Snow forecasts: Why the spread

Of all the progress made in meteorology, nailing snow totals remains an elusive pursuit.

The old standard that on average an inch of melted precipitation would yield 10 inches of snow – a snow-liquid ratio of 10 to 1 -- has long since melted away.

That conversion still appears on the National Oceanic and Atmospheric Administration’s snow-liquid table.

And simply applying the conversions in this table, given the temperature forecasts tomorrow’s snow ratio would be about 15:1.

“That sounds about right,” said Alex Sosnowski, senior meteorologist with Accu-Weather Inc., which is calling for 3-6 inches around Philly.

That would pop out of 0.24 inches of liquid, which would convert to 3.6 inches of snow. A little more liquid, and-or, a little more fluff would mean more snow, thus the forecast range.

That National Weather Service sees twice as much precipitation, 0.47 inches. Using the same 15:1 ratio, that equals just over 7 inches, right in the middle of the official 5-to-9-inch call.

Would that the atmosphere behave so simply and linearly.

Such 15:1 ratios are unusual around here. Tony Gigi, meteorologist at the National Weather Service in Mount Holly, found only six such cases in a study of 56 storms of 6 inches or more since 1950.

He found none that came in at 20:1, although the 30.7 inches recorded officially on Jan. 7-8, 1996, came mighty close.

One thing that snow researchers have found is that ratios depend on more than just the temperature at the surface.

Flakes vary in shape and in liquid content. Their sizes, shape, and type are determined during a crystal's long descent to the ground through layers of air of different temperatures, moisture levels, and wind speeds.

The best accumulators are dendrites, those classic hexagons so often replicated in holiday decorations. They are full of air and tend to pile up quickly.

All superficial indications are that Tuesday’s snow will have a high snow ratio. The storm is a “clipper” diving from the northwest, with the ocean getting involved at the end. And clipper snows tend to be less soggy than storms that draw the bulk of their moisture from the Gulf or ocean.

Temperatures are going to be quite low, in the teens for awhile during snowfall tomorrow night, and it will be cold through the atmosphere. All other things being equal, that should help the accumulation totals.

But all other things in the atmosphere rarely are equal; thus, the 3 to 6, and 5 to 9.

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