DETROIT -

As some of you have already seen, the advancing area of rain and freezing rain suddenly changed to a lead edge of heavy thundersnow.

The snow will last for roughly an hour to an hour-and-1/2, and drop a solid inch to an inch-and-1/2, before transitioning to freezing rain and rain. Lenawee and Monroe Counties will see mostly snow and rain, with less ice in between. Areas farther north, however, will see more of an ice zone between the rain and snow. But we all should end the day with just plain rain (and possibly more thunder).

Why the snow developed

Before getting into the Thursday night forecast, a lot of people are asking why the snow developed. Chuck briefly explained this concept on the air Wednesday, but it all boils down to something called evaporative cooling. The layer of air over southeast Michigan between the surface and about 7,500 feet aloft was ABOVE freezing, but very dry to start our day. As the area of rain moved northward and started falling into this dry air, raindrops started evaporating. That caused the temperature to drop (that’s why you feel cool when you step out of the shower or a pool -- the water is evaporating off your skin, which cools you).

The temperature in that layer consequently dropped below freezing, and the rain suddenly changed to heavy snow. This doesn’t happen very often here, but I have experienced it before. I saw this dry layer on Wednesday’s computer models, but those models also projected a much thinner layer than we ended up with, so I didn’t have much snow in my forecast.

Thursday night and Friday forecast

Alright, now let’s get back to the forecast and talk about wind. Very strong wind will ramp up with the approaching deep area of low pressure. As I mentioned in Wednesday’s forecast, there are two opportunities to get damaging wind: The first is when the cold front crosses our area (which I expect between midnight and 2 a.m.), when wind gusts could potentially approach 60 miles per hour.

After the front, rapidly rising pressure will generate sustained wind around 35 to 40 mph, with potential wind gusts between 50 and 60 miles per hour. I suspect there will be some power outages Thursday night and Friday morning as a result.

I’ll tweet some updates during the afternoon -- you can follow me @PaulGrossLocal4.

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