Lowland Snow Tonight? It’s Possible For SW Washington and NW Oregon

Yes – the rumors are true. There is a chance of lowland snow tonight into Monday morning for Southwest Washington and Northwest Oregon. Some spots over Washington have already seen some snow showers this morning, but the main threat – and the focus of this blog – will be the snow potential Sunday night/Monday morning over Southwest Washington and Northwest Oregon, with some heavy, very wet snow even possible over the Portland city center.

Impacts should be minimal over Portland proper. Road surfaces are warm and any snow will melt upon contact, but snow/slush could accumulate on roads further north and with a little elevation, where temperatures are expected to be colder. Snow will begin to melt in earnest by mid-morning, when precipitation begins to lighten, and melting will accelerate in the afternoon when temperatures rise into the 40s and we start seeing a few sunbreaks. The Cascades, on the other hand, will experience winter-like conditions tonight/Monday morning, and I wouldn’t be surprised to see a few power outages along the Mt. Hood Corridor due to heavy snow weighing down branches and causing them to snap.

Here are the main points of the blog:

1. A very unusual (but not unprecedented), cold April storm is currently strengthening off the Oregon Coast, will make landfall near the North/Central Oregon Coast tonight, and will track across the Willamette Valley early Monday morning.
2. This storm will bring a chance of lowland snow from near Centralia South to Clackamas, with the in the Kelso/Longview Area potentially seeing the heaviest snow. Snow amounts will be highly variable and could range from a trace to places like downtown Portland to 8 inches over the higher locations of the Northern Oregon Coast Range.
3. This storm will also bring gusty winds to 40 mph in the lowlands and 60 mph along the coast, and 1-2 feet of snow to the Oregon and southern Washington Cascades.
4. Impacts for the Portland metro area will be minimal, as any snow in the city will not stick to roads. The exception is the West Hills and potentially some spots of Clark County. For the entire area, melting will begin soon after the morning commute when temperatures warm and precip rates lessen, and it will accelerate throughout the day.

Current Conditions:

We currently have a very cold and very deep upper-level trough centered off of Vancouver Island, with a 170 mph just stream just offshore directing an intensifying low pressure system into North/Central Oregon.

Cold upper-level troughs like these are pretty common during the spring, particularly during La Nina years. The mid and upper levels of the atmosphere take much longer to heat than the surface in the spring, and they are also slower to cool off in the summer. This is why Snoqualmie Pass can see snow well into May but does not see any snow before October, and is also one of the reasons why the Great Plains see the most tornadoes in the spring – with warm/moist air at the surface and much cooler air aloft, the atmosphere is highly unstable, leading to intense convection and very powerful thunderstorms.

But in order to get snow all the way down to sea level during April, we need more than just a strong upper-level trough with very cold temperatures aloft. We also need a strong surface low pressure system to develop alongside this trough and make landfall just to the south the area, bringing cool winds with a northerly component and precipitation. Third, this precipitation with this low needs to be extremely heavy, as heavier precipitation creates dynamic cooling, which is where the phase change from snow to rain takes heat energy out of the atmosphere and lowers the snow level. And finally, the event needs to occur overnight or in the early morning hours when the high April sun is not shining.

Such a pattern is expected to occur Sunday night through daybreak Monday for Southwest Washington and Northwest Oregon. The current low is expected to intensify and make landfall near Newport shortly after midnight, and will then track ENE across the Willamette Valley towards the Blue Mountains. In the process, it will bring cooler air and heavy precipitation into Southwest Washington, the Coast Range, and portions of the Portland metro area, and the dynamic cooling from this heavy precipitation may help lower the snow level all the way to sea level, particularly for Northwest Washington.

Here’s the snow map from 5pm Sunday to 5pm Monday. Keep in mind that these are the snow totals “in the air” and not the snow totals that actually accumulate on the ground, so ground accumulation will be much less. I imagine the heaviest-hit sea-level locations over Southwest Washington could see an inch or two on grassy surfaces, but I wouldn’t be surprised if there were widespread 6+ inch amounts over the Northern Coast Range and even parts of the Willapa Hills.

That’s all for now – gotta head to night shift! Do some snow dances and with any luck some of you will wake up to snow tomorrow morning!