Major Snowstorm and Icestorm Increasingly Likely for Western Washington and NW Oregon

It now appears likely that Western Washington and NW Oregon will experience a series of significant winter storms from midday Thursday through Saturday evening, bringing snow, sleet, freezing rain, extremely strong east winds, and a prolonged period of subfreezing temperatures. I’ve been dying to write this post – I’ve been absolutely swamped the past several days – but now I finally have some time to dig into weather models and take a closer look at this event. The meteorology with this event is really fascinating – it’s essentially a “textbook” pattern for heavy snow/sleet/freezing rain, with brutally cold air over the Columbia Basin, a perfectly positioned, very moist system that has tapped into some subtropical moisture, and extremely strong offshore gradients through the Columbia River Gorge that allow this frigid air to make its way into Western Washington and Oregon.

Nearly every single time snow is in the forecast, the media goes crazy and overhypes snow chances. But so far, it seems like this event, if anything, has not gotten enough media attention. A lot of uncertainty still exists, particularly with regards to precipitation type in the Portland metro area, but I have high confidence that this will be a very impactful storm for many areas.

But before we delve into the meteorology and weather geekery, here are several “key points” for the upcoming series of storms.

• Precipitation type will vary substantially by region.
  • Western Washington and the Columbia River Gorge should see snow for most of the period. The exception is Thursday morning for SW Washington and the Western Gorge, where precipitation may begin as rain, freezing rain, or sleet before quickly changing to snow.
  • The Portland metro area will likely see a huge variance in precipitation type over relatively short distances. The NE metro will likely see several inches of snow, but the SW metro could see just an inch of snow but a ton of freezing rain.
  • There is a MAJOR freezing rain threat from Woodburn south to Corvallis and west towards the coast (especially the eastern foothills of the Coast Range). Snow is possible for Woodburn but Salem south will likely be all sleet or freezing rain.
• These storms will bring a LOT of precipitation, with approximately 1-2 inches of liquid equivalent falling over much of SW WA/NW OR.
  • Places that stay all snow could easily see 10+ inches from this series of storms.
  • Freezing rain is a very difficult forecast, but some models show up to 0.5 – 1.0 inches of ice falling during this period over the Central/South Willamette Valley and Coast Range foothills, which would cause widespread power outages.
• PDX snow amounts: I’m thinking Portland could see between 2 to 15 inches of snow and 0 to 0.5 inches of freezing rain over the entirety of this event. This huge range is due to the variable types of precipitation expected over the Portland metro area. Areas to the north and east (especially east of I-205) have the best chance of seeing heavier snow totals, while the southwestern metro will likely see much less snow and get much more freezing rain.
• The Portland metro will likely experience sub-freezing temperatures from Thursday night through Sunday morning. 
• There will be very gusty easterly winds in the Portland metro area and especially Columbia River Gorge from midday Thursday through Saturday
• The combination of heavy snow and 75+ mph wind gusts will result in blizzard conditions in the Columbia River Gorge and could close I-84 for at least a day or two (hat tip to Fox 12 Chief Meteorologist Mark Nelsen for this insight – see his most recent blog post on the storm here)
• Timing:
  • Precipitation will begin Thursday morning and likely switch to snow, sleet, or freezing rain by Thursday afternoon for the Portland metro area.
  • Snowfall will be nearly continuous through Friday and will intensify Friday evening/Saturday morning before tapering off Saturday evening
  • By Sunday afternoon, temperatures should rise above freezing. Another system will come through Sunday night, but precipitation should be in liquid form.

Current conditions:

In my last post, I talked about the “polar vortex” moving into the United States and how the Rockies shielded the Intermountain West from the truly frigid arctic air associated with this polar vortex. Since that post was written on 2/1, models have shifted the polar vortex slightly further to the west, allowing a small fraction of that frigid air to funnel through passes and gaps through the Rockies into Interior British Columbia.

Below are the temperatures as measured at 5pm Pacific Time. As you can see, the primo cold air is still east of the Rockies, but the inland Pacific NW is pretty darn chilly. With a recorded daily high 25 degrees, Spokane is still about 40-50 degrees warmer than the comically cold temperatures over north/central Montana immediately east of the Rockies, but still 13 degrees below its average high of 38.

