Snow. An icy lacquering for parts of the region. A potentially disruptive “flash freeze,” Below-zero wind chills. Based on the forecasts, the atmosphere evidently doesn’t care that it’s a holiday weekend.
Computers continue to bicker over the details of the track of a storm they have been tracking for a week, and precisely what effect that path will have on local temperatures and thus precipitation types.
In the macro picture, meteorologists are confident that snow will start in the immediate Philadelphia area late in the day Saturday, and at some point mix with and transition to rain as temperatures rise early Sunday. (And, yes, they still expect an inch or so of snow late Thursday and early Friday. Not taking any chances, the Archdiocese of Philadelphia announced that its high schools and parochial schools in the city will open two hours late on Friday.)
Then temperatures are going to crash in a hurry Sunday afternoon, and that would solidify all the assorted muck that hadn’t been washed away or dried up by wind into concrete-hard ice.
“The flash-freeze is going to be the big problem,” said Dave Dombek, a meteorologist with AccuWeather, which is predicting 1 to 3 inches of snow at the front end of the weekend storm, and perhaps an inch or less during the rapid cool-down cycle Sunday as the storm pulls away and draws in Arctic air.
“That might be the bigger concern,” agreed Jonathan O’Brien, a meteorologist at the National Weather Service office in Mount Holly, which was seeing about 3 inches falling in and around Philadelphia. The weather service has posted winter-storm watches as nearby as Lancaster and Berks Counties.
Temperatures early Sunday are expected to crack the 40s as the storm draws in warmer air off the Atlantic as part of its circulation. But the region will be lashed by punishing northerly winds as it departs,
“It’s conceivable you have a drop of 15 to 20 degrees in three to six hours,” said Dombek. Any storm leftovers will be frozen in place, and won’t go anywhere for a while. Temperatures Monday — Martin Luther King Day — might not get out of the teens.
PJM Interconnection, the regional grid operator based in Valley Forge that serves 13 states, has issued a cold weather alert for its 13-state territory, advising its plant operators to be prepared to call in additional staff if needed.
Temperatures around Philadelphia are forecast to fall into the 20s before nightfall Sunday, and tumble toward single digits overnight.
Precisely what kind of mess remains on the ground after the temperature plummets Sunday remains one of the unresolved issues. It will depend on how much snow falls at the outset; how much ice follows, which would hinge on how long cold air remains dammed at the surface when the precipitation changes to rain; how heavily the plain liquid rain falls; and how much snow crowns the remnants.
Dombek said areas to the north and west of the city likely would see some icing, and the Lehigh Valley could get hit with a significant ice storm.
After the freeze up that way, he said, “you’ll need dynamite to get rid of it.”
Ice accrual is difficult to predict, however. If temperatures are well below freezing, freezing rain can build up quickly on branches and wires.
If temperatures are borderline, a light rain might cause a buildup, but heavier rain would likely run off. That, of course, could cause road-flooding problems.
And let us not forget that wet road surfaces beget black ice, sometimes for several days, when temperatures are below freezing.