The National Weather Service has posted a flood watch for Philadelphia, the seven neighboring counties on both sides of the river, and northern Delaware, in effect from 10 a.m. to 10 p.m. Friday.
Rivers and stream levels are well below flood stage, and the predicted precipitation totals – under a half-inch – aren’t particularly menancing.
However, rapid melting of the snow-and-ice pack is possible tonight and tomorrow morning, and the bulk of the rain could fall in a hurry, said Gary Szatkowski, meteorologist in charge of the National Weather Service in Mount Holly.
“We’re just a little bit worried that we have convection coming through,” he said.
“It was decided that a flood watch would be prudent,” Mount Holly meteorologist Joe Miketta said in the afternoon forecast discussion.
Melting conditions are going to be ideal later on, and more snow and ice should disappear after dark than did in the daytime.
In and of itself, sun is not a great melting agent when the air is relatively dry, points out Nolan Doesken, a Colorado climatologist who is an expert on snowmelt. Snow can stay on the Rockies even when temperatures reach the 50s.
The key to melting, he said, is the dew point – that’s the temperature at which water vapor comes out of hiding and condenses. The dew points are forecast to shoot into the 40s tonight.
When the dew point rises, more water condenses, and as water vapor condenses atop snow, it releases latent heat that can rapidly melt it.
After the mammoth snowstorm of January 1996, a sudden warm-up and rise in dew points erased a substantial snowpack, engorging rivers and streams.
What followed was widespread, destructive flooding, even though the rain totals in Philadelphia were under an inch.
Szatkowski said the areas along the fall line, that’s where the coastal plain begins to rise toward the Piedmont, including parts of Philadelphia, might be especially vulnerable.
Some of the elevated areas to the north and west of the city have dense snow-and-ice packs that this morning were holding over a month’s worth of rain.
Another issue is ice on rivers and streams. Backed up water can cause flooding behind ice jams, and then can break through and cause flooding downstream.
If the bulk of the rain does come at once, expect widespread and rapid road-ponding in any area with snow-and-ice covered drains.