Lehigh Valley under winter weather advisory Wednesday

BETHLEHEM, Pa. — A storm system is forecast to approach the Lehigh Valley on Wednesday and bring with it a variety of wintry hazards, prompting the National Weather Service to issue a winter weather advisory.

But the storm –expected to bring several inches of snow to the area – will likely fall somewhere between a nuisance and a minor event when all is said and done.

It also likely won’t add much to our low seasonal snow total thus far: 1.4 inches and counting (officially measured at the weather station at Lehigh Valley International Airport in Hanover Township, Lehigh County).

• The Lehigh Valley is under a winter weather advisory Wednesday
• Snowfall rates potentially exceeding one inch of snow per hour are expected early on
• The storm is expected to change over to all rain as it draws in warm air from the southeast

The advisory is in effect from 7 a.m. to 4 p.m. Wednesday, with snowfall rates potentially exceeding one inch per hour early in the day. That would create hazardous conditions on area roadways, along with poor visibility.

The snow is expected to continue through late morning before changing briefly to a wintry mix and then to rain into the afternoon and evening hours, south to north.

"The greatest uncertainty in precip type evolutions remains in the region between I-78 and I-95,” the weather service morning forecast discussion said, noting colder air “will remain entrenched at the surface” longer in the Lehigh Valley area.

The Southern Poconos and highest elevations of Sussex County, N.J. could also see some higher amounts, according to the weather service.

National Weather Service

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Weather Prediction Center

But closer to the Philadelphia metro area, just an inch or two is possible, if that. And overall, there is still some uncertainty with the total amounts across the board.

It all has a lot to do with a funny-sounding term for a winter storm – the “warm nose.” It’s a layer of warm air up in the atmosphere that can melt falling snow back down to semi-frozen precipitation such as sleet or freezing rain, or just rain.

Or to put it a different way – precipitation has to fall through thousands of feet in the atmosphere to reach the ground. But each layer of the atmosphere has its own temperature. The more cold air the precipitation encounters, the more frozen it will be when it gets to the surface and lands.

The more warm air the precipitation encounters, the more likely it won’t be snow hitting the ground in the end.

That change in air temperature – determined by location and elevation – is expected to mean the difference between snow, sleet, freezing rain and rain across the region on Wednesday. It’s also why the weather service cautioned early on it would be monitoring trends to see how fast the warm nose of the storm surges north.

“When we talk about a front end thump of snow, it is not just a colorful term that I’m trying to use here,” said meteorologist Bobby Martrich of EPAWA Weather Consulting, in his daily forecast video. “The thump of snow refers to some intense frontogenesis, which is an added lift mechanism that gets some heavier rates when the precipitation is still snow.”

The EPAWA snow map has the Lehigh Valley in the 2 to 4 inch range – something Martrich said in his latest forecast could still be refined.

“It is going to change over to rain, no two ways about it,” he said.