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Metro Detroit weather: Coming snow, freezing rain trigger winter weather advisory

Winter weather advisory extended until 8 a.m.

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DETROIT – Snow will increase across the area late this afternoon and evening, and then begin a transition to sleet and freezing rain before changing to all rain toward morning. This transition will occur from south to north, as this is the direction some warmer air is coming from.

So, those of you in our southern areas will see the changeover to ice and then rain earliest, while those of you in the north will see the changeover last -- perhaps not even until late tonight. And you’ll also get the most snow -- perhaps 2 to 4 inches.

Because of the snow and ice that will fall tonight, the National Weather Service has issued a Winter Weather Advisory until 8 a.m. for all of southeast Michigan. As we discussed yesterday, our one concern for tonight was how dry the air would be when the precipitation arrives.  

Well, those fears were realized this afternoon when Ben Bailey and I started looking at surface maps, which showed temperatures in the mid-20s (-4 degrees Celsius), but dewpoint temperatures only around 12 degrees (-11 degrees Celsius).  

Remember, the greater the difference between the temperature and dewpoint, the drier the air is. And the drier the air is, the greater the cooling is (or slowing of the warming) as the precipitation develops and starts evaporating or sublimating into that dry air.  

So that’s why the changeover will take longer than we expected yesterday. By dawn Wednesday, most of us will be near or a little above freezing. Wind tonight will blow from the south-southeast at 15 to 20 mph -- if we end up with a higher-end ice accumulation, then that wind could bring down some limbs and power lines. Hopefully we’ll stay just below that threshold.

Alright -- let’s dive a little deeper into this and talk about why we’re seeing tonight’s wintry mess.  Take a look at this graphic:

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When the entire lower atmosphere is below freezing, it’s pretty easy to understand that everything that falls from the clouds will be snow, and will stay snow all the way to the surface.  Now, let’s introduce an increasing wedge of warmer, above freezing air, aloft:

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In this graphic above, south is to the LEFT and north is to the RIGHT.  The warmer air is moving in from south to north (left to right).

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Far out ahead of the warmer air, where there’s only a thin slice of it, the snow falling from the clouds stays all snow.

But, as that warm air wedge thickens a bit, the snow falling from the clouds melts, but then falls back into the below freezing air and freezes into little balls of ice. That’s called sleet, and is what you hear pinging off your windows when it falls.

Then, as the wedge gets even thicker, those snowflakes falling from the clouds melt into drops, but don’t have time to freeze before reaching the ground. However, the air temperature near the surface is still below freezing, so those drops freeze on contact. This is called freezing rain.  

Finally, as that warm air wedge thickens to the point where it’s above freezing from the cloud to the surface, everything falls as rain.  So, the progression you see from right to left on the above graphic is what will be happening aloft here tonight.

Wednesday

Although there still could be some wintry weather far north of I-69 first thing Wednesday, most of the day will be rain for us. Highs should approach 40 degrees (4 to 5 degrees Celsius) in the southeastern half of the metro area, with the northwestern half probably holding in the mid to upper 30s (3 degrees Celsius).   It’ll be breezy -- perhaps even windy by the end of the day.

Wednesday’s sunrise is at 7:55 a.m., and Wednesday’s sunset is at 5:36 p.m.

Cloudy Wednesday night, with lows in the mid 20s (-4 degrees Celsius), which means that any standing water could potentially freeze into ice by Thursday morning.

Thursday

It will be mostly cloudy on Thursday, with some snow showers possible by mid to late afternoon. Highs will be in the low 30s (-1 to 0 degrees Celsius).

Temperatures then will go way back below average through the weekend and through next week, with periodic snow shower chances. Monday, however, could be the day we see the next snow that we have to shovel. Stay tuned.


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