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Metro Detroit weather: Details on weekend's winter storm

First look at Christmas weather

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DETROIT – I talk about this every winter, and it’s appropriate to talk about it again today: salt doesn’t always work.

In one of the chemistry courses I took as a meteorology undergrad at the University of Michigan way back in -- well, way back -- we discussed the properties of salt. One of those properties is that the chemical reaction that occurs when salt encounters ice slows down considerably as the temperature drops farther and farther below freezing, to the point where, when it approaches or drops below 10 degrees, or minus 12 degrees Celsius, salt is basically rendered ineffective (keep in mind that this is at night. Salt is effective at these colder temperatures in the daytime due to a boost from solar radiation).

So, Wednesday night, after those snow squalls moved through, the road crews did a pretty good job salting. During the evening, the salt worked. However, some of the brine (saltwater) on the pavement refroze in places as temperatures plummeted. Snow that didn’t get salted until much later really didn’t melt much due to the cold temps. Fortunately, the sun came out today, so that residual salt started doing its job. However, temperatures will be even colder tonight, so watch out for refrozen damp pavement that salt melted during the day today.

Initially clear skies Thursday night will start to add some cloudiness, mainly after midnight. This will be a much better radiational cooling night than last night due to the lighter wind, so expect temperatures to drop to near 3 degrees (-16 degrees Celsius) in the city and colder in the suburbs during the time skies are clear. Once the clouds increase, temps will come up a bit toward dawn. Southwest wind at 4 to 8 mph will create wind chills of -5 to -10 degrees (-20 to -23 degrees Celsius) for kids at the morning bus stop.

Skies will become cloudy on Friday, with a chance for a bit of light snow by late afternoon -- hopefully with minimal impact on the afternoon rush hour, but it’ll be close. Highs will be around 20 degrees (-7 degrees Celsius). South to southeast winds at 7 to 12 mph will create wind chills between 0 and -10 degrees (-18 to -23 degrees Celsius).

Friday’s sunrise is at 7:56 a.m., and Friday’s sunset is at 5:02 p.m.

Snow will rapidly increase Friday evening, and will become moderate to heavy at times. Average accumulations across the area should be around 3 inches south of I-94, and 3-to-5 inches from I-94 to M-59, and 5-to-6 inches north of M-59. Temperatures will rise Friday night, and we’ll start our Saturday in the mid-20s (-4 degrees Celsius).

Saturday is still tricky, but some things are finally coming into better focus.

Snow will change to sleet and freezing rain, and then to rain for some, but not all. I’ve looked at a lot of computer model upper air forecasts this afternoon, and it appears that the rain-snow line may make it to 8 Mile, or even a little farther north -- perhaps halfway up into Livingston, Oakland and Macomb Counties.

I hope that Friday's models give me better confidence in this, but, based upon today’s models, those of you north of M-59 may see mostly snow, or only have a brief changeover.

Even though precipitation amounts will be lighter during the day on Saturday than on Friday night, our North Zone may pick up an additional inch or two of snow Saturday into Saturday night. My one concern is if the models push of warm air doesn’t verify as strong, that would give more of us a shot at some freezing rain. We certainly don’t want any ice on our trees, because it’ll become breezy Saturday night into Sunday morning, and that would bring down some limbs and create power outages. I’m not saying that this will happen, just that it’s a possibility I need to monitor.

Meanwhile, those of you in our South Zone could end up being mostly rain during the day Saturday. One thing I can say for sure is that we’ll finally get a reprieve from the bitter cold (for one day, but we’ll take it), with highs reaching the low to mid-30s (0-2 degrees Celsius).

To help you plan, here’s how our in-house RPM model is depicting the scenario:

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Saturday night should be all snow and snow showers for us, with some minor accumulation possible.  Temperatures should start falling sharply behind the cold front passing through, and bottom out in the mid-teens (-9 degrees Celsius).

It will be breezy and colder on Sunday, with some sunshine developing, especially by afternoon. Temperatures will only rise a few degrees into the upper teens (-7 degrees Celsius).

Christmas week travel weather

Monday, Tuesday and Wednesday all look dry for travel around the state, with mostly sunny or partly cloudy skies. After a bone-chilling start Monday morning with temperatures near 0 degrees (-18 degrees Celsius), highs rise into the mid-teens (-10 degrees Celsius), then into the mid-20s (-4 degrees Celsius) on Tuesday and into the low 30s (0-1 degrees Celsius) on Wednesday.

A weak weather system might pass through on Thursday with some snow showers. The nature of this system is highly uncertain right now, but today’s models aren’t portraying anything significant. Highs will be near 30 degrees (-1 degree Celsius).

Friday looks partly cloudy, as does Christmas Eve day. Christmas Day is also looking dry right now. Keep in mind that this is pretty far in advance, and could change. If you have travel plans, check my weather article each day in case there are any changes.

Humble thanks

I have received overwhelmingly positive comments and feedback (some national) for Thursday's climate change webcast. During the webcast, it was the number one item being viewed on ClickOnDetroit.com, and its Facebook reach was nearly 350,000 people (I’m not exactly sure what that means, but the newsroom tells me it’s significant).

I’m grateful that so many of you took the time to watch, and I’m hearing that a lot of people are sharing the link to the recorded version so family and friends can learn the truth about global warming without politics or preaching.

If you missed the webcast and want to see it, click here.

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