Beginning late last week, the GFS (American) model began showing a transition to much colder weather for early-mid January. However, the normally-superior ECMWF (European) model did not, and given how far in advance the forecast was, it was a bit premature to throw the “s” word around. After all, we’ve seen the disastrous repercussions of hyping snow events!
Pre-snowstorm in Portland, the only thing sold out so far at the grocery store was kale. Plenty of bread and milk remain. #pdxtst pic.twitter.com/grYk1EaVj4
— Jesse Friedman (@JesseCFriedman) February 8, 2019
Note: Downtown Portland only saw a trace of snow from this system, resulting in many furious Portlanders (Northeast Portland and the airport saw a few inches though)
But over the weekend, the European model began to shift towards the GFS, and as of Sunday night, both are in good agreement that much colder weather will take hold over the Pacific Northwest by next weekend or the first half of the following week.
Before we speculate on what could happen next week, let’s take a look at the weather expected for the next several days. We currently have a strong jet stream over the area with an “atmospheric river” looming just offshore. This atmospheric river will head straight into Western Washington tomorrow and deliver 4-6 inches of liquid precipitation over the Olympics and Washington Cascades, increasing the potential for light flooding on susceptible rivers. Seattle and Everett will see significant rainshadowing from the Olympics, so much lighter precipitation is expected there… only a few 10ths of an inch. On a side note, I don’t think the media emphasizes rainshadows enough – I’ve seen many region-wide forecasts of heavy rain and flooding that fail to mention the existence of a huge rainshadow over highly populated areas. Communicating these microclimates is crucial here in the Pacific Northwest!
Credit: WeatherTogether Models
Despite the heavy rain and flooding concerns, this atmospheric river originates from the North/Central Pacific instead of the subtropics, and as a result, snow levels will only rise to 5,000 feet instead of the 10,000 feet we can see with our “Pineapple Express” atmospheric rivers that originate near Hawaii.
Snow levels will fall back below pass levels Tuesday evening as a cold front comes through and chilly, post-frontal showers will keep the snow falling in the mountains. By Thursday morning, the Washington Cascades and Olympics will likely have seen 1-3 feet of snow from 2500 – 5000 feet, and over 4 feet of snow above 5000 feet! And with snow levels dropping to 500 – 1500 feet Wednesday night/Thursday morning, I wouldn’t be surprised if some of the Cascade foothills saw a rain-snow mix as well (North Bend, Index, Sandy, etc.)
Credit: University of Washington
Chilly, rainy weather continues Friday and Saturday, with TONS of snow in the mountains. But the real interesting weather – and the motivation for this blog post – begins Sunday/Monday.
On Sunday, a deep upper-level trough bringing arctic air will approach the Pacific Northwest. The chart below shows the upper-level (500mb, around 18,000 ft) heights and height anomalies midday Sunday. This pattern, with a huge ridge offshore and a deep trough carrying arctic, Canadian air over the Pacific NW, is a classic pattern for lowland snow.
Credit: WeatherTogether models
The below chart shows the 850 mb (~5,000 ft) temperature. Note the absolutely frigid airmass knocking on our doorstep.
By Monday/Tuesday, this airmass has moved over us, and we are in a deep freeze. Though the brutal cold stays east of the Rockies and the Cascades further protect the I-5 corridor from frigid weather, both Seattle and Portland would see very cold weather in this scenario, with highs on either side of freezing. The coldest spots in Montana (Cut Bank, Yellowstone, and others) would drop into the -20s or below, with windchills below -50 degrees!
The Million Dollar Question: Will We See Snow?
One of the reasons why snow is so hard to come by in the Pacific Northwest is because our cold weather tends to be dry and our wet weather tends to be warm. To get the magic combination of moisture and cold air, you need a shortwave trough arriving from the north to strengthen just off the Washington Coast, pulling cold, dry air from Canada over the mild Pacific for a short enough period of time to allow it to pick up some moisture without warming substantially.
The image below is taken from George Miller’s book “Pacific Northwest Weather: But My Barometer Says Fair!” I picked up this book in middle school and it was a formative book in developing my interest in meteorology. I can’t recommend it enough.
Though it’s too early to try and track individual storms that could cause snow, the GFS ensembles paint a relatively wet/snowy scenario, with above-average precipitation while this cold air is in place. In other words, it looks likely that some folks in Western Washington/Oregon will get snow next week, but it’s too early to tell who those people are at this point, and we probably won’t have a good idea until the end of this week.
To summarize, expect very rainy weather today/tomorrow for Washington Monday/Tuesday (apart from significant rainshadowing over North/Central Puget Sound), with lighter rain over Northern Oregon. A cold front Tuesday night will usher in a chilly, showery that will persist through Friday/Saturday, and our big pattern change to colder weather comes on Sunday/Monday, with this cold weather persisting through at least next Wednesday. Models show a decent amount of precipitation during this time as well, so some lowland areas will likely get snow, but it’s too early to pin down what areas those are.
In any event, no need to load up on the kale this week. But if we are still seeing snow in the models by Friday/Saturday, it might be worth a trip to the supermarket. 🙂
Have a great week!
Charlie
