mudslides and avalanches across more mountainous regions.
Recent snowfall totals from weekend storms across the Cascade Mountains range from 6 to 15 inches since Friday. As this next round of rain falls over the existing snowpack, this could increase runoff into area rivers. Rivers to watch for flooding include the Skagit and Snoqualmie, with each forecast to rise into the major flood stage late Tuesday night into Wednesday morning.
Urbanized areas such as Seattle and Portland should monitor travel conditions, due to a heightened risk of urban flash flooding because of already saturated conditions.
Accompanying the risk of excessive rainfall is the chance of strong winds approaching 40 mph in areas west of the Blue Mountains. Wind and rain can make for very poor visibility and frequent travel issues.
Atmospheric river events are a common phenomena for the Pacific Northwest region, as multiple occur during the autumn and winter seasons every year. The active pattern for the Pacific Northwest is expected to continue, with NOAA’s Climate Prediction center forecasting an 80-90% chance of above average precipitation for the month of December for the region. Atmospheric River events produce on average 30-50% of the west coast’s annual precipitation.
