A New Look at Snowfall Forecasts for Southeast Michigan

Snowfall and rainfall amounts typically occur in a range of values over a set time period.  For example, let us imagine that a snow storm has hit the area and dropped anywhere from 6 to 9 inches of snow in Wayne County.  It would be nice to know in advance of the storm what the probability is of hitting that 9 inch mark, since obviously snowfall amounts can vary tremendously over very short distances.  Consider another example when the rain/snow line is located across Wayne County with significant snowfall expected just north of this line. The forecast might call for less than an inch of snow, while a lesser chance still exists of exceeding 4 inches.  Again, this range of possible outcomes in advance could be useful to a wide variety of residents across Southeast Michigan .    

In the National Weather Service’s forecasts, snowfall is the average amount of snow expected over a given time period (usually 12 hours).  For instance, a 5 inch snowfall in the forecast gridded database would be expressed as a 4 to 6 inch snowfall in the text based forecasts.  Despite our best efforts, we all know that there is always going to be a margin of error in any forecast.  It would be nice to be able to somehow estimate what the probability is of that 4 to 6 inch forecast turning into 10 inches of snow.

The science team at the Detroit/Pontiac forecast office has developed a method to assign probabilities of specific snowfall and rainfall amounts being exceeded and have placed this information on the web.  Using the above snowfall forecast as an example, this web page would actually give a probability of the above forecast amounts exceeding 10 inches.  This was accomplished using statistical analyses and is located on our web page at www.crh.noaa.gov/dtx/ProbSnow.php.   

You will notice on this page that the snowfall values are displayed on a map of the 17 counties in the Detroit/Pontiac county warning area. The forecast periods are broken into columns. At the top of the page, the mean, minimum and maximum values are displayed. The bottom of the page features the probabilities of exceeding certain values. Please note at the top of the page that the duration, min, max and start and end days are all adjustable. By clicking on the button that says prob SNOW county time series will display a time series of the probabilities for each county. The buttons that say prob QPF postage stamp maps and county time series show exactly the same type of info as the snowfall, only they are for liquid precipitation (either liquid equivalent snow or pure rainfall amounts).


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