Good evening everybody! I hope ya’ll are having a great Sunday thus far. I know I have. Why, you ask?
It’s been snowing like crazy in the mountains.
Stevens Pass reports 20 inches of new snow over the past 24 hours, while Snoqualmie pass reports 19. All of this has made for some very dicey travel across the passes. Chains were required at many of the passes across both Washington and Oregon this morning. Most resorts around the state are already open for limited operations, but Snoqualmie finally has enough to open Tuesday at Summit West.
Here’s an awesome shot from the Summit At Snoqualmie’s Alpental webcam this morning just before sunrise.
And even though Summit Central is not yet open, people are making the most of the snow over by the Silver Fir chair there!
Why so much snow? Allow me to explain.
We had a strong cold front pass through last night, bringing a healthy dose of rain to the lowlands with it. Unlike most of our fronts, winds aloft were from a more westerly direction instead of southwesterly, and this caused the Olympic rainshadow that normally hangs out by Sequim to shift further south to Seattle. Looking at 24-hour precipitation totals below, many places outside the rainshadow (especially further south near the Washington/Oregon border) picked up close to an inch of rain. Within the rainshadow, places struggled to reach a tenth of an inch.
There was far more precipitation in the mountains, particularly the Central Cascades where the flow aloft was strongest and orographic (terrain-induced) precipitation was heaviest. Take a look at the position of the jet stream this morning over our area. The core of the jet is positioned right over the Central Cascades. Moreover, the flow is perfectly perpendicular to the north-south oriented Cascade Range. Strong, moist flow perpendicular to a mountain range will give you heavy precipitation every time. And if the atmosphere is cold enough, that precipitation will be in the form of snow.
We also had everybody’s favorite local weather feature helping keep snow around a bit longer: the Puget Sound Convergence Zone! The zone forms when onshore flow from the WNW splits around the Olympics (it’s too lazy to go directly over them) and converges back again somewhere in the Puget Sound region. The more northerly the flow, the further south the zone, and vice versa.
This convergence zone has been amazingly persistent and has more-or-less been around the entire day! Check out the picture from everybody’s favorite polar orbiting satellite, the TERRA.
Of course, the zone has kept the snow falling in the Central Cascades, but we’ve even had some flakes in the lowlands! Convergence zones can be quite intense, and intense precipitation cools the atmosphere because (a) evaporation of the precipitation into the surrounding atmosphere and (b) melting from frozen precipitation into rain. Both of these take heat energy out of the atmosphere and cool it. It’s the same process that occurs when you melt an ice cube: even though the temperature of the ice cube does not change, it clearly takes energy to melt it!
To illustrate this, let me show you the observed snow level at Troutdale, Oregon. After the cold front passage around 5 am this morning, the snow level dipped down to 2,000 feet by 7 am. No convergence zone here.
However, the convergence zone has helped deliver the goods to much lower elevations. I’ve gotten reports of non-sticking snow at North Bend (600 feet), Novelty Hill (550 feet), and Federal Way (500 feet). There are many more reports on the Seattle NWS office’s Twitter page. Some places near the center of the zone saw small hail and graupel (snowflakes with a coating of rime ice… they look like “snow pellets”). These are indicative of strong upward motion and an unstable atmosphere.
That’s what happened today. Let’s talk about tomorrow.
But first, we’ll start with a wide view of the current conditions over our area. After all, no sense forecasting the future if you don’t know what is currently happening!
The strong jet stream and cold front that brought us the heavy mountain snows and substantial rain in the lowlands has sagged south, opening up the door for much colder air with arctic origins. The mottled clouds to our northwest are evidence of cool air and an unstable atmosphere. At lower levels, this air has moderated as it has gone over the Pacific, so temperatures are nippy, not frigid. Here are the current temperatures as of 4:30 pm… most lowland locations are in the upper 30s to mid 40s.
