If you ask a forecaster what is one of the more difficult types of situations to predict they would likely tell you it’s a warm front.  A front of this type brings with it a surge of warm air no matter what the season  However, in winter, this often presents a mixed bag of precipitation ranging from rain to freezing rain to sleet and snow.

The exact and temperature profile of the air from the ground up to where precipitation is falling becomes critical when making a forecast within a warm front.  The image below gives you an idea what things will look like this evening from just south of the Maine-New Hampshire border and on up to interior southwestern Maine.

Warm fronts in winter typically bring a mixed bag of weather

Warm fronts in winter typically bring a mixed bag of weather. NOAA

Later today, as warm air rides up and over cold air at the surface, precipitation will begin to fall.   The air will be cold enough at the start of the storm for snow, even along the coastline. But enough warm air will surge northward at higher levels of the atmosphere for a change to mixed precipitation or even simply rain along  the coast.  A change is less likely inland where a winter storm warning has been posted.

Winter storm warnings and winter weather advisories are posted for much of southern Maine

Winter storm warnings and winter weather advisories are posted for much of southern Maine. Dave Epstein (NOAA data)

You should expect the precipitation to begin late this afternoon and early evening.  It could impact the evening commute, especially if you leave late.

To make matters more complicated, a small new storm will form on this warm front and act like an anchor.  It will slow down the advancing of the warm air and may even bring back the cold air, keeping a mixed bag of precipitation going longer.

Some forecasts have a high level of confidence, others lower. I would put this forecast on the lower end of the confidence scale.   In other words, in those areas where I have forecast snow followed by rain, such as in Portland, there could be a couple of inches more snow if the cold air is in more layers than forecast or a few more hours of rain and less snow if there is more warm air above the ground.

Snow will be heaviest inland during Tuesday Nights snow.

Snow will be heaviest inland during Tuesday night’s storm. Dave Epstein

One thing I am positive about, this won’t turn into a big snowstorm.  During the morning on Wednesday the precipitation will continue mostly in the form of rain and snow showers.  Freezing rain isn’t likely to be a problem although there could be some light glazing on the surface of the snow in parts of the interior at the end of the event.

As Wednesday afternoon progresses the storm will move out to sea, the precipitation will end and the skies remain cloudy.  The rest of the week looks storm-free along with temperatures above average.  There will be some melting of the new snow, especially on the pavement and south-facing slopes.

You can follow Dave Epstein’s Forecast on Twitter @growingwisdom


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