BETHLEHEM, Pa. — Record-high temperatures kicked off the month of November in the Lehigh Valley, but forecasters say Mother Nature is about to deal a cold — and potentially snowy — reality check to the region.
The National Weather Service has now issued a winter storm warning in effect from Thursday afternoon through Friday afternoon for Carbon and Monroe counties, with heavy, wet snow possible in elevations above 1,500 feet.
By 3 p.m. Thursday, the weather service expanded that warning to Northampton County.
The warning replaces the earlier winter storm watch that had been issued for those areas.
A warning is issued when a significant combination of hazardous winter weather is occurring or imminent.
Schuylkill County also was placed under a winter storm watch, which means there is the potential for significant and hazardous winter weather within 48 hours. It does not mean that significant and hazardous winter weather will occur, but that it is possible.
“Lower elevations may receive significantly less,” according to the weather service, which warns of hazardous conditions impacting the Thursday evening and Friday morning commutes.
The Lehigh Valley also could see its first measurable snowfall of the season, forecasters say, though it may be just a dusting in this “elevation-dependent system.”
Beware of ‘absolute nonsense’
EPAWA meteorologist Bobby Martrich offered a blunt assessment of some of the “snow maps” being shared on social media Tuesday depicting a large-scale storm straight down through the I-95 corridor.
“If I had a clown emoji I could put on here, I would put it right over this image, because that’s what this is,” Martrich said of a graphic suggesting 6+ inches of snow in the Lehigh Valley and over a foot to the north.
“This is why you have to make sure you trust the person, whoever the poster [of that information] is, because if they’re posting stuff like this, this is absolute nonsense.
“This is not happening in any shape or form,” he emphasized.
Martrich said a powerful cold front that will blitz the mid-Atlantic will indeed turn heavy rain to snow, but there’s a big factor at play.
“This is November. It’s not January,” he said, noting marginal temperatures will be another thing playing a big role in snow accumulations — or lack thereof.
That means accumulation will be very light outside of the mountainous areas.
The best formula to predict potential snowfall
Martrich and the weather service both agree that rain will transition to snow Wednesday night into early Thursday in some areas.
The heaviest snowfall should occur between late Thursday and Friday, but after that snow should become lighter and more intermittent before ending Saturday.
Martrich said one of the best tools in the forecasters’ arsenal is the model depicting “accumulated positive snow depth change, which is a lot more realistic.”
That model uses an estimated true snow-to-liquid ratio, accounting for snow that packs after falling and snow melt during accumulation. It shows a coating to an inch possible in the lower elevations, including the Lehigh Valley.
“I can see that happening, but it’s probably going to be Friday night,” he said.
The weather service said it also was keeping an eye on "a few simulations which imply snow could reach significantly further south across our region, even down to the I-95 corridor and Philly metro, particularly the Canadian models.
"For the moment we've mostly discounted that guidance as being outliers, but will be keeping it in mind as we watch further progress with this system," the latest forecast discussion said.
Brisk and cold will be the overall theme, with overnight temperatures plummeting into the low 30s and daytime highs topping out in the 40s.
This story was updated Thursday morning, Nov. 21, to reflect the winter storm watch had been changed to a winter storm warning.
