It's tough to get snow in Western Washington and Western Oregon. During the 1991-2020 time period, Seattle averaged 6.4" of snow a year, while Portland has averaged just 4.2". The reason why snow is so difficult to get is both because we have a mild climate with relatively few arctic outbreaks, and the arctic outbreaks that we do have stem from dry continental airmasses that lack moisture. To get snow, you need both moisture and cold air - and getting both of these in the same place at the same time is a very difficult task. 

The "classic" pattern for lowland snow is to have an arctic airmass already in place with a powerful, cool low-pressure system arriving just to the south of the forecast region. Areas to the north of the low will see a reinforcing blast of cold, northerly winds rushing to meet the low while still experiencing the moisture associated with the low. The most famous example of this in the 20th century was the "Friday the 13th" snowstorm of January 1950, where Seattle saw 21.4 inches of snow in a 24-hour period amongst 40 mph gusts and a high of only 19 degrees.

Snow on the streets of Seattle after the 1/13/1950 blizzard. Credit: MOHAI. Retrieved from Scott Sistek's article on the Jan 13 1950 Seattle Snowstorm

An article on the storm by Scott Sistek included a NWS surface analysis of the storm at the time - and the setup is perfect for heavy snow over Seattle, with a frigid, entrenched arctic airmass overhead and an intense, relatively cool cyclone moving just to the south.

National Weather Service Surface Analysis Chart from Jan 13, 1950. Credit: NOAA/NCEI. Retrieved from Scott Sistek's article on the Jan 13 1950 Seattle Snowstorm

Friday night into Saturday, some portions of SW Washington and Western Oregon have the chance of experiencing a chance of a highly impactful winter storm. This will be nothing like the storm of 1950 and is battling several factors. First off, this low will not be moving into an entrenched arctic airmass; rather, it will be arriving just as a low-level arctic blast is moving into Western WA and NW OR. Second, this low has subtropical origins and is associated with a much milder airmass aloft, introducing the chance for sleet and freezing rain as mild air overruns this low-level arctic blast. And to top it off - models have been all over the place with this system and continue to have very little agreement less than 60 hours out from the event. 

All of that said, here are some highlights and things we *do* know.

• Arctic air will filter into Western Washington Thursday night and Western Oregon Friday morning. Friday will see decreasing temps throughout the day, with highs in the upper 30s/low 40s in the morning falling to the 20s by the evening.
• A mild Pacific storm will spread precipitation to the Pacific Northwest beginning Friday night, with a stronger storm following quick on its heals Saturday morning. There will be little/no break in precipitation between the storms.
• Precipitation forecasts have trended further south for these storms. Seattle is expected to stay completely dry and see no snow. Portland may see an inch or two but amounts are far less than feared earlier. However, one model - the GFS - drops a foot of snow over Portland, so there is still significant uncertainty in this track and the potential for much higher snowfall amounts. The Central/South Willamette Valley has the best chance of seeing snow/sleet and has a risk of experiencing a destructive freezing rain storm.
• High confidence in frigid temps for Seattle and Portland Friday/Friday night through early next week. Lows in the teens are possible Saturday-Monday mornings.
• The Portland metro area will see very gusty winds Friday night/Saturday as this storm approaches, with widespread 40-50 mph gusts in the metro increasing to 60-80 mph in the Western Gorge. This should result in subzero wind chills in spots Saturday morning.  Gusty east winds will continue through at least Monday/Tuesday next week.

Current pattern: 

We currently have a large, blocking ridge in the Pacific resulting in a highly meridional (north-south) rather than zonal (east-west) jet stream. To the west of this block, deep southerly flow is bringing subtropical moisture from Hawaii up to the Aleutian Islands. To the east of this block, a deep trough is allowing arctic air to seep south from Canada and filter into the Lower 48. Though most of the arctic air will stay east of the Rockies and eventually spread into the Plains and Midwest, some of this arctic air will filter into the Interior NW, with modified, low-level arctic air squeezing into the Fraser River and Columbia River Valleys into the Western Washington and NW Oregon lowlands, respectively. 

This arctic air is just beginning to move into the Northern Rockies now, will move into the Interior NW tomorrow, and will move into Western WA/NW OR early Friday, strengthening throughout the day. The high-resolution WRF model from the Univerisity of Washington does an excellent job of showing how the mountains help "dam" this frigid, dense arctic air on their eastern slopes and obstruct its flow. 

Arctic front moving into NW OR at 7am Friday. Note the strong pressure gradient through the Columbia River Gorge. Credit: University of Washington

Meanwhile, an area of low pressure will develop over the northern Pacific, tap into this subtropical moisture, and race towards the Pacific Northwest. However, models are struggling mightily with the exact location of this low. European, German, British, and Canadian models all bring it south of Portland with very little/no snow for the city of roses. The American model takes it much further north, towards the mouth of the Columbia, giving an all-out blizzard to Portland with up to a foot of snow, as well as several inches for Seattle. If there was ever a time to be patriotic - now's the time. 

Below are the snow maps from the 18Z GFS and 12Z Euro operational runs today. The 18Z GFS has a foot of snow for Portland. The 12Z Euro has nothing. It doesn't get much different than that.

48-hour GFS snow forecast for Pacific Northwest ending 7pm Saturday 1/14. Credit: Weatherbell

48-hour ECMWF snow forecast for Pacific Northwest ending 7pm Saturday 1/14. Credit: Weatherbell

How about the model ensembles - the "suites" of models with multiple forecasts? Perhaps these show better agreement and the operational runs are just outliers. Unfortunately, the ensembles show the same pattern as the operational runs, with one caveat. Most of the European members are dry, but a few show a major snowstorm for Portland.

24-hour snow forecast for Portland from ECMWF Ensembles. Credit: Weatherbell

The GFS ensembles are similar to the operational and most show a major snowstorm, with an ensemble mean of 8" in 24 hours.  

24-hour snow forecast for Portland from GFS Ensembles. Credit: Weatherbell

The next few runs will be critical for getting a better handle on the path of the low. If the GFS continues to trend south - as I suspect it will - it dramatically decreases the risk of a major snowstorm for Portland. But if the European, Canadian, German, and British models all of the sudden trend north, Portland could be in for a major event. Stay tuned!