While us Seattle Metro area residents see heavy rain today and tomorrow, our counterparts in East Portland, OR, the Gorge, and even the Cascade Mountains and Eastern Washington are seeing a significant Ice Storm event. Ice Storms are not common in this area, and a significant event such as the ones unfolding nearby are relatively rare. Occasionally as we transition from colder weather back to warmer weather via a warm front, sometimes the precipitation can begin as snow or freezing rain before transitioning to plain old rain. More often than not, this transition period is short and the potential for ice accumulation is slim, maybe seeing a tenth of an inch of ice before the transition is complete. However, occasionally the situation is a bit different, and cooler air lingers at the surface for a variety of reasons, and ice rapidly accumulates. This is what is happening in Portland currently. We will get to why that is, in just a little while.

First off, let’s start with “What is Freezing Rain?” Freezing rain only occurs when the weather conditions are just right, and the conditions that support such precipitation can be hard to get all at once here in the PNW, hence freezing rain being relatively rare in our area. All precipitation starts out frozen when it begins to fall from the clouds above. With normal rain, the layer below the cold cloud layer is much warmer so the frozen precipitation immediately turns into liquid and falls to the surface, reaching the ground as rain. Ice begins to form when frozen precipitation first falls through a layer of above freezing air and melts, before falling through a smaller layer of sub freezing air closer to the surface. When the precipitation passes through that secondary sub freezing layer, and refreezes in the air BEFORE hitting the ground, it will land as sleet. However, if the precipitation continues to fall through that layer without the drops themselves freezing, it can be “supercooled” to at or below freezing temperatures. When a supercooled droplet hits a surface that is at or below freezing (such as pavement, trees, powerlines, etc), it instantly freezes and coats the surface. This is why it is called freezing rain.

Here’s a quick little diagram I made. It’s not fancy, but it will give an idea of the layers and how sleet and/or freezing rain can form. The top layer in the clouds always starts with frozen precipitation, as snow. As it falls through the layer of warm air, it melts into rain drops. Finally, as it passes through one last layer of colder, sub freezing air, it either freezes back into sleet, or supercools, and will freeze immediately when it hits an already sub freezing surface.

So what’s the big deal with freezing rain, you ask? Well, that coating of ice can add up quickly if the conditions are right. Ice accumulations can cause a variety of issues. Freezing rain coats everything with glaze ice, causing dangerous walking and driving conditions on roadways, tree branches and sometimes even entire trees can be affected by the weight of the ice and can snap resulting in widespread power outages, and power and telephone lines can fall or break due to the weight of the ice all on their own. Trees don’t necessarily have to fall on power lines to create issues in an ice storm, the ice itself can be just as damaging when accumulations become significant. Over one span of power line with a quarter of an inch of ice can be 500lbs heavier than normal. That is a drastic amount. Ice storms can easily cripple an entire city if the accumulations surpass even a small amount such as one tenth of an inch or more. A big concern during ice storms is power outages. When temperatures are hovering just around freezing, people without power are susceptible to cold related issues, if they have no other heat source than electric heating. Pipes are also susceptible to freezing, and can burst.

Now, let’s take a look at Portland’s situation today, as an example. They are just coming out of an epic snowstorm last week that brought various amounts of snow to the area, ranging from 7-16 inches on average. That’s a lot of snow. Keep in mind that there is still snow on the ground as temperatures have stayed at or below freezing most of the week. Portland sits just southwest of the Columbia River Gorge, which plays a major factor in why this ice storm is occurring. The warm front moving through the area has moderating temperatures, however, there are very cold and strong east winds (winds originating from the east but traveling west) funneling into the Eastern Portland Metro area, which is keeping the temperatures right at or just below 32 degrees at the surface. The mid level atmosphere (where the warm front is traveling), is a warmer layer. So, the above graphic shows what takes place with the precipitation when it falls through the atmosphere toward the surface, resulting in freezing rain in areas where the surface temperature is at or below freezing. Ice accumulations in the gorge are of the biggest concern, where that cold east wind is ripping through, above 60 miles per hour or more. Here’s a picture from Anthony Sigliani of WeatherBug, showing how the gusts funnel through the gorge. See if you can find the 102 mph gust at Crown Point!

Currently, I-84 from Troutdale to Hood River is closed because of the potential for significant ice accumulation. There is already a dangerous sheet of ice in that area. The Gorge is very susceptible to ice accumulations right now, and the Oregon Department of Transportation is bracing for a crippling event. Ice accumulations could reach an inch or more. Remember earlier when I mentioned even a quarter inch of ice along a normal span of power lines can add 500lbs? Think of an inch or more worth of weight on those lines, it could be devastating. That kind of accumulation can bring down entire lines and even transmission lines which can cause widespread, prolonged power outages. The safety of lineman is a high priority, and until the ice begins to melt, they often are not able to begin any repairs.

Let’s take a quick look back at history.  Here’s a quick recap of some of our most serious Ice Storms across the United States. I used Wikipedia’s “Notable Ice Storms” article as a reference.

January 1961: Northern Idaho recorded an incredible 8 inches of ice from a single storm, a record which still stands today!

February 1994: Tennessee, Mississippi, and Alabama sustained a combined 1 billion dollars in damage from devastating ice storm.

January 1998: From January 5th-9th, the “North American Ice Storm of 1998” wreaked havoc throughout Nova Scotia, Eastern Ontario, Southwestern Quebec, and the Northeastern United States. This remains one of the most disruptive ice storms on record, leaving over 3 million Canadians in the dark for up to a month and a half.

Some areas of the Cascade Mountains and Eastern Washington are also seeing an ice storm over the next day or so, as temperatures are hovering just around freezing and the conditions are prime for freezing rain. If you are traveling to eastern Washington, or south towards Portland tonight or tomorrow (Tuesday-Wednesday 1/17-18), check for road closures and current conditions before you travel! Ice is incredibly dangerous on roadways, drive slow and safely in areas with a coating of ice. Ideally, it is best to pull over and wait for roadways to thaw.

Here are a few of my own personal images from various ice accumulations. The first isn’t from freezing rain, but it’s an idea of how the ice can coat a tree branch:

In these images, you can see how ice has coated the branches, and in the black and white image, a frozen droplet has formed. While ice storms can be beautiful, they can also be incredibly devastating. We had a very minor ice storm with minor accumulations here in the Seattle area in 2012. One of the things I distinctly remember, was sitting and listening to the sound of the ice clinking in the tree branches as the wind blew through it.

Here is a video of trees cracking after an ice storm, found on Youtube.