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What conversion functions exist to transfer precipitation forecast in mm to expected snowfall in cm? Simple rules of thumbs as well as more advanced methods would be of interest.

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  • $\begingroup$ Hello from Athens Greece! We are about to face here in Athens tomorrow morning till midnight a heavy snowstorm of approximately 76.2 - 127mm of snowfall in less than 24h with low temperatures at 26.6F rising up to 35.6F along with moderate winds of 8.70-16.78mph, so I want to ask the amount of the accumulated snow that are expected. $\endgroup$ Jan 23, 2022 at 12:29
  • $\begingroup$ @ArisKolovos usually snowfall forecasts will be the amount of snow (not the water equivalent liquid precipitation amount as this question asks about), and your values sound too large to be water equivalent. So you don't need this question. How much will accumulate will depend on ground temperature, wind, sunlight, etc. $\endgroup$ Jan 24, 2022 at 12:26

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The snow density, which is the "conversion function" you are looking for, of fresh fallen snow varies with atmospheric conditions, vapour saturation and temperature since these parameters determine the snow grain morphology (See this figure from SnowCrystals.com; I leave it as a link for copyright reasons). Densities can vary from as low as < 100 kg m-3 for cold conditions to maybe > 400 kg m-3 for wet snow. This essentially cover the spectrum for a normal snow pack. If you, by your question, are thinking about weather services reporting how much snow will come from a snow fall it is by no means a an easy thing to predict since you may "know" the amount of precipitation from a model, but you will have more difficulties knowing the density of the fallen snow unless you devise a classification based on the atmospheric conditions during snow fall (based on snow crystal structure as in the figure). Hence for many purposes, the snow pack thickness is of little value and most will want the water equivalent of that snow, i.e. how much liquid precipitation it corresponds to.

If snow is falling during drifting conditions, the density may become higher than the falling snow itself would have resulted in. This is because with wind drifting, snow crystals are reduced in size by crushing and can hence be more densely packed.

With time the snow pack will densify due to metamorphic processes. This will cause the snow to decrease in thickness with time. But that is perhaps a different question since yours were about fresh snow.

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@Peter Jansson provides a much more thorough answer, but for a quick conversion of expected liquid water equivalent to expected snow depth:

1 inch liquid water = 10 inches snow for warm storms

1 inch liquid water = 20 inches snow for cold storms

or

25 mm = 25 cm for warm storms

25 mm = 50 cm for cold storms

Often a certain climatology will trend one way in these numbers, e.g. maritime winter climates such as in California tend to get warmer storms so you generally get between 10 and 15 in/in and in Utah/Colorado which have continental climates you generally expect 15-20 in/in unless it's late spring.

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