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It's possible temperatures will not get as cold Sunday in Portland as they did January 9, when this photo was taken at Columbia View Wesleyan Church at Northeast Halsey Street and 167th Avenue. Stephanie Yao Long/Staff

UPDATE 11:20 a.m. Threat of flooding; Winter Storm Watch and Flood Watch issued.

There is a 100 percent chance of snow Sunday in Portland with accumulation of up to 6 inches by Sunday night and a foot by Monday morning, according to the National Weather Service.

However, a variety of factors must converge for the substantial amount of precipitation expected Sunday to arrive as snow, said weather service hydrologist Andy Bryant.

But the best forecast at this point calls for snow Sunday night. Lots of it. And continuing into Monday morning up until 10 a.m.

For perspective of what might be in store, it was 52 degrees at 2 p.m. Saturday near downtown Portland. That means the temperature will drop 20 degrees -- as the National Weather Service predicts -- to the freezing point of 32 degrees by Sunday night. Snow mixed with rain could begin earlier Sunday, when the high temperature is expected to be 42.

More perspective: By 2:30 p.m. Saturday, the downtown temperature had dropped 7 degrees in a half hour, from 52 to 45.

That is the difficulty. If it is snow it is a major impact. If rain, little impact. Not much in between here. https://t.co/GZThavKAle — NWS Portland (@NWSPortland) February 4, 2017

Here are the areas where we are most concerned about heavy/wet snow. Keep in mind, this area could shift a bit by tomorrow. Rain elsewhere. pic.twitter.com/Fxvs1oj3rh — NWS Portland (@NWSPortland) February 4, 2017

"Monday morning commute could be ..."

Bryant paused before continuing to speak.

"If we get as much snow as we think we might," he said, "we'd be pretty much at a standstill Monday morning."

In the winter storm watch issued at about 9 a.m. Saturday, the National Weather Service said low pressure "will drape a front across Western Oregon, while low pressure northeast of Hawaii adds increasing moisture to the front (Saturday night) and Sunday ... meanwhile, cold air is expected to undercut the moisture which may cause the rain to change over to snow down to the lowest elevations Sunday.

"The combination of subtropical moisture and arctic cold has the potential to result in a major snow event for portions of southwest Washington and northwest Oregon. However, the details regarding snow levels and amounts remain highly uncertain as even minor changes in the evolution of this pattern can have major consequences for what is experienced on the ground."

On top of that, there could be flooding.

The National Weather Service issued a flood watch Saturday afternoon for parts of Northwest Oregon and Southwest Washington.

"Periods of heavy precipitation will cause sharp rises on several rivers in Northwest Oregon Sunday and Monday," the weather service says in the watch notice. "There is also the potential for some localized flooding in the Portland and Vancouver metro area Sunday, but this depends on how much precipitation falls as rain and how much as snow on Sunday."

Again, while the forecast calls for snow, Bryant said it's possible temperatures will not reach 32 degrees. It's possible that it could be 35 degrees on Sunday with a lot of rain.

"The big question is will it be cold enough at (Willamette) Valley elevations to get snow?" Bryant said.

If the snowstorm indeed arrives, it could provide the boost needed to make Portland's winter of 2016-17 among the snowiest ever.

For Saturday, expect rain, up to a half inch, with a high near 49. Saturday night, additional rain accumulation of up to three quarters of an inch is possible.

--Allan Brettman

503-294-5900

@allanbrettman