Weather Wisdom

Sleet and snow create hazardous travel this morning

The predicted messy morning is well underway as snow, sleet and rain have moved into the area. The precipitation is coming down moderate to heavy depending on where you are and you can hear the pinging of sleet from Boston to Worcester.


The sleet is going to last for a while this morning meaning travel will be hazardous until the warm air is scoured out at the ground. The image below, from WeatherBell Analytics and shows the frozen precipitation, purple, occurring at 10 a.m. over a wide area. The green, rain, has likely pushed into Boston, at least downtown by then.

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You can follow my updates here and on Twitter @growingwisdom during the storm.

Roads Worse Early
Travel is going to be difficult for a while this morning, but since this event was well predicted the crews are out and roads will mainly wet by mid-morning across eastern and southern areas. If you can wait a bit to leave this morning, it might save you an accident. Sleet is like tiny icy ball bearings and can be difficult to gain traction. Take care when walking as well.

Sleet Counts As Snow
Point of note, when the accumulations of this storm are given after it ends, sleet will be included in the snowfall measurements. So although many wonít have an inch of pure snow, you could see an inch of frozen precipitation, this doesnít include freezing rain. Sleet is 3 to 4 times as dense as snow. This means to get an inch of sleet you need a lot of water and it will be very heavy to move.

As warmer air continues to stream north, the sleet will eventually change to rain. However, over inland areas, especially west of Route 495, the air right at the ground should remain below freezing. This will create a freezing rain situation.

Ice storms are far more dangerous than snow or sleet and in the Pioneer Valley of Massachusetts and northward into Vermont, enough ice could accumulate to bring down powerlines. Todayís situation is marginal. I donít think we are looking at widespread power issues, but if you are headed west, be aware travel on secondary roads could be difficult at best for a while.

icing boston32141.png
Why Not More Snow? The reason for the sleet, not snow, is the air above us is above freezing. As the snow falls from the clouds it falls into this warm layer and melts. Once snow melts to rain it can never change to snow again, but it can refreeze into a ball of ice, sleet, or hit the ground and then freeze, freezing rain. This morning we have sleet early and then later in the morning, there will be pockets of freezing rain, especially west of Route 495.
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The image below is from the weather balloon sent up at Chatham last night. The red line is temperature. If you notice that little sideways V shade where I wrote ďnose of warm airĒ, that is the reason for the sleet this morning.

warm air.png

Once the warm air moves in at all levels, rain will be the primary form of the precipitation. This will continue this afternoon and wash away much of the snow, sleet and ice.

New Year's Weather
The weather is still cloudy Wednesday with seasonably chilly air and highs near 40 degrees. A bit of rain overnight Wednesday will yield a milder and partly sunny day on Thursday. Highs will reach well into the 40s!

New Yearís Eve is pleasant with temperatures falling through the 30s along with light winds. It will be a great evening to head into Boston and enjoy ushering in 2016.

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