Blame the snow for all the fog shrouding Philly
A temperature record could fall Friday, and snowflakes could return Monday morning.
The fog that has shrouded the region and turned it into an impressionist landscape since early Wednesday, dropping visibility to less than a quarter-mile at times, is likely to persist at least through Friday morning, forecasters say.
“I don’t really see it getting out of here,” said Alex Dodd, lead meteorologist at the National Weather Service Office in Mount Holly.
If you feel as if your head and body have been in the clouds the last few days, they have been. And blame the fog on the defeated snow that has vanished beneath it.
» READ MORE: Here's a visual look at the end of Philly's snow drought
What’s causing the fog?
Technically, this is “advection fog,” caused by warmer air oozing over a cooler surface: In this case, snow and cold ground. As the air cools, it can’t hold as much water and the vapor condenses into cloud droplets.
“It creates that cloud right at ground level that’s super-persistent,” said Dodd. Warm air rises over cold air, but in this instance the cooler air is near the ground.
Plus, the winds that howled and blew snow back onto roads and driveways and sidewalks during the weekend have been taking it easy during the workweek, varying from “calm” to 6 mph since Wednesday morning. “There’s no source of wind,” said Dodd.
Fog first was reported at Philadelphia International Airport around daybreak Wednesday, and was dense during both the morning and afternoon commuting periods on Wednesday and Thursday.
“In some cases it’s as bad as snow or ice,” said Dave Dombek, senior meteorologist with AccuWeather Inc.
On average in Philly, January is the peak month for fog that reduces visibility to a quarter-mile or less, which happens to be the blizzard visibility criterion.
Where did the snow go?
That same phenomenon that helped generate the fog — warmer air condensing into water droplets over the cold snow — also evidently set off a riot of melting.
When vapor condenses, it releases latent heat that accelerates melting. One of the classic examples of this phenomenon occurred in January 1996 after Philly’s biggest snowfall, 30.7 inches. A warm-up that preceded a rainstorm all but erased the snowpack, leading to significant flooding.
Officially, Philly emphatically ended its two-year snow drought last week, with 7.9 inches of the elusive white stuff: That’s more than Minneapolis has had this season.
“It was nice and crisp and clean,” said Dodd.
By Thursday, it was all but unrecognizable. “It’s a little bit dreary,” he said.
Any more snow in the future?
Philadelphia has a shot at a record Friday with a forecast low in the mid-40s. The record high minimum temperature for a Jan. 26 is 46 degrees, set in 1916. Lingering rain is due to shut off, and high temperatures are expected to approach the mid-50s.
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However, the region is entering the climatological peak for snow, and even though temperatures are forecast to average above normal through the first week of February, computer models have been hinting at snow chances, including one early Monday.
» READ MORE: A lot of Philadelphians evidently welcomed the return of 'winter'
More rain is forecast for Sunday, but as a coastal storm pulls away it could draw in just enough cold air to turn rain to wet snow before it ends Monday.
Dombek said it might be no more than “garbage flakes.”
At most it would produce an inch or so around Philly, the weather service’s Dodd said, and more likely would be a soggy coating on the grass that once again has turned to sponge.