An atmospheric river of moisture will affect southwest Washington and northwest Oregon Wednesday through Thursday as a well organized frontal system slowly moves across the region. Latest model guidance has increased the rainfall potential, with 1 to 2 inches of rain expected for the interior lowlands, and up to 3 inches possible for the coast. The heaviest rain will likely be in the Willapa Hills, as well as the Coast Range and Cascades north of Oregon Highway 22, where 2 to 4 inches of rain are expected and up to 5 inches possible. The heaviest rain appears likely to occur Wednesday night into Thursday morning.
The main concern with this event will be the potential for urban flooding in areas of poor drainage. Officials and property owners should do what they can to clear storm drains of fallen leaves today before the rain arrives tomorrow. Recently burned areas are also of concern, as rainfall rates may be sufficient to cause minor debris flows. While a few small streams may jump their banks due to the heavy rain, early season low flows should keep mainstem rivers from flooding during this event.
After a brief break Thursday night, the next front and atmospheric river are expected arrive later Friday. Latest indications are that rainfall will not be as heavy with this second system, but this may change as details become clearer.
Stay tuned to weather.gov/portland for the latest forecast and warning information. This product will be updated by 4 PM Tuesday.