Some of us in the metro area will see our first snow of 2018 tonight. It’ll happen under the cover of darkness though. Most of us will be sleeping peacefully through the white stuff and wake up to cool rain showers Wednesday morning.
So what’s up with this possible “surprise snow”? A weak system is approaching the Pacific Northwest this evening. Obviously it’s too warm to snow right now, and I DO NOT expect freezing overnight. BUT, we have a very dry airmass over us. When the air is this dry, temperatures drop quickly. Evaporating precipitation removes heat from the air; it’s called evaporational cooling. Model soundings and cross-sections show that cooling and generate snow in some areas. Take a look at our RPM forecast at 2am:
It goes for at least a dusting above 500 feet or so around Cornelius Pass and north, plus much of Clark County. Also the foothills east of Happy Valley, Gresham, and Oregon City could see at least a dusting. So keep in mind many of these areas would wake up to some white covering the ground, but again, I think it won’t affect your AM commute unless you live up above 1,000′ in those areas. The WRF-GFS from UW shows a similar setup with those same areas getting some snow overnight.
After 3-4am a stronger onshore flow has kicked in west of the Cascades lifting the sticking snow level up around 2,000 feet or above.
Let’s move on to our next threat of lowland snow…that would be Sunday.
HIGHLIGHTS FOR THIS WEEKEND AND BEYOND
- There is no sign of a widespread snow event in the lowlands through at least Sunday/Monday next week
- But snow showers (even to the lowest elevations) are possible at any time from late Sunday through Monday
- Don’t alter any of your plans for Sunday-Tuesday next week for now. This could easily end up being a “dusting on the barkdust” sort of event for many of us.
- The coldest airmass we’ve seen since Christmas moves into northern Oregon and southern Washington Sunday night and Monday
Models are in great agreement that we’ll see a cold upper-level trough digging south out of Western Canada Saturday & Sunday. Initially that means lots of cold showers and mountain snow…that’s through Sunday morning/midday. During this time we’ll have strong onshore flow (southwest wind) which will keep snow levels way up around 1,500′ or higher. So there is little chance for low-elevation snow through at least Sunday morning/midday. 850mb temps are a little too warm as well, ranging from -3 on Saturday to -6 or so midday Sunday. Typically this time of year you want to see at least -6 for low elevation snow at night, and -7 to -8 to see it during the daytime.
By Sunday evening and then continuing through at least Tuesday, it appears the airmass over NW Oregon and SW Washington will be cold enough to support the possibility of snow. 850mb temperatures drop to around -8 to -9 on both the GFS/ECMWF. The operational ECMWF was even a bit colder, although its ensembles are more reasonable. That said, the ECMWF model totally dries us out Sunday night and doesn’t really generate any lowland snow.
The GFS is similar, with very little at the lowest elevations. This is because there just isn’t all that much moisture. That GFS only produces 0.15″ precipitation over us from Sunday afternoon through Monday. So moisture availability appears to be the main issue if you are looking for snow days either Monday or Tuesday. That’s because the upper-level ridge is so close to us…the systems coming down from the north & northwest are typically quite dry.
Just beyond our 7 Day forecast, the ECMWF model shows a wave of moisture moving inland next Tuesday evening and Wednesday. There is no cold Gorge wind so the flow quickly turns onshore; warming us up well above freezing by Wednesday. So taken at face value, there is no obvious “great setup for snow” in the lowlands the next 7-8 days…just a very cool period beginning Sunday.
A series of these cool/cold systems will drop down over the Northwest beyond next Tuesday…we’re into a cool pattern the rest of the month!
I’ll be keeping a close eye on things of course the next few days. There are always last-minute twists and turns in these marginal snow situations.
Chief Meteorologist Mark Nelsen