Whether it happens or not, the Flood of 2010 already is historic in one sense.

"I don't ever remember talking about floods a week (in advance)," said veteran National Weather Service hydrologist Bill Drzal at a news briefing Tuesday.

The flood of 1996 arrived so suddenly that then-Mayor Tom Murphy and several lawmakers criticized the weather service afterward for not giving Pittsburgh more warning.

The weather service and Army Corps of Engineers have used an array of high- and low-tech tools in anticipating a one-two punch from melting snow and steady rain later this week.

Last week, an airplane flew over the region, measuring differences in gamma radiation compared to when the ground was free of snow in autumn. Weather service and corps personnel also trudged out into the snow to take measurements. That allowed the weather service to divine the moisture content of the unmelted snow in the Laurel Mountains and other higher elevations.

And that might be the most compelling information to come out during the past several days -- some areas still have the equivalent of 10 to 12 inches of rain piled up on the ground, waiting to melt, Mr. Drzal said.

"That's going to start melting in the next couple of days," he said.

On top of that, "significant" rain is expected starting today and intensifying on Friday and Saturday. Even without rain, there is enough melting snow to cause flooding, said Werner Loehlein, Corps of Engineers chief of water management.

Much of the rain and melt will be captured in 16 reservoirs scattered throughout the district, all of which have been drawn down to their lowest practical levels. But the reservoirs control only about one-third of the 19,000-square-mile area that drains into Pittsburgh's three rivers.

"We can reduce the risk of flooding but we can't eliminate it," Mr. Loehlein said.

He said the district's three biggest floods were in March 1936, before the reservoirs were built; June 1972, when water lapped onto the outfield of Three Rivers Stadium; and January 1996, when water reached the rail line that passes Station Square. The 1972 flooding would have been 12 feet higher and the 1996 water level 10 feet higher without the reservoirs, Mr. Loehlein said.

Mr. Drzal said he did not think this year's flooding would be as severe as that of 1996 but said it could rival the remnants of Hurricane Ivan, which caused widespread damage in September 2004.

He said the region might get an inch or two of rain but it will be spread out over a longer time than the torrential 1996 storm.

"I'm fairly certain that were going to be above flood stage in Pittsburgh," he said. "The rivers are going to come up over the weekend. I think it will be a more gradual rise."

Mr. Drzal said the weather service hopes to give 12 to 24 hours' advance warning of any major flooding.

Forecasting flooding before the rain has fallen is difficult, he said. The weather service's model for predicting river stages works better after the rain is on the ground.

"It's now just a forecast based on a forecast," he said. "There is just a ton of uncertainty in this."

Local officials have sought to raise public awareness of the flood threat since Thursday, when Pittsburgh Mayor Luke Ravenstahl urged residents to prepare. Allegheny County hosted a meeting of more than 200 emergency services personnel representing about 100 municipalities at its crisis command post in Point Breeze on Friday.

"It's the calm before the potential storm," said Bob Full, county emergency services chief, who added that authorities are using the 1996 flood as their planning model. "Our philosophy has always been plan for the worst, hope for the best."

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