Meanwhile, take a look at the current pattern over the Pacific. While much of the US is engulfed in a brutal “polar vortex,” another popular weather buzzword – an “atmospheric river,” lies offshore. This can be seen in both the “precipitable water” model image and the water vapor satellite imagery.

So we’ve got brutally cold, dry air inland, and a deep, subtropical moisture source in the Eastern Pacific. The million dollar question of course is whether we can get this cold air and subtropical moisture in the same place at the same time. And right now, it’s looking like we will do that in spades.

Thursday-Saturday Storm: Meteorological Synopsis

By Thursday morning, the atmospheric river that is currently well offshore will begin to spread precipitation into the Pacific NW. The heavily annotated image below shows the precipitable water and upper-level heights at 4am Thursday 2/11. Note how the atmospheric river has strengthened and move east, while the polar vortex has moved further west. Western Washington and NW Oregon are in the “sweet spot” and experience both cold, arctic air and subtropical moisture, creating the potential for a major winter storm.

Here are the UW WRF sea-level-pressure/temperature and precipitation charts at 4am Thursday morning. Note the MASSIVE temperature contrast between the atmospheric river offshore and the frigid air over the Columbia Basin and Western Washington. Anything falling in Western Washington should be in the form of snow the entire day, while areas south of Eugene should see just rain.

The Portland metro area south to Corvallis is a much trickier forecast. We’ll have subtropical air at mid and upper levels, but frigid arctic air at the surface pouring through the Columbia River Gorge. When we have this type of “overrunning” event, where mild, above-freezing, Pacific air rides over a layer of dense, subfreezing surface air, we can get sleet or freezing rain. Sleet occurs when the layer of warm air initially melts the snow, but the layer of cold surface air is thick enough for this rain to refreeze back into little ‘ice pellets’ (not to be confused with the tiny hail we occasionally get with intense showers, which is also sometimes referred to as ‘ice pellets’ but is formed by a different process). Freezing rain occurs when the layer of cold surface air is relatively shallow and supercooled (below 32F) raindrops freeze into an icy glaze upon contact with a subfreezing object at the surface.

One way we can look at the potential for snow vs. sleet vs. freezing rain is to look at forecast soundings, which are just temperature profiles of a column of the atmosphere at a given location. The UW WRF model creates soundings for many of the airports across the Pacific NW. Compare the soundings at Cascade Locks, Portland, and Salem at 10am Thursday, when cold easterly winds will be rushing through the Columbia River Gorge into the Willamette Valley. Note how all three locations have sub-freezing temperatures at the surface, but the temperature profiles aloft are very different. Cascade Locks has a deep layer of frigid low-level easterlies, while Portland has a much thinner layer of cold, low-level easterlies. However, temperatures remain below freezing throughout the atmospheric column in both locations, so each would see snow. On the other hand, Salem has a thick layer of above-freezing temperatures at mid levels and just a very, very thin layer of subfreezing air at the surface, resulting in freezing rain there.

Friday and Saturday:

After Thursday’s atmospheric river passes through and gives snow/sleet/freezing rain to the area Thursday afternoon and evening, Friday should feature periods of moderate snow/sleet/freezing rain with extremely strong easterly winds. Precipitation and winds are expected to intensify Friday night/Saturday morning as another atmospheric river takes a similar track and delivers even heavier precipitation totals to the area. Below are the sea-level-pressure/temperature and 3-hour precipitation ending at 4am Saturday. It looks like Thursday’s storm on steroids! There’s still a lot of time for this storm to strengthen or weaken and confidence in the details is lower than Thursday’s, but at this point it looks like it could feature a similar snow/sleet/freezing rain distribution as the first system Thursday afternoon and evening with even higher amounts.

That’s all I have for tonight… gotta save some gas in the tank for tomorrow’s (shorter!) blog! Stay safe and prepare accordingly for this storm by charging your devices, altering/canceling travel plans, and making sure you have all the items on a “winter preparedness checklist” like the one below.