Gusty East Winds and a Light Wintry Mix On Tap for Portland

I’ve been watching the potential for snow, sleet, and freezing rain in the Portland area Thursday night into Friday as a weak warm front runs into a cold air mass over the area. The front has trended weaker and weaker with recent model runs, so while some snow and freezing rain still appears possible, amounts are looking quite light. Even a little bit of snow and freezing rain can make for very slick roads, so exercise caution if you plan to travel tonight or Friday, and if possible, stay home.

The bigger story over the past day has been the winds. Winds have been gusting in the 40s all day throughout the Portland metro area, and even higher winds have been measured at Vista House at Crown Point. A 78 mph (not 77 as the graph below shows) gust was recorded at 4:56 pm yesterday, and we have seen nearly 24 hours of 20+ mph sustained winds at Portland International Airport.

Blustery winds will continue throughout the day as cold, dense air over Eastern Washington/Oregon rushes toward lower pressure west of the Cascade Crest. The super-high resolution 1.33km WRF-GFS shows 35 knot sustained winds over the Portland area continuing through early Friday morning. I think this may be a little too high, but sustained winds of 30 mph with gusts 40-50 seem likely.

Easterly winds still persist late Friday evening, though they will be much weaker then. Models show southerly wind affecting the Portland metro area by midnight Saturday, but easterly flow will continue in the Columbia River Gorge for much of Saturday.

A Note About Gap Winds

There is a common misconception that winds are the highest in the middle of the gap where winds are “squeezed” the most by the surrounding terrain, accelerating the winds via the Venturi Effect. In reality, the strongest winds tend to occur at the EXIT regions of gaps, not in the middle.

There are two reasons for this: (1) large, or synoptic-scale pressure gradients due to features like extratropical cyclones and large ridges of high pressure, and (2) differences in pressure driven by differences in temperature across a gap.

Large scale pressure gradients cause strongest winds at the exit region of the gap because winds accelerate along isobars, lines of constant pressure. As pressure decreases along the gap, the wind accelerates along these isobars, reaching a maximum velocity at the exit region of the gap.

Differences in temperature cause differences in pressure via the hydrostatic relationship, which says that the rate at which air pressure decreases with height is proportional to the density of the air. The ideal gas law tells us that colder temperatures increase the density of the air. The result is that for a given pressure aloft (for example, 500 mb), the pressure at the surface will be determined by the temperature of the air below this level.

Many gaps provide a passageway for relatively cooler air to rush towards relatively warmer air, and the Columbia River Gorge is a prime example. As the gap widens at its exit point, the depth of this cool air decreases, and because surface pressure is dependent on the temperature of the air mass, you see a rapid decrease in pressure at the exit region of the gap, resulting in locally higher winds there.

Gap flow is fascinating. There are times when there can be light winds throughout Eastern Washington as a persistent cold pool sits over the region but winds at Crown Point can gust over 100 mph. And it’s not just the exit region of the Columbia River Gorge that gets strong easterly flow; Washington foothill communities such as North Bend, Enumclaw, and Gold Bar can get very strong winds as well. Bellingham and the Northern Interior can get strong gap flow from the Fraser River Valley when an arctic air mass moves into the region. Even the western exit region of the Strait of Juan de Fuca can see enhanced gap flow.

Westerly gap flow is very common during the summer, but since winds are coming from the west, the highest winds are east of the Cascades. The predictable rise in winds most summer afternoons and evenings creates world-class windsurfing conditions for the Columbia River Gorge and provides abundant wind energy for areas downwind of gaps in the Cascades, especially near and around the gorge.

Snow and Freezing Rain:

Models have consistently trended weaker and weaker with the incoming front Thursday evening slated to bring us snow and freezing rain. However, it still appears as though the Portland area will see at least some snow, even if it is just a dusting, and up to .2 inches of freezing rain. Yesterday’s “Weather Story” from the Portland NWS provides a nice overview of the timing and effects for different regions, but things have trended even drier since then. Now, it appears that the South Willamette Valley will see little or no snow and a tenth of an inch of ice, the North Willamette Valley will see up to an inch of snow and .2 inches of ice, and the Columbia River Gorge will see 2-6 inches of snow with up to a half inch of ice.

Even though snow lovers may be disappointed with the downtrend in snow amounts, it’s been fascinating to witness such an extended period of strong easterly winds. It’s been a great winter to be a weather geek in Portland!

For more information on gap flow, read Cliff Mass’ ‘Navy Mesoscale Primer’ on Gap Winds here.