Winter storms are often difficult to forecast, especially when it comes to coastal areas.  That is because the coast is often an area where temperatures in the atmosphere are right on the fence between rain and snow. 

 When making a snow forecast, one of the things we look at is liquid water accumulation.  To make this easier to think about, just imagine that this storm was warm enough to be all rain. The liquid accumulation would be the amount of rain in the forecast. 

 There is still a big range of liquid water in the forecast for southern New England.   Many of our computer solutions seem to be honing in on 1 to 3 inches (with isolated 4 to 5 inch amounts).  When translating this to snow amounts, we use a 10:1 snow to rain ratio as a general guideline. That means this 1 to 3 inches of liquid water would translate to 10 to 30 inches of snow.  For inland locations, especially north and west of Providence, we are going with 12 to 24 inches, but some isolated amounts of 2 to 3 feet are possible.

 But here is where it gets tricky.  Every storm is different, so the 10:1 ratio can’t be used for every situation.  For our upcoming storm, temperatures may be mild enough at the coast to create a wintry mix with a combination of a very wet snow, sleet, or even rain.  If this mixing happens for much of the time, we can throw that 10:1 ratio right out the window.  That means that 1 to 3 inches of water might amount to a foot of snow….or even less. 

This first map is a scenario if it stays all snow at the coast.  I don’t go all the way up to 30 inches at the coast because the snow will likely be a little wetter, so the 10:1 ratio is a little bit too high.

This next map is a scenario if a decent amount of mixing to rain or sleet occurs.  Of course, all of this is subject to change so stay tuned! –Pete Mangione