Flood watch issued as snow melt hastens

During the weekend we were surprised to see grass reappearing in Rittenhouse Square, a sure indicator that the Great Melt has accelerated.

Still, away from the Center City heat island, the snowpack has some life left, with generally 6 to 10 inches of it hanging tough north and west of the city as of Monday morning, according to the Middle Atlantic River Forecast Center.

Of more significance, that snow was holding from 1.2 to 1.9 inches of water, and the National Weather Service advises that it could all liquefy in a hurry at midweek, setting off some  "minor" and perhaps "moderate" flooding.

With its late-afternoon update, the weather service has posted a flood watch for Wednesday afternoon through Thursday afternoon for the entire region.

Rapid snowmelt and an inch of rain in a six-hour period Wednesday would be enough to send some small streams out of their banks, the weather service's Mount Holly office noted.

Of particular concern is a surge in the overall moisture content of the air that could aim a torch on the snowpack.

The forecast calls for dewpoints – the temperatures at which invisible water vapor condenses -- to crest past 50 for several hours on Wednesday.  Temperatures right at the snowpack would be well below 50.

When vapor condenses, it gives off latent heat that speeds up the melting process. That happened to the nth power within 10 days of the record snowstorm of Jan. 7-9, 1996, erasing almost all the snow even before a raindrop fell.

What resulted was catastrophic and widespread flooding.

The threat in this case is not in that league. However the weather service says that some flooding is possible along the Schuylkill, said weather service hydrologist Ray Kruzdlo.

Other flooding suspect include the Cristina and Passaic Rivers and the Neshaminy, Brandywine, Perkiomen, and Chester Creeks.