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Tuesday, December 3, 2013

One Piece Of The Puzzle

I think rain will change over to a mix of snow and sleet and then eventually snow Friday night into Saturday morning. Here's one of about a million things I look at when making a forecast.

You're looking at a cross section of the atmosphere modeled by the ECMWF or "European" model. On the Y axis you have pressure in MB. As you go up in the atmosphere, the pressure gets lower. So the bottom of the y axis would show you what's going on at ground level. The x axis is location as if you're looking at a slice of the atmosphere from New Haven (far left) to Windsor Locks (far right). The colors show temperature at various levels of the atmosphere.  Anything above 0 degrees is painted in green and yellow.

Notice how Friday night we start off as rain because as snow falls from the clouds it enters a big layer of above freezing temperatures and melts.
Rain Friday night
Here's where things get a little more complicated. Look at New haven on the left side first. Snow falls into a warmer layer and melts into rain.  Then just above the surface the rain refreezes before reaching the ground, likely forming sleet. In Windsor Locks on the far right, snow only falls into a marginal layer of above freezing temperatures.  Here the snow may actually survive the trip down to the ground.

Changing to Sleet/Snow
Then by Saturday morning the  layer above freezing is completely washed out allowing any leftover snow to reach the ground!
Snow/Sleet mix to all snow

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