Breaking down SE Michigan, Metro Detroit snow forecast

Local 4 meteorologist Paul Gross explains how he forecasts snow amounts



As I mentioned yesterday, our entire weather situation tonight is based upon changing thermodynamics.  Rain falling into dry air and evaporating, as well as snow falling from the clouds and then melting, BOTH cause cooling in the atmosphere.  This is changing our afternoon rain into snow and, as expected, this initial burst of snow is a very heavy one.

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However, that changeover to snow is happening two-to-three hours later than expected.  Since there's only a certain amount of moisture in the atmosphere available to fall as rain or snow, the delay in the changeover will cut back a bit on my expected snow totals (note - later in this article I explain how I forecast snow amounts...it's pretty interesting).

But this doesn't mean we won't have any weather problems. First of all, once the snow starts falling and the temperature drops to near freezing, an icy mess could develop underneath the snow.  That could be really treacherous come Wednesday morning. 

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Second, this will be a heavy, wet snow.  If you are elderly, have heart problems, or are at risk for heart problems, you shouldn't be shoveling this snow. 

As far as total snow accumulation from tonight's snow and the periodic snow we get on Wednesday, I think an average 4-5 inches total snow can be expected across the heart of the metro area.  Those of you in the Winter Storm Warning area will probably average about 6 inches, while those of you in Lenawee and Monroe Counties should see less...perhaps only 3 inches of snow. 

So how do I forecast snow?  There are so many factors with this storm (which is not the typical winter storm that we get around here) that I would have to write an essay for you. But here are the highlights:

The first thing to consider is what we meteorologists call precipitable water.  This is the amount of water vapor in the atmosphere available to be turned into rain or snow.  My computer models over the past couple of days have consistently predicted 0.80-0.90" of precipitable water for this afternoon through tonight, so I was justifiably concerned.

Next is how much of that precipitable water will fall as rain, and how much will fall as snow.  This was the trickiest part of today's forecast, and I explained earlier in the article how the changing thermodynamics impacted things.

Once I decide how much of that precipitable water will fall as snow, I then have to calculate how much that snow will pile up, and that's based upon temperature.  If you recall the snow that fell in those very cold temperatures earlier in the month, those were fluffy snowfalls that accumulated a couple of inches from just one-tenth of an inch of water (a 1-to-20 ratio).  Today's snow is falling in much warmer temperatures...a roughly 1-to-8 ratio...and also will be compacting due to its own weight from the extra water content, so even if we get 0.6" of liquid falling as snow, it'll probably only accumulate 4-5 inches.

Then, I have to take things like lake effect into account, which can add to the snow totals.

There are many other factors that influence how much snow we get (some predictable, some not), but this gives you a pretty good idea how I do it.  Sometimes it's an easy job (last Friday's 2-3" forecast panned out exactly), and sometimes I pull my hair out trying to make my forecast.

By the way, I'll be out reporting live in the elements Wednesday morning on Local 4 News Morning starting at 4:00AM.  I'll have you covered with the most up-to-date weather and road conditions when you wake up ... see you then!

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