The Army Corps of Engineers said today that its network of 16 reservoirs is drawn down and ready to receive runoff from melting snow and rain forecast for the rest of the week.

But corps officials and other local authorities warned that many areas are vulnerable to flooding in the coming days, and a National Weather Service forecaster predicted that rivers in Pittsburgh will rise above flood stage this weekend.

Weather Service hydrologist Bill Drzal said the task of forecasting the coming events is difficult because so many variables, including precipitation and rate of snow melt, are involved.

"There is just a ton of uncertainty in this," he said at a briefing at Allegheny County's emergency operations center.

Mr. Drzal said models of rain storms later this week are still evolving but that he expects "significant" rain, most heavily on Friday and Saturday.

He said he expected river flood watches to be posted late Wednesday or early Thursday.

Mr. Drzal said at present he does not expect flooding as severe as that of 1996 but said it could rival that produced by remnants of Hurricane Ivan in 2004.

He said if major flooding is to occur, the weather service hopes to give 12 to 24 hours warning.

Werner Loehlein, chief of the Corps of Engineers' water management branch, said reservoirs currently have most if not all of their flood-catching capacity. But those reservoirs affect only about a third of the area that drains into Pittsburgh rivers.

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

Bob Full, Allegheny County's emergency management chief, said local officials are using the 1996 flood as their planning model and said residents in low-lying areas should be preparing now for flooding.

"Our philosophy has always been plan for the worst, hope for the best," he said.

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