Thursday Looking A Little Snowy For Some, Downright Nasty for Others

It now appears as though Portland will experience a significant winter storm tomorrow, with gusty winds, a few inches of snow, and a changeover to sleet and freezing rain in the evening. Over Seattle, I believe we could see a trace to two inches of snow Thursday night before precipitation changes to rain early Friday morning, but the situation is looking less snowy than feared earlier.

We’ve got a lot to talk about, so let’s get right to it.

There are a myriad of winter weather watches, advisories, and warnings currently up for the Pacific Northwest. Seattle currently has a Winter Weather Advisory for 1-3 inches of snow Thursday night before shifting to rain Friday morning, and Portland has a Winter Storm Warning for 2-4 inches of snow, a quarter inch of ice, and extremely strong east winds gusting 40-50 mph.

 

Let’s start by looking at the current conditions over the Pacific.

Compared to last night, the massive trough that brought us all this cold air in the first place has moved east, and we are currently under a quick-moving, very weak ridge of high pressure ahead of our incoming Pacific storm. Because the trough has moved east and we are seeing onshore flow at the mid and upper levels of the atmosphere, the temperatures aloft are warmer now than they were 24 hours ago. However, offshore gradients at the surface have kept cold, dry northeasterly winds funneling through the Fraser River Valley, resulting in a slightly cooler day at the surface for most of Western Washington with highs only reaching the upper 30s in most places. Below are the current conditions throughout our region as of 5:56 pm.

This type of setup – cool continental air at the surface with moist, warmer Pacific air aloft – is called an overrunning event (meteorologists like to add extra words to make themselves feel important) and is the most common way for us to get region-wide snow here in the Pacific Northwest. The amount of snow we get depends on the (a) depth of the cold air currently in place, (b) the characteristics of the storm off the Pacific, and (c) the track of the storm itself.

Like many overrunning events, this storm will bring very strong easterly winds, lowland snow, and even freezing rain to a couple areas across our region. These are all very significant hazards, so let’s go over them one at a time.

Winds:

The model above shows the temperatures at the 925 mb level of the atmosphere (remember, meteorologists use pressure as a vertical coordinate), which is approximately 2,500 feet above sea-level. Additionally, it shows the sea-level pressure (the black lines) and the sustained winds at 10 meters above the surface (the wind barbs). As you can see, there is still a large amount of cold air over Washington, Eastern Oregon, and the Columbia River Gorge, with a large east/west pressure gradient due to higher pressure east of the Cascades and lower pressure offshore. The strong warm front (where the green turns to yellow) off the coast of California is the storm that will impact us tomorrow.

This pressure gradient is creating very windy conditions along the western exit region of the Columbia River Gorge. One place famous for these winds is Vista House at Crown Point. Indeed, winds have been rising there much the day and are currently gusting over 70 mph. They will continue to increase this evening as pressure gradients tighten even further.

By 10 pm this evening, the warm front has inched further northward and east/west pressure gradients increase as a result.

 

 

By 7 am Thursday, the low is off the Oregon Coast and gradients have increased even further.

Gradients this high would suggest 40-50 mph gusts in Portland, 50-70 mph gusts near Troutdale, and up to 100 mph gusts at Crown Point. There is also a very large gradient further north against the Washington Cascades, and this type of setup commonly gives very strong easterly winds to the foothills as dense, cold air flows through gaps in the Cascade crest and accelerates as it rushes into Western Washington. Taking a look at the map below of estimated sustained winds at the same time, you can clearly see areas of higher winds downwind of low-elevation gaps in the Cascades, most notably the Columbia River Gorge and Stampede Gap (near Snoqualmie Pass).

These winds will decrease Thursday night as the storm passes north of our area and the flow shifts to onshore. But between now and then, things will be very windy on the western Cascade foothills and Columbia River Gorge. We will also see strong offshore winds along the Strait and off the coast, where Gale Warnings are currently in effect.

Snow:

There is good model agreement that Portland and most places in Western Washington will see some snow Thursday. Unlike Monday, Thursday’s event will have widespread precipitation as opposed to isolated showers, and the air in place when precipitation arrives will be cool and dry enough for snow to fall. The humidity of the air matters because snow will often “sublimate” if the air is sufficiently dry, and this process takes energy and thus further cools the surrounding atmosphere.

By Thursday morning, snow starts spreading across Northwest Oregon. Though this model shows just rain or freezing rain for the Willamette Valley, the Portland NWS office still has a Winter Storm Warning over them for 1-2 inches of snow and up to .25 inches of ice. Models like the WRF-GFS below almost always mix out cold air too quickly during these events, particularly in valleys, so I assume that the Portland NWS is taking this into account when making their forecast.

 

By Thursday night, this snow spreads into Western Washington.

24-hour snowfall totals ending Friday morning range from approximately 4 inches along the Kitsap Peninsula near the Olympics to nothing on the Cascade foothills and further north near Bellingham. The reason for the lack of snow on the Cascade foothills is due to the strong downslope winds drying the air, making it easier for snow to sublimate before it hits the ground. I believe the models are overdoing this effect, and I’m forecasting an inch or less of snow for the foothills.

It’s worth mentioning that models have trended lighter and lighter with the precipitation, leading to progressively lighter and lighter snowfall amounts in the models. Though it appears that most places throughout Western Washington will see a brief period of snow Thursday evening, amounts are less than feared earlier – likely a trace to 2 inches instead of the 1-4 feared yesterday. The exception is Hood Canal, where the offshore winds will actually help increase snowfall as they run into the Olympics. Expect up to 4 inches of snow there, which is also a decrease compared to yesterday.

Freezing Rain:

 

Because overrunning events involve warm air moving over cold air, we can see a transition in precipitation types as the layer of cold air becomes thinner and thinner. For most of Washington, the transition from snow to rain is fairly rapid, with little in the way of sleet or freezing rain. The same cannot be said for Portland, however.

Even as the air aloft warms, cold easterly winds through the Columbia River Gorge can continue to keep surface temperatures below freezing well after the precipitation has turned to rain. When that happens, you get this.

At this point, ice accumulations from this storm are forecast to remain under .25 inches, so I am not worried about a major ice storm like the one pictured above. Still, even a quarter-of-an-inch of freezing rain can make for extremely treacherous travel conditions. You do not want to be driving in this stuff.

Regardless of what happens, I’ve had a great time tracking the winter weather with you all this week. Thanks for reading my blog, and be sure to check out our Facebook and Twitter pages for all the latest not only this storm but the wild world of weather in general.

Have a nice night!
Charlie