A spring nor’easter is on the way—here’s what to expect

A pedestrian walks in the Public Garden Friday.

To say this storm is difficult to forecast is certainly an understatement. Depending on where you get your weather information, you might think a major snowstorm is coming or it’s just going to be a windy spring rain event.

But we can all agree on one thing: This storm will bring raw and wet weather that could last from Friday afternoon through Saturday evening.

What’s making this forecast so confounding is predicting the temperature profile.

If temperatures are below freezing on a snowflake’s journey to the ground, it will hit the surface as a snowflake. However, if there are warm layers present as the snow is falling, the snowflake can partially melt, fully melt, or even melt and then refreeze into sleet. Once snow melts, it can never turn back to snow, no matter how cold it becomes.

Warmer air aloft can change snow to other forms of precipitation.

During this storm a warm layer is forecast to develop at about 8,000 feet, which will melt the snowflakes. The narrowness of this warm layer is what’s making the forecast so challenging.

Temperatures at all levels of the atmosphere are critical in determining precipitation type.

The rain may refreeze into a sleet pellet or simply fall to earth as rain. If the layer were thicker, I’d have more confidence in a rain event. But cold air from above this layer could fall into the warm one and cool it, creating more snow.

There are other forecasting models that predict more snow. I am leaning toward a warmer forecast with more sleet and rain rather snow and sleet, thus why my snowfall totals are somewhat lower than others. If the storm brings in more cold air, then you’ll see more snow (and I’ll be wrong).

Snowfall potential for the next two days.

Total precipitation is forecast to be at least two inches for nearly all of southern New England. Areas north and west of Boston could receive slightly more, from three to six inches. The South Shore and the Cape will see mostly rain, with a slight coating up to an inch of snow and ice.

The highest opportunity for snow-covered roads is from about 10 p.m Friday night through about 10 a.m. Saturday. While snow will fall after this time, the sun angle and time of year will most likely prevent it from sticking to the roads. Therefore final snow and sleet totals may be officially higher than what you’ll actually need to shovel.

There will be a mixed bag of precipitation falling Friday evening across the region.

During the day Friday, you can expect some light precipitation, mainly rain, with some snow. There won’t be enough accumulation to cause any travel issues. Overnight, travel will be most affected north and west of Interstate 495, but some roads in the Boston area could become slushy. On Saturday, the morning hours present the highest likelihood of snow or sleet on the roads, with improving road conditions later in the day for a while. But roads may become covered in snow again later in the day if the precipitation lasts into Saturday evening.

The storm will finally end Saturday evening, so plan on precipitation falling for a long time. When it’s all over, two inches or more of water will be on or in the ground, which could range from rain and sleet to snow.

To look on the bright side: This storm will make a big dent in the drought that’s plaguing our region.

Sunday is sunny and milder with plenty of melting. Another storm threatens with rain on Tuesday — but could drop more snow on area north and west of Boston.

You can follow David Epstein on Twitter @growingwisdom.