Multiple waves of heavy rain across the Pacific Northwest over the next few days will likely lead to flooding along rivers and possible mud/rock slides. Today, a Moderate Risk of Excessive Rainfall has been issued for parts of northwest Oregon.
An impressive atmospheric river (Pineapple Express) continues to bring widespread heavy rainfall over much of the Pacific Northwest, which could easily result in urban and river flooding through the afternoon and evening hours with 2 to 4 inches of rain possible.
FLOOD WATCHES REMAIN IN EFFECT THROUGH WEDNESDAY MORNING...
A slow moving front with a tap into an atmospheric river of moisture will continue rain through Wednesday. The rain will be heavy at times, especially over the coastal mountains and the Cascades. A front stalled near the northern coasts this morning,
and the highest rainfall totals have been observed in the Willapa Hills and north Oregon coast range where around 4 inches of rain has fallen in the last 24 hours. The area of heavy rain will move south and east through the day. Rivers and small streams are rising in response to the heavy rain, and many will rise close to or above flood levels today and Wednesday. The heavy rain may also result in urban flooding today,
especially in low lying areas with poor drainage. Saturated soils may also lead to land slides.
A sustained plume of deep subtropical moisture from the central Pacific, as depicted by recent GOES water vapor imagery, continues to be advected towards the Pacific Northwest coast today as a cold front slowly approaches the region.
The region remains downwind of an intense 250mb jet streak promoting upper level divergence aloft. There is a large area of PWs in excess of 1 inch offshore, with values as high as 1.5 inches in the core of the warm air advection.
Precipitation will fall entirely as rain outside of the mountains as the temperatures will remain sufficiently above freezing. By the end of this event, coastal areas with favorable topography could be measuring 8+ inches of rain. The longevity of rainfall rates of roughly 0.4-0.7 inches per hour in mountainous terrain has potentially significant implications on hydrologic impacts, including big water rises on area creeks and streams.
Much of this region has already endured well above average rainfall over the past few weeks, particularly across western Washington and northwest Oregon with rainfall at
least 200% of normal for some areas. Due to over-saturated soils, there is also a heightened risk for mudslides near steep terrain.
Rainfall over the Pacific NW over the last two weeks.