Snow Update

The Portland metro area should see some light snow some tonight! Most folks will just see a dusting, and up to an inch could fall above 500 feet (especially over Western Washington County) and east of I-205. But despite the light amounts, it’s wonderful to start December off with some snow.

We have a complex upper-level pattern, with one upper-level low off the Washington Coast and another several hundred miles west of San Francisco. The upper-level low off the Washington Coast is bringing moist, SSW flow northward, but is also slowly drifting to the SE as it spins counter-clockwise around the California Coast low. The result is moderate precipitation from Salem southward that fizzles out into mid/high level clouds as you head further north into Washington.

If we take a look at the radar, you can see this band of precipitation spreading northward. The heaviest stuff is currently south of Salem, but it should reach the PDX metro by midnight.

Unfortunately, this southerly flow is relatively warm, and ODOT cams show snow north of Eugene only above 500 feet, with elevations below that seeing rain. Surface observations show above-freezing temperatures nearly everywhere at sea-level (including the Portland metro area), with relatively high dewpoints where precipitation is currently falling and much lower dewpoints to the north. The Portland metro area is particularly dry, with dewpoints in the upper teens.

Due to the lower dewpoints over the Portland metro and Columbia River Gorge, these areas will experience stronger evaporative cooling than areas to the south when precipitation arrives. Evaporative cooling is the process by which precipitation falls into a dry air column and evaporates/sublimates. This change of phase from liquid/solid (respectively) to gas takes a significant amount of energy, causing the atmosphere to cool. Evaporative cooling also increases the dew point by increasing the amount of moisture in the atmosphere. The point where the increasing dew point and decreasing temperature “meet” due to evaporative cooling is known as the “wet-bulb temperature,” and wet-bulb temperatures are below freezing for much of the Portland metro right now, increasing the potential this precipitation to be in the form of a rain-snow mix or non-sticking, wet snow down to sea level for the Portland metro area.

We can thank the “East Wind” for this super dry air. The East Wind machine has indeed been raging – Vista House at Crown Point has already seen gusts over 70 mph, and these winds should continue to increase through tomorrow morning.

Combining these East Winds and the evaporative cooling they cause with the precipitation overhead, we get this map of snow accumulations from the latest UW WRF model.

Forecast

• Note the tiny little sliver of snow over the Portland metro area. I think the WRF is overdoing these amounts – I think most spots in Portland will get a dusting, with up to an inch possible above 500 feet and east of I-205 where the East Wind is stronger.
• The Coast Range will see higher amounts, but temperatures will rise above freezing there rather quickly tomorrow morning. If you need to cross the Coast Range Sunday, it’s best to do it after noon.
• The biggest impacts from this system will be in the Columbia River Gorge. Troutdale east to Bonneville Dam could see 0.5-2 inches of snow followed by 0.1-0.2 inches of freezing rain and 60-70 mph gusts will rake the “usual suspects” like Corbett, Rooster Rock, and of course Vista House/Crown Point.
  • Snow accumulations will be even higher as you head further east, with 3-6 inches possible between Cascades Locks and The Dalles, but winds will be much weaker, and freezing rain, if any, will stay to the west of Hood River.
• Areas below 500 feet south of Oregon City are not expected to see any snow tonight

Timing

Precipitation should begin around 11pm-1am for those living in the PDX metro and in the Columbia River Gorge and should mainly be over by mid-morning. Snow will only last a couple hours for the Portland metro before switching to freezing rain (mainly for the northern metro) or plain ol’ rain.

Snow will hang on longer as you head further east. In fact, the WRF shows another band of showers moving through in the afternoon, and with the East Wind still howling through the Gorge, this should be in the form of freezing rain near Bonneville and snow east of Hood River.

What About Western Washington?

As usual, I’m out of town for the snow in Portland (I’m up on Whidbey Island with my parents and dog). Unfortunately, the precipitation from this system won’t reach us until after sunrise, and most of us will just see rain. The exception is Lewis and potentially Thurston County, which could see a dusting as this precipitation moves through shortly before sunrise.

I hope y’all had a wonderful Thanksgiving! Thanks for reading, and wherever you are, I hope you see some flakes tonight.
Charlie