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Don Paul: 4 inches of snow in forecast may not be 4 inches on the ground

Since sticking my neck out has been part of what I do for a living for 42 years, I might as well do it again. In many places, actual snow on the ground by Wednesday evening will be less than what is predicted in this Monday projection from the Buffalo National Weather Service:

(National Weather Service)

I write this not because I believe the forecast will be inaccurate. After all, these numbers are almost identical to what I talked about myself and showed on my broadcasts Sunday evening.

The problem with determining “ground truth” for snow amounts will be our thermal environment. Temperatures will be cold enough aloft, where the snowflakes form. But near the ground, readings will be above freezing at lower elevations on Tuesday. With marginal temperatures come melting. It may snow hard enough for snow to accumulate, but daytime Tuesday temps in the mid- to upper 30s on the Niagara Frontier makes some melting inevitable. Because the snow will be slushy and water-laden with such readings, its sheer weight will cause it to compress, producing some heating and melting at the bottom of the snow. And there will be some melting due to the above-freezing temps of the air.

So if meteorologists know that a 6-inch snowfall won’t end up as 6 inches on the ground by late Wednesday, why predict that amount?

We do it for consistency in our climate records. Meteorologists, observers and trained snow spotters make every effort to measure fresh snowfall before melting and settling have caused reduced snow levels. This often requires multiple measurements with each fresh coating. If we have to wait until melting had taken its toll on the snowpack, we can still take a sample core and melt it for its liquid equivalent to estimate how much snow fell. When that core conversion is done, we need to calculate the liquid to snow ratio. To do that accurately, we still need to measure the observed fresh snow to get an accurate ratio before melting and settling has occurred. This can be done with the snow collected in a manual rain gauge, IF there hasn’t been considerable blowing and drifting. Otherwise, with drifting, a core sample can be taken from the ground.

Many people have heard that 1 inch of rain equals 10 inches of snow. That is a big oversimplification and is inaccurate in many, if not most, snowfalls. The low-density, smaller-flaked snowfalls that occur in frigid snowstorms have much less liquid content, compared to the ratio which will occur in Tuesday’s marginal temperatures. Fluffy, low-density snowfalls have flakes that lie atop one another like lattices and scaffolding, with lots of air space between flakes. In very cold snowfalls, 1 inch of liquid can equal 30 inches of snow.

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In the October 2006 lake-effect storm, with marginal temperatures, the precipitation was very heavy and very water-laden. We saw snow-to-liquid ratios as low as 6:1. That weight on top of foliage still on trees led to the disastrous impact of that snowstorm.

So, how is snow properly measured? Ideally, a snowboard is used, painted white so it doesn’t absorb any weak solar input. The board will be situated above the ground, away from buildings and any reflective heat sources. Measuring directly from the grass can be less accurate, because blades of grass can cause an artificial inflation of the measurement, pushing the snow up higher.

One of the major exceptions to the snowboard application is something that happens often: snowstorms with blowing and drifting. In that case, the best we can do is eyeball an area in which snowfall seems fairly uniform, then use a ruler to make multiple measurements and average them out.

We all have noticed there are days in which multiple snow events occur. This graphic illustrates preferred measurement technique from a snowboard.

Rather than go into infinite detail on accurate snow measurement, here is the tutorial developed by NOAA and its National Weather Service for trained observers and spotters. Weather geeks, soak it up!

As for this week’s snow, it will be elevation-driven. As the moist air is forced to ascend the hills, such as the Chautauqua Ridge inland from Lake Erie, it will cool further and the topography will actually squeeze more moisture out of the air. Accumulations will be much heavier on these hills and ridges, especially those facing into the wind, than at lower elevations. Elevation-driven snow can sometimes leave lower elevations with virtually no accumulation. Several years ago, we had an April storm in which as much as 18 inches of slushy snow piled up on some interior hills, with mainly rain and no measurable accumulation near Buffalo.

This time around, we believe some snow will accumulate at lower elevations, most of it falling Tuesday night into early Wednesday, when the surface temperature drops to the freezing mark.

But even on the hills where plenty will pile up, you ski bums will have to make due with high-liquid-content snow. Translation: not a bit of powder to be found.

Temperatures later in the week into next weekend will be conducive to more melting, especially factoring in more rain by next Sunday.

Heavy, wet snow forecast for WNY through Wednesday; storm warning posted south of Buffalo

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