We will cool overnight, but the combination of onshore flow and mostly cloudy skies will keep many locations from reaching the freezing mark, particularly near Seattle proper and near bodies of water. Outlying locations and places with a bit more elevation have a better chance at seeing sub-freezing temperatures.
We may see a few isolated showers overnight, but the real action comes around sunrise when a more consolidated band of precipitation comes into the area associated with a weak low pressure system off our coast. This is our first chance at snow.
The models do show some snow with this feature, first arriving at approximately 5 am and ending around 9. Here are the 3 hour accumulations ending 7 am and 10 am from this morning’s WRF-GFS model run by the UW.
There is still a considerable spread in the ensembles for this feature though. Take a look.
Because temperatures are really on the fringe for snow, snow amounts with this feature will vary substantially with elevation. The Seattle National Weather Service is calling for 1-2 inches of wet snow above 500 feet, with less than an inch of snow below 500. Accordingly, they have a snow advisory for much of the region from 4 am to noon. Most of Seattle will see little accumulation aside from the highest hills, but folks on the Eastside with some elevation have a better chance of picking up an inch or two. Overall, this will not be a big event.
After noon, we will warm up to the upper 30s/low 40s and any further accumulations should be minimal. Passing showers could drop a slushy dusting at some of the higher elevations above 500 feet, but any accumulations should melt after the shower ends. Like the convergence zone that occurred today, heavy showers can also locally drop temperatures.
Monday night is when things get interesting. The super-high resolution 1.3 km WRF-GFS model shows our trusty Puget Sound convergence zone developing in the evening and persisting through the night.
The setup with this convergence zone is a little bit different than the one we saw today. As the wind barbs show, winds off the coast will be from the NE, not the NW like we saw today. However, we will still have southerly flow through Puget Sound as winds wrap around the low pressure system that brought showers earlier Monday. At the same time, a very cold air mass will shift southward into British Columbia, allowing frigid air to rush through the Fraser River valley into Western Washington. I expect we will see some snow where the cool southerly winds and cold northerly winds meet.
I am very keen and even a bit concerned about this event for several reasons. First, because this band of snow will be quite intense and temperatures will be cooler tomorrow than they were today, snow could fall down to sea level. Second, the current timing on this feature is the evening commute, though that could very well change. Third, with road surfaces still relatively warm, any snow that falls will turn to slush on the roads, but after this convergence zone comes through, temperatures will plummet and we could see some serious icing on the roadways. This band will slowly shift southward as the night goes on.
There is a ton of uncertainty with the evolution, strength, and timing of this feature. At this point, the Everett to Tacoma corridor has the best chance at seeing snow. The highest accumulations would be on the foothills with elevation.
Regardless of what happens with this front, we will have have very strong northeast winds funneling out of the Fraser River valley into the Northern Interior as the arctic air mass moves southward. This is due to a strong pressure gradient between the arctic high to our north and the relatively lower pressure over our area. Current sustained winds appear to be in the range of 20-30 mph for many folks, and with temperatures dropping into the 20s Tuesday morning, windchills could be in the single digits in spots.
Additionally, these northeasterlies will pick up moisture as they travel over the Strait of Juan de Fuca and rise as they hit the Olympics, giving snow to the northeast foothills there. The NWS currently has a Winter Storm Watch Monday afternoon through late Monday night for up to four inches of snow for this area, with the potential for snow down to cities like Sequim and Port Angeles.
In conclusion, this is a tough forecast and snow amounts will vary significantly by region. Regions with higher elevation and away from the water will likely see the most snow. A convergence zone Monday night looks likely, but it is certainly not guaranteed. I am more concerned about the zone Monday night due to roadway icing after the zone passes and colder air infiltrates the area.
Tuesday and Wednesday are cold with high temperatures in the mid 30s, and then another tough forecast is on tap for Thursday. But I’ll save that for another blog. 🙂
Congratulations on making it to the end of this post! Do some snow dances, and hopefully Mother Nature will deliver the goods.
Have a nice night,
Charlie
