Chance of A Snowflake Tomorrow Morning for Portland

There is a chance of a dusting of snow across the Portland metro area tomorrow morning. Accumulations, if any, should remain under half an inch, and I doubt the morning commute will be affected, though you never know with Portland drivers. Still, it’s pretty insane that we are even talking about snow this late in the year. Daylight savings time begins next week, for goodness sake! And to think I had just about given up on winter at the end of January…

Let’s start by looking at an incredible visible satellite image from the now-operational GOES-17 satellite. A strong low pressure system is currently bringing heavy rain/mountain snow to California, and light precipitation with this system is expected to clip Northwestern Oregon tomorrow morning. And with temperatures falling to the lower 30s tonight for the Portland metro area and dewpoints in the teens, any precipitation that fall should be in the form of snow. Low dewpoints help due to a phenomenon known as evaporative cooling, where precipitation evaporates/sublimates into dry air. This change of phase requires energy, and the temperature drops as a result.

Current Conditions:

Temperatures are currently in the mid-upper 30s and dewpoints are in the 20s for most places along the I-5 corridor. The Portland metro is slightly colder and significantly drier due to a cold, dry easterly wind coming through the Columbia River Gorge.

This easterly wind is caused by a cold pool – and the resultant high pressure/offshore pressure gradients – over the Columbia Basin. Cold pools occur all the time from December through Valentine’s Day over the Columbia Basin, but it’s rare to see cold pools there in March. I can think of two reasons why we’ve had such a persistent Columbia Basin cold pool: first, we’ve had a large trough over our area with repeated shots of cold, Canadian air into Eastern Washington, and second, the Columbia Basin is still covered in snow, which reflects sunlight during the day and emits infrared radiation (heat) at night.

The upper-level pattern is quite complex. Here’s my “go-to” graphic for looking at the current upper-level conditions; the infrared satellite with overlaid observed winds (yellow, knots) and the 500mb heights (green, meters)/temps (red, Celsius) as modeled by the GFS. The deep upper-level trough off the coast of California is quite apparent, but there is also a pair of upper-level lows over and west of Vancouver Island. We have southwesterly flow aloft over our region due to the juxtaposition of these features, lifting the precipitation over the Golden State northward into the Willamette Valley.

Radar shows the heaviest precipitation over California with some lighter precipitation lifting northward into Oregon. As I’ve mentioned before on this blog, the radar coverage over Southwestern and Southeastern Oregon is absolutely horrendous, so there is some precipitation off the Southern Oregon Coast that is not being captured in this graphic.

Fellow WeatherTogether blogger Mark Ingalls has a great post detailing the need for better radar coverage over the Pacific Northwest. Read his post here, and be sure to “like” his Tri-Cities Weather page on Facebook, even if you don’t live in the Tri Cities. 🙂

Forecast

The National Weather Service expects less than one inch of accumulation for the Portland metro area, and I agree with this assessment. In fact, given how paltry radar looks right now and how low dewpoints are, I doubt most places will see even a quarter of an inch, and some places, particularly those west of downtown, may see no snow at all.

With sub-freezing temperatures expected overnight, any snow that does fall should stick to the valley floor. You certainly won’t need to pack chains, but just keep in mind that there could be a couple slick spots tomorrow morning if snow does fall.

If we don’t get any snow Wednesday morning, we have another chance of wet snow Thursday morning, with amounts once again under an inch. After daybreak Thursday, we should be done with snow until the 2019-2020 winter.

Thanks for reading, do some snow dances, and have a great night!
Charlie