We saw some snow yesterday, and although it didn’t stick in most places, it certainly gave it the ol’ college try. With all the frantic talk about a few flakes flying through the air, it’s easy to forget that some models were advertising a more widespread and significant lowland snow event on Thursday. Let’s open that can of worms now.
Let’s start by doing the only thing I love more than weather forecasting… looking at what the weather is like right now! I’ve shown this chart in some of my recent blogs, but as a refresher, it is an infrared satellite picture overlayed with both modeled 500 mb heights in meters and the temperatures at the 500 mb level of the atmosphere. Remember, in atmospheric science, we often use pressure as the vertical coordinate instead of height, and millibars (mb) are a unit of pressure.
The most striking feature is a huge trough over not only our region but much of the Continental U.S. This trough is directing very cold air south through Canada into the United States. Things are downright frigid as you go further east; my friend Mark Ingalls of St. Anthony, Idaho reported a temperature of 1 degree this morning, while Old Crow in the Yukon currently has an “Extreme Cold Warning” for temperatures of -50° with windchills dipping down to -65°F! Even sourdoughs from the Klondike Gold Rush would agree that a -65° windchill is a tad uncomfortable.
The Rockies (and to a lesser extent, the Cascades) prevent truly frigid air from infiltrating into our region, but we are still a tad nippy this afternoon, with current temperatures in the upper 30s for most locations around Western Washington. Note the colder air and stronger northeasterly winds up by Bellingham and the San Juans… this is our famed “Fraser River Outflow” and is the primary way that modified arctic air spills into Western Washington.
The current hi-resolution MODIS imager aboard NASA’s TERRA satellite shows scattered cumulus clouds over the Pacific and Southwest Interior, and lots of snow cover throughout the Pacific Northwest. Cumulus clouds are convective, meaning they are formed by air that freely rises without the assistance of some sort of larger feature (like a front) to force it upward. In order for an air parcel to rise, it must be warmer (and therefore less dense) than the surrounding atmosphere. It’s no coincidence that the cumulus clouds here are mainly located over the relatively warm, ~50 degree Pacific Ocean.
Before we go on to the forecast, I want to call attention to a really fascinating feature in this image. If you look carefully, you can see a line of cumulus clouds pointed NW over Admiralty Inlet in Northern Puget Sound. I’ve enlarged this feature below for your convenience.
This line of clouds is actually evidence of two air streams converging over Puget Sound. Because Puget Sound is warmer than the surrounding land, the pressure at low levels is slightly less due to air being less dense. Cooler air over the surrounding land slowly comes to “fill” in this area of low pressure, and when the different air streams meet, they force each other to rise, forming clouds in the process. This is an example of a “land breeze,” where wind flows from land to sea. In the summer, the ocean is far cooler than t land, so we get the reverse: a sea breeze.
Tonight will be the coldest night of 2016 as clear skies will allow for excellent radiational cooling. Expect morning lows to dip into the upper 20s to start your day in the Seattle area, with outlying areas and areas further north dipping into the low 20s or even upper teens. Additionally, we’ll still have a bit of Fraser River Outflow for the North Interior, making things feel even colder up there.
Wednesday will be relatively similar to today, with increasing high clouds ahead of our next system as the day goes on. And this next system could be a doozy, particularly for the Portland area.
A moderate-strong low pressure system will approach from the southwest, directing a strong warm front into our area. Normally, this would amount to a typical wet and windy day for the Pacific Northwest. However, since we will still have cold air in place, precipitation will likely start as snow.
Models have been all over the place with regards to snow amounts. The local WRF-GFS, which I will show here, was consistently showing nothing for the lowlands until this morning. Now, it shows little for the foothills but nearly a foot for the Kitsap Peninsula. The reason for this large east/west gradient across the lowlands is due to the strong easterly downslope winds expected in the Cascade foothills, which will limit snowfall in places like Enumclaw but help increase it near Hood Canal where these winds bump up against the Olympics.
Going further south, you can see a tongue of snow sticking out from the Columbia River Gorge into the Portland metropolitan area. This is due to very cold and dense air rushing westward through the Columbia River Gorge as the low approaches the coast. Expect very strong winds in the Gorge during the late Wednesday night to Thursday time frame. The Portland National Weather Service forecast office has issued a Winter Storm Watch highlighting the potential for 1-4 inches of snow down to the floor of the Willamette Valley followed by a period of freezing rain and sleet while 45+ mph gusts race through the Columbia River Gorge.
I’ve got to run to a climate conference now, but I’ll have much more on this tomorrow.
Charlie
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