'Wintry precipitation of some variety': Friday storm sets up sharp snow, sleet divide across eastern Pennsylvania

BETHLEHEM, Pa. — Forecasters often know up to a week in advance if conditions are likely to be favorable for things such as winter storms, but their ability to pinpoint exactly who’s getting snow and who’s not sometimes can be much more limited.

Friday will bring one such scenario.

EPAWA meteorologist Bobby Martrich called it one of his “least favorite things to forecast,” describing the setup where eastern Pennsylvania will be a battleground between warm and cold air.

That will set up a boundary that will mean the difference between snow, sleet and freezing rain on Friday night.

“It’s almost impossible to get that boundary exactly right,” Martrich said, describing a possible scenario emerging in which “you’re getting heavy snow in Easton and you go [west] over to Fogelsville and it’s heavy sleet.”

Timing, precipitation

Martrich and his team will make a first call on the storm around 2 p.m. Wednesday.

But here’s what is currently known:

Forecasters are looking at a general start time of 3 to 6 p.m. Friday for precipitation to move into the region.

“We are near 100 percent confident that most of our area will receive wintry precipitation of some variety,” the National Weather Service said in its latest forecast discussion.

But the exact amount of snow, sleet and ice that any given location will receive? It’s uncertain, and it’s likely to stay that way until you can see it out the window.

“Folks with travel plans from Friday afternoon through Friday night should expect impacts to their plans, as road conditions will likely deteriorate during this time with snowy and/or icy roads,” the forecast discussion said.

Why the forecast is so challenging

The setup for Friday night’s winter storm involves the placement of cold, dry air from Canadian high pressure needed for wintry precipitation in our region.

But the forecast headache comes from the “warm nose” in the storm — an invisible layer of warm air sandwiched between two layers of colder air that significantly changes precipitation, turning snow into sleet, freezing rain or rain.

It does so by melting snow as it falls through the warm layer before re-freezing on its way down to the surface.

How it works

• The setup: Cold air sits at the surface, but a layer of air several thousand feet up is above freezing (the "warm nose").
  
• The process: Snow forms in the cold upper atmosphere, falls into the warm layer, and begins to melt.
  
• The outcome:
  
  • Sleet: If the warm layer is thick and the surface cold layer is deep enough, the melted snow refreezes into sleet before hitting the ground.
  • Freezing rain: If the surface cold layer is very shallow, the liquid raindrops don't have time to refreeze and instead freeze on contact with cold surfaces, creating a glaze of ice.
  • Rain: If the warm nose is very thick or the surface cold layer is very shallow, the snow melts into rain that reaches the ground as liquid and does not refreeze.

Martrich said the area with the best chance to see all snow will be the Poconos and points north and east, including northern New Jersey.

The Lehigh Valley could start as snow, transition to sleet and then end as snow, Matrich said, calling it a “realistic expectation” for how the storm could play out.

Ensembles suggest the area could end up with 3 to 4 inches of snow, but time will tell.

"But again, it's going to be one of those deals where, like, Phillipsburg, New Jersey, is [possibly] getting six inches of snow and Macungie, Pennsylvania and southwestern Lehigh County only has like two or three," Martrich said.

"It's very possible. It's going to be that sharp of a cutoff."