Today was just about the warmest we’ve seen so far in December with temperatures around 50. As the front moves south of us this evening, the rain shuts off, skies partially clear, and colder airmass returns. Expect temperatures down around freezing late tonight and any wet roads will freeze. There’s a good chance some roads dry though before turning icy. Still, be careful during the morning commute!
Tomorrow will be a pleasant, cool, and crisp December day with lots of sunshine and not much wind most of the day. Around sunset, cold modified arctic air will arrive on a gusty northeast wind. This will be the cold airmass that’s been bottled up in Washington the past few days. By Thursday morning the east wind will be blowing hard in Portland and temperatures will be the coldest we’ve seen so far this season…down into the lower 20s! That’s a wind chill in the single digits, something we haven’t experienced in a few years. Assuming we don’t get above 29 Thursday, that will be our coldest day since January 2017.
Thursday will be dry until sometime in the afternoon as a few flurries start to fall. Then as we head into the evening hours the flurries change to ice pellets and finally freezing rain, depending on location in the metro area. By Friday morning the entire metro area and everywhere else west of the Cascades will be a frozen, icy mess. More freezing rain falls through the day Friday and that means we’ll basically be in ice storm conditions all day long Friday. Assume all roads will be frozen the entire day. Travel will be very difficult in the entire I-5 corridor.
Saturday should dawn with the same frozen conditions for just about the entire I-5 corridor and metro area. Freezing rain continues to fall at times Saturday in much of the metro area. But at some point later in the day a warming southerly wind should push all areas except central/east metro above freezing. As the sun sets Christmas Eve, a general thaw should be in progress except in those areas. But many areas could be out of power due to thick ice accumulation in the 48 hours before that time.
Rain (or freezing rain depending on location) will continue at times Saturday night through Sunday. The cold east wind should be mainly gone by Sunday morning, so expect ALL of the metro area to thaw Christmas Day. Temperatures in some areas could approach 60 degrees…it’s a very warm airmass overhead through the weekend.
AT THE COAST
The first area to thaw will be the northern Oregon and southern Washington coastline. Expect only a brief period of freezing rain Thursday evening and night, mainly NORTH of Lincoln City. The southerly wind takes over Friday and temps rise above freezing even way up on the Long Beach Peninsula by midday Friday.
IN THE VALLEY & NORTH TO LONGVIEW
This will be the 2nd area to warm up…most likely not really starting until Saturday. As we’ve seen in past ice/snow events, a very thin layer of cold air will remain down to about Albany/Lebanon for freezing rain off/on Friday. As long as wind remains northerly (through Saturday morning), temperatures won’t warm much. This is how it looks for these areas, including north Clark County to Longview. Most likely frozen from tomorrow night until Saturday morning. Now keep in mind the freezing level Saturday will be up around 7,000′ or so. When the south wind DOES arrive, it’s going to be a dramatic warming! Spots could go from 32 to 55-60 in just an hour or two! Up to ½” is possible from Thursday evening through Saturday morning in the valley, luckily nothing like the 1-2″ in spots during the February 2021 event. I think Christmas Eve (after sunset) should see pretty good travel conditions away from the Portland metro area (both north and south)
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PORTLAND METRO AREA
As always, since this area is most exposed to cold air pouring out of the Columbia River Gorge, it’ll be the last to go above freezing.
– West of the West Hills, and south of about a Downtown to Clackamas line, temperatures should rise above freezing at some point Saturday. It could be a dramatic warming in those places too…32 to 55 in a short period of time! Clark county, away from the Columbia River, should see at least some thawing Saturday too, even if it takes until later in the day. For these areas, it’s possible you’ll be able to drive around comfortably after sunset on Christmas Eve. We’ll see how that works out.
– Downtown east toward the Columbia River Gorge? I have a feeling this part of Portland could stay frozen until Sunday morning, or at least until sometime overnight Saturday into Sunday. The problem is that A LOT of rain will have fallen down into the subfreezing air from Thursday night through Saturday evening. Hopefully it won’t be like December 1996, but I could see a lot of icing with this event, maybe well over ½” or even an inch of icing. This could be a big ice storm for the east metro area with at least an inch of icing and widespread power outages. Merry Christmas…
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COLUMBIA RIVER GORGE
There’s no way the Gorge will see thawing through Christmas Day, so this is a 3+ day long event. Snow at first, and I-84 should “just” be snowy for the first part of Friday. Then ice pellets and finally a change to liquid rain freezing on contact later Friday. From that point through Sunday, it’s all freezing rain in the Gorge. Quite a “Merry No Power Christmas” for some of you!
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WHY IS OUR FORECAST DIFFERENT?
Apparently our forecast looks much different than other forecasts for Friday and Saturday, which is a little unusual. Significantly colder and hanging onto freezing rain through Christmas Eve for at least some of the metro area.
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Usually I’m the one downplaying snow chances with my “warm bias”.
But this is a setup I’ve seen time and time again, even the highest resolution models scouring out the thin cold layer too quickly. Sure, it’s better than 20+ years ago, but still issues. Check out this gem from a blog post in December 2016. It’s almost like Mark from 6 years ago is leaving me a message… “After countless events like this, I should have realized (again) that unless models forecast a significant southerly wind push up the valley, or a decent westerly wind in the Gorge, temps won’t warm up quickly at all” Blog post is here: https://fox12weather.wordpress.com/2016/12/09/cold-air-stuck-in-gorge-ice-snow-continue-tonight/
During that event, which wasn’t even as cold as this one is forecast to be, it only warmed up 2-3 degrees in a 24 hour period. So…we’re going very conservative with the warming Friday through Saturday, and thinking the Gorge stays frozen. Even the ECMWF model has 9 millibars easterly pressure gradient through the Gorge Saturday morning, and still 5 in the afternoon. The cold air will still be blasting into the east metro area on Saturday. Only on Sunday does it briefly go flat, but then returns to 5-6 millibars easterly Monday and Tuesday. I see a long duration ice storm out there with I-84 potentially closed for several days over the Christmas weekend.
That’s it for now…enjoy the dry weather tomorrow!
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