One Last Thing to Consider

Remember the snow-to-liquid rule? 

Generally, one inch of liquid precipitation = 10 inches of snow.

This ratio is true as an average but it is never a rule.  The ratio changes based on the temperature in the lowest layers of the atmosphere, including here at the surface.

We notice the difference immediately, but not in a mathematical way - in a physical way.  Remember the storm in December?  The first thing my friend said when he walked outside and picked some up was: "it's not snowball snow".  Indeed it wasn't.  It was loose and powdery and didn't pack well.  This means that it was more ice crystal than liquid.

The 4-5 inch storm on Tuesday night was different.  That snow packed almost as soon as you picked it up.  It was the kind that stays stuck to tree branches and leaves them looking beautiful in the morning sun.  That snow had a lot of water content.

Our snow from this megastorm will have more water content than the first, but less than the one on Tuesday night.  I expect ratios of 10:1 or maybe 12:1 from this storm whereas Tuesday night the ratios were more like 8:1 and back in December the ratios were 15:1 or more.

From the same amount of water, wet snow produces lower accumulations and dry snow produces greater accumulations.

So - if the snow is "wet", it will pack down and the measured accumulations will be somewhat lower.  If it's mostly "dry" snow, like we expect, then we have a real chance to be in the mid to upper 20's in snow totals from this storm.

At this hour:  moderate snow in Arlington, 33 degrees.  Roads and sidewalks are now getting icy.  Take care out there...