depends on your climate and the specific type of snow
light fresh snow could be 1 inch rain = 1 foot snow

Thanks cvg, I've assumed 1/12 for my rough calcs.

Do you have a source I can cite?  No offense, but I'll be sealing these calcs eventually.

Have a great weekend.
--Steve

I don't have a source, as I am working in Phoenix and not normally concerned with snow...

The most common conversion used is that 10 inches of snow will melt to one inch of water. That is a very rough approximation, however. The density of snow on the ground depends on many factors. The conditions of temperature and humidity in the cloud determine the type of snow crystals that form.  In summary, 0.1 inch of water can yield as little as 0.4 inch of snow or as much as five inches of snow under extreme conditions. More commonly, 0.1 inch of water yields from 0.6 to 1.1 inches of snow.

Snow water equivalent can be presented in units of kg/m2 or meters of depth of liquid water that would result from melting the snow. SWE is the product of depth and density:
SWE = depth (m) x density (kg/m3) (units: kg/m2)
SWE = depth (m) x density (kg/m3) / density of water (kg/m3) (units: m)

Recommend The Snow Booklet by Nolan J. Doesken and Arthur Judson. Published in 1996 by Colorado State University
or
http://www-nsidc.colorado.edu/data/swe//

Be careful with your calculation as a conversion.  You must also consider a worst case snow melt as well - frozen ground, very warm or even rainfall precipitation for a relatively low time period.  In this scenario, widespread flooding can occur in the watershed in a similar fashion as a very intense rainfall event.

From experience, a very typical winter (snow fall), but a short and intense melt period can cause considerable stresses on a drainage system.  Use a very conservative approach with this conversion.

KRS Services
www.krs-services.com

Snow melt is the melting of compacted, thawed and refrozen snow. Snow melt doesn't run off until it percolates through the entire depth of existing snow. I have seen the snow melt equal to 75% of a rainfall event. Under estimating snow melt has been a major problem of hydrologists since they started estimating. I would use .8 inches of water for each inch of lost snow. Just as an aside, next spring in Feb. and March take samples of undisturbed snow fields. Take it in the lab and melt it. the snow collected early will have less water per inch than the snow collected late in the season. Disturbed snow will have a higher water content.

For all things snow and ice:

http://www.crrel.usace.army.mil/

That would be the Cold Regions Research and Engineering Laboratory in Hanover, NH

Check with your state structural engineering association and see if they have a snow load manual.  The WSEA in Washington state upgraded their manual a few years ago to account for the increased density of snow on the bottom of a snowload vs the snow at the top.  If there is a similar manual for PA that might give you a pretty good estimate just convert weight to cubic feet of water.  The other advantage a manual like this has is it is based on a lot of historical data and it has data broken down for probably 100 sites.

DPA