PORTLAND, Ore. (KOIN) – All eyes are still on the weekend as we prepare for colder air and a threat for low elevation snow.
Before we get there, we do have some more clouds moving in as a weak area of low pressure brings some rain in on Thursday. Why is this important? It’s not going to have a major impact but it will keep us cooler, instead of a system that could potentially warm us up with mild Pacific moisture like the atmospheric river event.
This is leaving the bar already low for the weekend, which means we won’t have to watch our temperatures drop all that much to get cold.
Right now the visible satellite is displaying the clouds moving in from the north and the high pressure to the south keeping some regions still clear, like California. Clouds continue to increase from the north to the south through the day.
WHAT TO WATCH AS THE WEEKEND NEARS?
Where is the cold air coming from and how cold are we actually going to get? When we were watching the forecast evolve the last seven days, you could see the signs of colder air coming in from Canada. That hasn’t necessarily changed.
For a successful setup in the Willamette Valley, you really need to get the cold air pumping through the Gorge to the Willamette Valley. That cold air needs to then spread through the lowland and settle into the valley floor. The thing about the Willamette Valley, if you get the cold air in, it will be tough to scour out. Unfortunately, it is quite the process to get the cold air in place!
As of Wednesday, fairly temperate marine air is impacting areas of SW Canada, with a large area of low pressure to the north of Alberta. What would be ideal for the Columbia Basin to turn cold would be for polar air to start moving south heading into the weekend. We then turn on that east wind and “open up the freezer door” as they like to say. We likely have a taste of that Sunday through early next week. However, I’m not sold on the temperatures being cold enough for the valley floor to have much action on Sunday. It may be a different story later in the week.
Why? Well notice with this map below that the real cold air actually moves farther east and we don’t necessarily tap into the coldest polar air out of British Columbia or southwest Alberta. We could use cold arctic air to really make an impact. I don’t think the Columbia Basin is going to find cold enough air by Sunday. Once that system moves in on Sunday and the wind shifts, history shows us that we are too warm for snow to the valley floor. We really want that cold purple core to be farther west.
This is a weather model of early afternoon on Sunday. The area of low pressure is moving in around the tip of the Olympic Peninsula and the front is still moving inland. Temperatures likely in the upper 30s at this point of time. The wind pulling mostly from the south, keeping the coldest air out of the valley. If you look east towards the Columbia Plateau, you’ll notice it’s not all that cold there either. At this time, it looks more like a cold rain event for the valley with snow levels above the floor.
WHERE AND WHEN COULD THERE BE SNOW?
SUBJECT TO CHANGE (FORECAST WEDNESDAY 1.20.21)
- Rain in Portland. Potential for a morning mix. Too warm for the afternoon. Moisture falling apart late.
- Rain and snow mix for neighborhoods ~500 feet and above. Light snow accumulation for some grassy areas.
- Snow possible for southwest Washington counties (Clark & Cowlitz). Foothills around 1,000 feet. Potential for accumulating snow.
- Fresh snow for the mountains and ski resorts. Snow levels below the passes. Snow travel Sunday for the passes.
You can see the difference from the morning to the evening where the moisture spreads east leaving snow for those east of the Cascades. We will stick out our classic cold rain event in the valley. Places to watch, may be the hills of the central Gorge.