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Expect tricky travel conditions Friday morning as freezing rain and sleet set in

04:17
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A transition from mild to cold air Friday will bring a lot of rain and eventually a change to wintry precipitation, causing tough travel and some school closures.

Watch the loop below as the rain changes to freezing rain in purple, and then sleet in orange, before ending. (All of this takes place between 7 a.m. and 7 p.m. Friday.)

A wintry weather scenario is in place for Friday across New England. (WeatherBELL)
A wintry-weather scenario is in place for Friday across New England. (WeatherBELL)

Big picture, a cold front slipping south from northern New England is going to put an end to the melting conditions we've been experiencing the past 24 hours. As this front approaches, an area of rain — some of it perhaps moderate to even heavy — will push through southern New England.

If the front was moving quickly this would be the end of the story and skies would clear. However, a little wave of low pressure will develop along the front after it passes. That means there will still be precipitation falling when the cold air moves in.

Temperatures fall during Friday to below freezing over most areas by early afternoon. (WeatherBELL)
Temperatures fall during Friday to below freezing over most areas by early afternoon. (WeatherBELL)

I expect Greater Boston temperatures to go below freezing between 6 a.m. and 9 a.m. Friday. That's when the rain will transition into freezing rain and eventually sleet. It may end as a brief period of snow. But the main concern is that freezing rain and sleet.

A cold front will sweep the mild air out to sea Friday. (NOAA)
A cold front will sweep the mild air out to sea Friday. (NOAA)

I don't think we're going to see a lot of accumulation with this system, but there will be enough to cause icy travel. The map below gives an idea of what to expect, with the most changeable conditions occurring during the morning commute and throughout the first part of the afternoon.

I expect the precipitation to come to an end before the evening commute. By then, everybody will be below freezing and there will be many slick areas.

Expect lots of changes on Friday as colder air arrives from the north. (Dave Epstein)
Expect lots of changes on Friday as colder air arrives from the north. (Dave Epstein)

Here's why we're getting freezing rain and sleet: Temperatures will be below freezing at ground level, but above freezing at higher levels of the atmosphere. If you went above that, it would be cold again.

Think of it as a "warm-layer sandwich." That warm layer melts the snowflakes, and since a snowflake can't turn back into snow after it melts, the rain either freezes into sleet pellets or when it hits the ground.

The profile of the atmosphere Friday shows a warm air sandwich wich creates freezing rain and sleet. (TropicalTidbits)
The profile of the atmosphere Friday shows a "warm-air sandwich," which creates freezing rain and sleet. (TropicalTidbits)

We will clear out for Friday night. Saturday and Sunday both look cold and dry with plenty of sunshine. If you don’t remove the slop that falls Friday, you will have to wait until later next week for readings to get above freezing again.

Related:

Headshot of David Epstein

David Epstein Meteorologist
David Epstein was WBUR's meteorologist.

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