BETHLEHEM, Pa. — It could it be a white Christmas, or at least a white Christmas Eve, for parts of the region.
Whether it is depends on how much snow falls Friday into Saturday, according to the National Weather Service.
“A complex weather system will take aim at the area today, resulting in quite a difficult and challenging forecast,” NWS meteorologist Joe DeSilva said in the latest NWS forecast discussion.
DeSilva and many others will keep a close eye on a system from the west as it interacts with a storm developing offshore.
Overall, they say two key things need to happen to bring measurable snow to the area:
- The system from the Great Lakes moving east needs to take on a negative tilt. In weather parlance, it simply means the system is tilted to the west with increasing latitude. A negative-tilt often is a sign of a developing or intensifying system.
- An inverted trough needs to develop on the northwestern flank of the offshore storm.
Inverted troughs are seasonal phenomena that are responsible for unexpected high-intensity snowfall.
If Friday’s inverted trough develops as much as guidance indicates, there should be a localized area of heavier precipitation over part of the area.
“Unfortunately, these inverted trough set-ups are very difficult to predict, especially with respect to an exact location,” DeSilva said.
“So until the trough actually develops, it is unknown if there will be a more targeted area for heavier precipitation. Based on guidance, however, the greatest confidence is over the northern half of the forecast area."
How much snow is expected?
In terms of snowfall amounts, not much has changed from the thinking forecasters have leaned into over the past few days.
The weather service said the greatest snowfall totals will be 2 to 3 inches (locally, up to 4 inches) across the Pocono Plateau of Carbon and Monroe counties and extreme northern New Jersey, where a winter weather advisory has been issued.
For the Lehigh Valley and parts of northwest New Jersey, the forecast holds 1 to 2 inches of snow expected.
EPAWA meteorologist Bobby Martrich has the Lehigh Valley circled for “possible snowfall enhancement” if the inverted trough forms, but otherwise has the area destined for a coating to 2 inches in his latest updates.
Timing, temperatures
Light precipitation is expected to overspread the region from south to north later this morning into the afternoon.
As we head into the evening, “the forecast becomes even more interesting, especially across the lower elevation areas,” the weather service said.
The strengthening offshore system will begin to pull away from the area, but the formation of an inverted trough would cause precipitation to continue through the overnight hours.
Winds also will back to more northwesterly on the backside of the storm, forecasters said, letting colder air pour into the region.
“This will allow temperatures to dynamically cool after sunset,” the weather service said.
Additionally, Canadian high pressure will “increase its influence across the region, ushering in an Arctic air mass.”
The Lehigh Valley can expected wind chills in the teens and 20s on Saturday, and in the single digits Saturday night.
Temperatures are expected to be even colder on Sunday and Monday, with highs stuck in the 20s and wind chills in the teens and single digits, with feels-like temperatures below zero in the Poconos.
It means any snow on the ground may not have an opportunity to melt until Tuesday, with highs in the mid-30s expected and a few snow showers possible.
