First, let's take a look at the forecast. Yes, there was snow in the forecast. Last Friday we had a "40% chance of rain or snow showers". Not very definitive or confident. But by Saturday, we had a forecast for Tuesday of "60% chance of snow". By Sunday we'd increased the probability to 70%. Snow accumulations were expected to be less than 1". So we knew snow was coming, and expected it to be light. Yes, the snow turned out to be more like 1-2", but that wasn't the reason for the slippery commute.
Here's a radar image at 7:20am, showing the narrow band of snow.
At this time of year, timing is everything when it comes to snow, especially if you're talking about accumulating snow. Our snow this morning came by at just the right time. Here's the weather observations from Spokane International Airport (latest time is at the top):
The snow started at 6:41am (actually very light snow started at 6:02am, but we won't spend the time to explain why in this blog). So you can see from the air temperature column, that temperatures Monday at 5:53pm were 35F, and then cooled to 30F overnight before the snow started. We were actually forecasting a low of 26F at the airport. You've probably heard that clouds act like a blanket and trap the earth's heat. Last night was a little cloudier than we thought, so the temperature didn't cool as much as expected.
But for snow to accumulate on the roads, you really need to know what the temperature of the pavement is. This can be considerably different than the air temperature, which is measured at 6 feet above the ground. Luckily, the state Departments of Transportation have sensors that measure the road temperature. So here's a graph of the temperatures at four sensors around the Spokane metro area (some have 2 sensors, so there's 2 sets of dots):
Well, like we said, clouds act as a blanket. As the clouds from the storm moved in overnight, the roads stopped cooling off. But the ground below the roads is still quite warm at this time of year. At our office, we have a temperature sensor 4" below the ground, and it's currently at 40 degrees. The DOT sensors also have sub-surface sensors, and they were reading 50 degrees. This is the left-over warmth from the summer. As we get later into winter, these temperature will also cool into the 30s. But the ground holds onto the warmth, so it doesn't cool as quickly as the air or the surface of the ground.
So what does the sub-surface temperature have to do with anything you ask? When the clouds moved in last night, the surface temperature stopped cooling. And the warm ground below actually warmed the pavement surface during the night. Impressive?
OK, so what? Shouldn't the snow have just melted when it hit the roads? And that brings us to the 2nd interesting point on the temperature graphs. Why did the pavement temperatures cool just before 8am, when the sun was coming up?
In order to melt snow or ice, you need heat, right? As the snow started to fall on the pavement, the 35F pavement melted the snow. But that process removes heat from the pavement. And that's why the pavement cooled a few degrees around 7am. The pavement temperature dropped back to freezing, with water and wet snow on it. Guess what happens. Ice. The melting snow re-froze on the roads, forming a nice layer of packed snow, slush, and ice.
Now, it some areas, the roads were just wet, and were no problem. But the pictures below (compliments of KREM-2 and KHQ TV's web site) showed that snow accumulated on some roads.
Highway 291 between Spokane and Suncrest was closed for several hours this morning due to ice and accidents.
The intensity of the snow also played a role. If the snowfall was very light, the roads would have been able to melt it without cooling, because the warmth from below the roads could have kept the pavement warm. But the rapid snowfall at commute time allowed the snow to accumulate on the roads faster than it could be melted. Car tires aided the process, packing the partially melting snow.
If this same event happened in the afternoon or evening, the road temperatures would have been warmer and all the snow would have melted. And if this event happened in December or January, the sub-surface ground would have been too cold to warm the pavement overnight. Thus, the pavement would have been below freezing when the snow started to accumulated, and it wouldn't have melted. This would have given us the more typical packed-snow that we're used to, which is much easier to drive on.
Like we said, when it comes to snow, timing is everything.