That's a toughy sometimes…Because the dividing line between when rain freezes and snow melts is 32 degrees Fahrenheit. You could be forecasting the exact correct amount of precipitation (which would be all forms of water falling from the sky melted down into liquid), but still be 'wrong' because you didn't get the right precipitation types down.

Think of it this way, in a typical storm event, let's just say 1" of liquid precipitation falls from the sky on an area. Well, if it were all rain, that would be simple-- 1" of rain would be in the forecast. If, instead, the precipitation is frozen as snow, then there is less water content in each snowflake than there is in each rain drop (obviously). Normally, the ratio is 10:1 …. Which means for every 1" of liquid precipitation, 10" of snow would have to fall and be melted down to equal that. But…even that isn't constant, because it varies by air temperature. (The full breakdown of that can be found on our website here at http://www.cbs3springfield.com/weather/classroom/weathercharts - just look for 'liquid equivalents). Not only all of this, but there's also sleet, which has it's own ratios that have a little more water content per pellet than a snowflake, and furthermore 'freezing rain', which falls like rain but freezes as ice as soon as it hits the ground. In a storm like the one we're talking about, perhaps there'd end up being as much as 1/4"-1/2" of ice as a result (which would be huuuge problems).

So a forecaster has to figure out what the temperatures are going to be during the event (because if they change, the precipitation type can change too). Then they need to figure out just how much total precipitation is going to fall (and when, so they can line it up with the temperatures to see how much of what type we'll get). Keep in mind that the temperature on the ground is just one element. A little higher up in the sky, the temperature is different, which can greatly affect what type of precipitation falls from the sky (these different temperatures with altitude is the ONLY way to get sleet / ice to fall from the sky).

So once the forecaster comes up with that, they can issue a forecast to the viewer, but keep in mind, there's a lot of viewers all across diverse terrain; some up high, some in valleys, and their experience is going to differ. It's almost impossible to pinpoint it for everyone, which is why a forecaster would break it down into manageable ranges by area-- something like 2"- 4", etc.

But, any little mistake with temperatures and such, and the forecast looks TOTALLY different (for example, 1" of rain looks nothing like 10" of snow, even though it's the same total liquid precipitation).