Though Pittsburgh is still digging out from more than 20 inches of snow, with more snow expected this week, National Weather Service meteorologist Lee Hendricks has already started thinking about what's going to happen when all this snow melts.

"Compared to a normal winter, this is an incredible snow pack," he said.

"If we have a rapid warm up with rain on top of the snow, that could seriously cause some widespread problems on the ... rivers."

But temperatures will stay at or below freezing for the remainder of the week, and Mr. Hendricks said there will hopefully be a gradual warm up after that to decrease the threat of major flooding.

"Nothing good happens fast in hydrology," he said.

But melting snow is still a distant prospect. The more immediate concern is the snow expected to arrive Tuesday.

The National Weather Service issued a winter storm watch that said a storm system could bring snow accumulations of six inches or more Tuesday morning through Wednesday afternoon.

And memories are still fresh that the weekend's snowfall, which dropped 21.1 inches of snow on Pittsburgh, was worse the National Weather Service's dire predictions.

On Thursday, the day before the snow started falling, the National Weather Service predicted Pittsburgh would get 6 to 12 inches. They later changed that prediction to upwards of 15 inches.

"We ended up still getting beaten by five inches on that," Mr. Hendricks said.

A heavy snowfall of 16 inches or more only happens in Pittsburgh about once every 15 years, he said.

"It's a fairly rare event, and it's almost invariably an East Coast storm, like this was," he said.

Even with 21.1 inches recorded, last weekend's storm was still No. 4 in Pittsburgh's history, at least since 1876, when record-keeping started.

Two massive back-to-back snowfalls would be extremely unusual, Mr. Hendricks said, and the weather system expected to arrive Tuesday is not looking nearly as severe as the weekend's.

See the PG's weather page for the latest conditions and forecast -- and the Countdown till Spring clock. Commuters also can access the updated traffic conditions.

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