Thanksgiving weekend is wrapping up and now it’s on to the last few days of November and then December. Right on cue, a colder airmass is arriving today with cold showers. I’ve seen a mix of hail/rain at home.
Snow hasn’t been too heavy in the Cascades so far, but intensity picks up a lot tonight through midday Monday. Cascades are the easy part of the forecast. Expect another 10-15″ at pass elevations (a bit more higher up) tonight through Monday. Add in a colder airmass and that means tomorrow morning you cold have 20+ miles of snow to drive through. That’s much tougher than just a few miles over the summits. Snow will stick down to around 1,000′ in the Cascade foothills tonight; I’m putting out my “snow board” for the season, anticipating a brief dusting here at home…maybe. This plus another 12-24″ Tuesday night through Thursday means ski areas should all be able to open this coming weekend with many feet on the ground!
Highlights for the lowlands
If you have a short attention span, or in a hurry, this is what I’m thinking over the next week or so…
1) Get used to lows/highs in the 30s/40s the next 7-10 days. Not really that cold, but these are mid-winter temps; a bit cooler than average for this time of year
2) There’s NO SIGN of an “arctic blast” west of the Cascades for at least the next week. No “pipe-busting” cold or anything colder than what we’ve seen so far this season. There’s also no sign of any significant cold east wind through the Gorge this week; typically that would help us get something frozen (snow or freezing rain). Not this week.
3) But, we have a few “close calls” with low elevation snow over the next week. Mainly late tonight, Tuesday evening/night (for some), and Wednesday night/Thursday AM. There’s a good chance that none of these produce measurable snow in the vast majority of the I-5 corridor from Clark county down to Eugene
4) The issue is that when it IS just barely cold enough for snow to stick to sea level this week, the precipitation is mainly gone. The heavier precipitation periods will be a bit “warmer”.
4) That also means that at this point I don’t see snow impacting your life Monday, Tuesday daytime, Wednesday, or Friday through Sunday in the lowest elevations. That could change, but it’s what I’m seeing right now
We are under the influence of a chilly northwest flow in the upper-atmosphere. A cold upper trough is passing through tonight, with 850mb temps down around -5 to -6 and 1000-500mb thicknesses around 520-521dm.
Typically, to get snow in the lowest elevations, with onshore flow, we want to see -7 to -8 at 850mb and 520 or lower thickness. So at best the lower elevations with see snow mixed with rain overnight like we saw a few weeks ago…weak sauce. By the time it get’s quite cold (tomorrow evening and tomorrow night), the precipitation is all gone and we stay dry. I notice the freezing level bottoms out around 1,800′ tomorrow midday, so with relatively light showers early tomorrow, I wouldn’t expect sticking snow in the metro area below 1,000′…if it even gets that low.
Then Tuesday night and Wednesday another cold upper-level trough digs down from the north, but this one is headed farther west of us at first…Tuesday here:
That means the really cold (arctic) air is dumping out over the Gulf of Alaska. It does spin up a nice bomb cyclone near the northern tip of Vancouver Island. Looks like a drop from 1012 to 978mb out there from Tuesday AM to Wednesday AM. But a surface low in that position pulls up a southerly wind over us = snow-killer in the lowlands. That said, as the precipitation starts Tuesday evening, it could easily start as snow but not stick out in Columbia county in Oregon and Cowlitz county in Washington. I’ll be watching that closely, but rarely has that pattern produced anything very interesting snow-wise in the metro area. So expect plenty of rain and big mountain snow during this time. Here’s the GRAF model’s snow forecast through Tuesday night. This sure doesn’t say “I’ll be sledding early this week in Portland”.
One more way to look at it is the cross-section from the UW-WRF model. The zero line (32 degrees) is blue. Above that line it’s at/below freezing. I didn’t draw correctly on Tuesday, obviously we are above freezing most of that day. You can see the southerly wind (barb-looking things) pushing freezing level up to around 3,000′ later Tuesday night
By Thursday, that trough is STILL just barely moving onshore.
A lot of the energy has gone south us and into California. That delays the arrival of cooler air again and modifies the cold airmass offshore a bit further. At the surface an elongated band of rain/snow has become almost stationary over the West Coast Thursday
Another gray day with possibly a rain/snow mix in the lowlands, or just rain. Some models HAVE been implying we could get into a situation with heavy precipitation within the frontal boundary changes somewhere in Oregon over to all snow. I’ll be watching for that possibility closely; this model run says that may happen in SW Oregon; notice all the snow down there.
By Saturday, that trough is gone and another is swinging south; offshore like the Tuesday/Wednesday trough
You get the idea…much of the action is happening well west of us over the ocean. This spins up another wet frontal system for a solid period of cold rain sometime Friday through early Saturday, with showers to follow Sunday. 3-4 days ago models were keeping these troughs much closer to the coastline and bringing cold air down directly over us. Another way to see how they adjusted, a chat showing 6 hourly minimum temperatures out in time. Each horizontal line is one run of the ECMWF model. Notice the Thanksgiving morning run (12z 24 Nov) was showing very cold weather later this week with modified arctic air coming in from the north (upper part of circled area). Then in the 6 model runs since that time, ending with the latest, you see we’re back much closer to normal with a forecast for later this week. No arctic air moves south, but maybe some close calls with snow in the hills and mixed in the lowlands.
Maybe a better real world translation is: “3-4 days ago I was worried my wife would have issues getting to work in the early mornings this week. Now I’m not worried at all. Maybe an inch or so sometime in the next few days at 1,000′, but that’s about it”.
That’s it for now, I’ll be at work all week long!