Yesterday ( Monday Feb 1 ) we were forecasting about 1 inch of snow for the Twin Cities Metro area yet 3 to 4 inches of snow fell. Today people may be wondering why the forecast was wrong? Forecasting snow is always tricky as the moisture content ( amount of water in the snow is always different ). For example: Rain is rain - a half inch of rain at 40 degrees is the same as a half inch of rain at 75 degrees but snow is very different because 4 inches of snow at 20 degrees is much drier and has less water than 4 inches of snow at 32 degrees. 4 inches of snow falling at 32 degrees will be heavy and wet - the heavy wet snow will quickly compact and may even seem like less than 4 inches of snow even fell. 4 inches of snow at 20 degrees will be light and fluffy and not compact so the depth of the snow can occur much faster.
Last example: 4 inches of snow at 32 degrees compared to 4 inches of snow at 20 degrees may look like this. The heavy 32 degree 4 inch snowfall is only 2 inches in depth and the moisture content is almost a half inch of water. The fluffy 20 degree 4 inch snowfall is 4 inches in depth and the moisture content is less than a quarter of an inch of water.
The bottom line in snow forecsating is in order to forecast snow accumulations accurately you have to look at the variables. The variables are how much moisture is available, what is the tempertaure and how long is the snow going to fall?
Click on this link Snowfall Totals for snowfall totals around Minnesota from the National Weather Service in Chanhassen.
Jonathan Yuhas KARE 11 2/02/2010