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Analysis of Wednesday’s Snowfall

Posted at 12:47 PM, Dec 27, 2012
and last updated 2012-12-27 14:23:31-05

GRAND RAPIDS, Mich — All it took was one band of snow at the wrong time to cause a bit of chaos on West Michigan roads Wednesday evening.

The snow started picking up during the afternoon and spread from southern Michigan into much of West Michigan just as the evening commute started.  The initial burst of snow made highways icy and slippery before crews could start to get salt or chemicals down on the pavement.  Numerous major accidents were reported from the Grand Rapids area through Kalamazoo and elsewhere.

Without getting too deep into the meteorological details, a band of heavier snow developed on the very back edge of the overall storm system that moved through the Midwest.  Here are some radar images from that time, starting at 3:00 PM Wednesday.  You see the leading edge of the snow just entering the Grand Rapids area:

At 6:00 PM, the main band stretched from around South Bend and Cassopolis up through much of Kent County:

By 11:00 PM, snow was diminishing for most of the region:

When it was all said and done, most of West Michigan ended up receiving about one to three inches of snow on average.  Here is the snowfall reports map from the National Weather Service office in Grand Rapids.  You can see that the amounts increase once you get past Jackson, as most of the Detroit metro area picked up five to eight inches of snow.

Now, let’s go back and compare that to what some of the forecasts were.  Wednesday morning, I was using the FOX 17 Future Track HD computer to show estimated snow amounts:

While it did forecast that band of one to three inch snowfall, it placed it about 50 miles too far to the south and east.  The snow that was expected along I-69 shifted westward back into the US-131 corridor.  Here is another, more complicated graph showing the snow forecasts from several different computer models for Grand Rapids:

Four different runs of computer models agreed that Grand Rapids should pick up about an inch of snow.  So where did things go wrong?  Well, for one, they did get the timing of the snow correct, falling mainly from mid-afternoon through the evening and ending by around midnight.  But they underforecast the amount of precipitation that fell.  The official liquid-equivalent amount in Grand Rapids ended up as .17″, where most forecasts were for around .10″.  Those extra hundredths of an inch may not sound like much, but they can mean substantial differences in snowfall when you compound them with changes in another factor– snow ratios.

The snow ratio is the expression that compares the amount of measured snowfall to how much melted liquid it produces.  Tradition teaches many people that an inch of rain is equal to a foot of snow (or ten inches, as some learn the saying).  That gives a snow ratio of 10:1 (ten inches of snow from an inch of liquid) or 12:1 (a foot of snow from an inch).  However, snow ratios can range from only two or three to one (heavy, wet, slushy snow) to twenty or even thirty to one (extremely light, fluffy snow).

Despite recent advances in techniques, snow ratios are notoriously difficult to forecast.  So, even if the precipitation amount was correct, the computer forecasts for Grand Rapids would have likely only told us to expect 1.7″ to 2.0″ of snowfall.  The actual total was 3.1″, and that extra inch can be credited to how “dry” or “fluffy” the snow was Wednesday night.  The dynamics within that band of snow were just right to create bigger flakes with a lot of air in them.  So rather than 10:1 or 12:1, the actual snow ratio was around 18:1.  Again, minor differences like that can really change a forecast.

Outside of the weather itself, several manmade influences exacerbated the overall impact of the snow.  First, roads had not been pretreated, and without much snow this season so far, did not already have some residual salt or chemicals on them.  Thus, the road conditions deteriorated more quickly than they may have otherwise.

Second, without any snow of this magnitude yet this season for most of the area, drivers are still not adjusting to winter habits.  Police departments tell FOX 17 every season that the biggest factor in weather-related accidents is overconfident drivers; that is, people going too fast for conditions or not leaving a safe following or stopping distance.

West Michigan drivers should get plenty of practice, though, in the coming week.  Snow may leave another inch or two of accumulation around the area Friday afternoon and evening, with additional snow in the forecast as colder air moves in for much of next week.  You can check out the full forecast on the Weather page.