Rain VS Snow

General Discussions: ASK John: Rain VS Snow

By snomeister on Wednesday, December 24, 2008 - 09:44 am: Edit Post

Hey John, Merry Christmas Eve! Is it easier to predict rain vs snow. I beleive it must be because we do not always get the snow that is predicted, but we always seem to get the rain.


By admin on Wednesday, December 24, 2008 - 10:03 am: Edit Post

It is a little easier to forecast rain vs. snow, but the main reason is because of persons tollerances of the forecast.

Most forecasts call for rain and folks just look for it to rain (or not). They typically do not put much concern into how much rain will fall and most forecasts do not get into the details of how much rain will fall. Thus, a forecast will call for rain and most folks will only pay attent to if it rains or not and not pay attention or care if .10" fall or .75" fall (hence the way you said: we always seem to get the rain, not "we seem to get as much rain as they predict").

Now take that same scenario and switch it to snow. .10" of liquid equivalent snow would be an inch or two in most cases, maybe as much as 3-4" if it is very cold and the snow really fluffs up. .75" of liquid equivalent snow could be as much as 7-10" of snow or maybe even as much as 15" if it is cold and the snow is really fluffed.

I dare anyone to not notice the difference between 1-2" of snow and 7-10" of snow!

When looking at a storm, the models really do not struggle any more with precip that will be snow vs. precip that will be rain. Same level of errors. It is mainly that the errors with snow are much more noticeable.

So I guess in a nutshell it is the perception of the forecast error, rather than the actual forecast error that makes the difference between a rain and snow forecast.

-John


By snomeister on Wednesday, December 24, 2008 - 10:11 am: Edit Post

Wow John, that makes a lot of sense. I bet your exactly right. Thanks for the info.


By jack_in_ct on Wednesday, December 24, 2008 - 03:07 pm: Edit Post

Thanks, John! You actually answered a question I was going to ask within this answer (comparison of rainfall to snowfall amounts).


By sabercat on Sunday, January 04, 2009 - 07:36 am: Edit Post

Hello John,
I don't understand what "liquid equivalent snow" is.
During the summer, if it rains a 1/2 inch, approximately how much snow would that equate to? Assuming typical mid 20's and here in the midwest.
Thank you for your time, Ken


By admin on Sunday, January 04, 2009 - 11:04 am: Edit Post

sabercat-

Liquid equivalent snow is just the amount of water you are left with when you melt down a snowfall or snowpack.

In general, when temps are around 26-30 degrees, the liquid equivalent to snow will be around 10:1 to 12:1, meaning 10 or 12 inches of snow to one inch of water. This is just a generality though. Even within that temperature range it can be as low as 6 or 8:1 or as high as 14:1.

I have seen ratios as low as five to 1 and as high as 40:1.

-John


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