The worst weather to hit Oregon in decades is likely to bring some flooding, but the biggest concern is storm surge.
It could bring up to a foot of water in some areas.
The National Weather Service issued an advisory Monday afternoon, saying the heaviest rain could fall in Portland and surrounding areas.
The worst is likely in the downtown core, but there could be some scattered flooding, the weather service said.
The storm is forecast to strengthen, making the area prone to flash flooding.
The weather service warned residents to be on the lookout for power outages and power outage warnings are in effect.
The storm is expected to weaken and be mild in the area of Portland by the end of the day, but that is not always the case.
The weather service issued a strong tropical storm warning for coastal Oregon starting at 5 p.m. local time, with winds of up to 60 mph.
The forecast track for the storm is for a low pressure center over the Strait of Juan de Fuca off the coast of British Columbia.
The storms could bring severe wind and rain to the Portland area, but it will likely be mild, according to meteorologist Steve McCauley of the Weather Channel.
The strongest storm surge could hit the Portland metro area and the northern parts of the Portland metropolitan area by Monday afternoon and Wednesday morning, according the Weather Service.
It is expected that the storm will bring heavy rain and high winds to some areas, but will also bring flooding.
The winds will be at or above 60 mph and gusts up to 40 mph, the Weather Bureau said.
The Weather Bureau also said there will be a moderate to severe storm surge in the Portland-Vancouver area.
There could be flash flooding in parts of Portland.
The high surf and gale force winds could bring storm surge to the area, the agency said.
If the storm does not weaken or clear by Monday evening, it could bring heavy downpours in the areas where the storm has already made landfall.
The Weather Bureau warned that the maximum storm surge from the storm could reach 6 feet, with potential flooding.