Why was Thursday's snowstorm such an icy mess for commuters?

For anyone driving on the roads in the Spokane/Coeur d'Alene metro area on Thursday (16 Dec 2021), the roads were rather icy in spots.  The Spokesman-Review published a few pictures online.  More than 100 accidents were reported by Spokane Police Department, leading them to urge people to stay home if possible.  

An Ice covered Grand Boulevard created the conditions for this multi-car pileup that also included a STA bus that slide into a power pole in the southbound lanes below 12th Ave, Thursday, Dec. 16, 2021. No one was injured, but southbound Grand was shutdown until tows could be arranged and the road de-iced by the city.

Colin Mulvany The Spokesman-Review

The snowfall amounts from this event weren't particularly noteworthy.  Here's a map of snowfall reports from the CoCoRaHS website.  In the Spokane area, 1 to 3 inches of snow fell.  Over in the Idaho Panhandle, snowfall was much more varied, with a few reports of 5 to 6 inches, but most folks in the Coeur d'Alene area received 3 to 5 inches.  In other words, this was a fairly typical snowfall event for our area.

Additionally, the temperatures were rather cold, and the snow was dry.  The kind of snow that is difficult to make into snowballs.  And that kind of snow usually doesn't cause icy roads.  So what caused yesterday's treacherous icy driving conditions?

To answer that question, we need to look at the road pavement itself.  Or more specifically, the temperature of the pavement.  Thankfully, in recent years the Departments of Transportation (Washington State DOT, Idaho Transportation Department) have installed pavement sensors at various locations.  These sensors measure not only the temperature of the pavement, but they can also sense whether there is moisture on the pavement, and if it's in the form of liquid or ice.

You can see one of these sensors in the pavement on the Highway 395 camera near Loon Lake.

Here's a graph of the temperatures on I-90 at the Perry Curves.  Note that the temperature was around 29F during the night, but then it warmed to 34F shortly after 9am.

According to the weather observation at nearby Felts Field, light snow started to fall just before 10am, and continued until 4pm, with air temperatures of 31F or 32F.  So as that morning snow began to fall on the road pavement, the 34 degree pavement was able to melt some or all of the snow, resulting in wet or slushy pavement on well-traveled roads.  But note that the pavement temperatures began too fall just after noon, which is typical for pavement temperature.  The pavement cooled below freezing around 230pm, and all of the moisture that was on the roads quickly froze, causing the icy conditions that snarled traffic.

Looking at the sensor at I-90 and Liberty Lake, we see a little different situation.  At that location, the road temperature was around 28F at sunrise, but only warmed to 31F during the morning, and then quickly cooled back to 30F during most of the snowfall event.  This cooling occurred right after the snow started.  In other words, the pavement remained below freezing for the entire snowfall event, so it's likely that there was little or no melting of the snow, and conditions at this location probably weren't as icy as other parts of the metro area.

So the icy roads on Thursday took a careful combination of several factors. The air temperatures were cold and stayed below freezing.  The mid-morning timing of the snow allowed some of the roads to warm above freezing, which melted and then refroze later in the afternoon.  Locations either north or south of the Spokane metro area either had the snow start too early to allow pavement melting, or started too late to melt and refreeze.  Had the snow in Spokane started a few hours earlier, the outcome would likely have been much different than what we experienced.