On Thursday evening, while Seattle was worrying about approaching snow, Portland was being hit by freezing rain: liquid precipitation that freezes on contact with the ground or other objects. Extending from the western Columbia Gorge into the northern Willamette Valley, the Portland freezing rain lasted from 4 PM Thursday until noon on Friday, causing power outages and closing I 84 for hours.
Bellingham has also had several hours of freezing rain, mainly late Friday and Saturday, with a touch of it last night.
So why is the western Columbia Gorge, Portland, and the Bellingham region vulnerable to freezing rain, while it is an exceptional rarity here in Seattle? How can liquid water freeze on contact?
Freezing rain is associated with supercooled water. Strangely enough, water can be cooled to below freezing 0 C (32F) without freezing. This is not a rarity: much of the water in clouds is supercooled. To go from liquid water (where water molecules are moving around but are bound together) to ice (where water molecules are found in a rigid, crystalline structure) does not happen immediately when temperature drops to freezing. Sort of like a preschool class when you tell the kids to sit down....it takes a while. And having a template for the crystal structure from some kind of particle (such as a freezing nuclei) helps things along--and sometimes such particles are lacking in the free atmosphere.
Nearly all precipitation in the Northwest starts as snow aloft, but as it falls in our normally mild air (even in winter) it melts and turns to rain (see figure, left hand side). And if the atmosphere is cold through depth, the snow can reach the surface (right side). But in some situation, a shallow layer of below freezing air occurs near the surface, while warm air (above freezing) exists above. Thus, snow aloft can melt into rain and then get supercooled just above the surface. When it hits a cold surface it then freezes, thus freezing rain.
In the Northwest, a set up for freezing rain can occur in and downstream of gaps in our mountains. In winter, cold air is often found over eastern Oregon and Washington, with the coldest air prevented from reaching the west by the Cascades. Yes, some air can try to get over the mountains, but it rapidly warms as it sinks on the western slopes (as it is compressed by higher pressure). Only in and immediately downstream of sea-level gaps, such as the Columbia River Gorge or the Fraser Valley of southern BC can primo cold air get across. This is illustrated by the temperature forecast for 1 PM Thursday from the UW WRF system for 925 hPa (about 2500 ft). Cold temperatures are blue and white, warmer temperatures are yellow and light green.
On Thursday, there was cold, dense air east of the Cascades crest, associated with higher pressure. As a result, cold air accelerated westward down the Columbia Gorge, something shown by aircraft observations at Portland that day (see below). The figure provides a time-height cross section, with time increasing to the left (from 4 AM Thursday to 4 AM Friday) and temperature (red lines) and winds shown. Heights are in pressure (850 is about 5000 ft). Strong easterly winds are apparent at the surface, while the winds turned southerly/southwesterly aloft and the air warmed to just above freezing aloft.
With supercooled water falling on Portland, trees picked up lots of ice, causing branches to fall and power outages. And some of the roads became dangerously slick.
Bellingham has had a lot of cold gap flow the past few days as cold northeasterly flow surged out of the Fraser River Valley (which taps cold air in the interior of BC). This morning is no different (see surface map at 11 AM is shown). Below freezing in the northeasterly flow north of Bellingham, result in continued snow over NW Washington. Some of the NE flow is moving up the northern of the Olympics resulting in snow near Port Angeles. In contrast, south of Everett the temperatures are in the low 40s.
A view of the cam at the Peace Arch near the US/Canadian border looks white with fresh snow, and the situation on Hurricane Ridge on the northern side of the Olympic Mountains shows deep fresh snow. Northwest meteorologists must always be thinking of the gaps in our mountains when forecasting snow and freezing rain.
Bellingham has also had several hours of freezing rain, mainly late Friday and Saturday, with a touch of it last night.
So why is the western Columbia Gorge, Portland, and the Bellingham region vulnerable to freezing rain, while it is an exceptional rarity here in Seattle? How can liquid water freeze on contact?
Freezing rain is associated with supercooled water. Strangely enough, water can be cooled to below freezing 0 C (32F) without freezing. This is not a rarity: much of the water in clouds is supercooled. To go from liquid water (where water molecules are moving around but are bound together) to ice (where water molecules are found in a rigid, crystalline structure) does not happen immediately when temperature drops to freezing. Sort of like a preschool class when you tell the kids to sit down....it takes a while. And having a template for the crystal structure from some kind of particle (such as a freezing nuclei) helps things along--and sometimes such particles are lacking in the free atmosphere.
Much of the liquid water in clouds is supercooled (below freezing).
Nearly all precipitation in the Northwest starts as snow aloft, but as it falls in our normally mild air (even in winter) it melts and turns to rain (see figure, left hand side). And if the atmosphere is cold through depth, the snow can reach the surface (right side). But in some situation, a shallow layer of below freezing air occurs near the surface, while warm air (above freezing) exists above. Thus, snow aloft can melt into rain and then get supercooled just above the surface. When it hits a cold surface it then freezes, thus freezing rain.
In the Northwest, a set up for freezing rain can occur in and downstream of gaps in our mountains. In winter, cold air is often found over eastern Oregon and Washington, with the coldest air prevented from reaching the west by the Cascades. Yes, some air can try to get over the mountains, but it rapidly warms as it sinks on the western slopes (as it is compressed by higher pressure). Only in and immediately downstream of sea-level gaps, such as the Columbia River Gorge or the Fraser Valley of southern BC can primo cold air get across. This is illustrated by the temperature forecast for 1 PM Thursday from the UW WRF system for 925 hPa (about 2500 ft). Cold temperatures are blue and white, warmer temperatures are yellow and light green.
With supercooled water falling on Portland, trees picked up lots of ice, causing branches to fall and power outages. And some of the roads became dangerously slick.
Bellingham has had a lot of cold gap flow the past few days as cold northeasterly flow surged out of the Fraser River Valley (which taps cold air in the interior of BC). This morning is no different (see surface map at 11 AM is shown). Below freezing in the northeasterly flow north of Bellingham, result in continued snow over NW Washington. Some of the NE flow is moving up the northern of the Olympics resulting in snow near Port Angeles. In contrast, south of Everett the temperatures are in the low 40s.
A view of the cam at the Peace Arch near the US/Canadian border looks white with fresh snow, and the situation on Hurricane Ridge on the northern side of the Olympic Mountains shows deep fresh snow. Northwest meteorologists must always be thinking of the gaps in our mountains when forecasting snow and freezing rain.
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