Unreliable long-range weather forecasts hamper flood prevention efforts and attempts to control Lough Neagh water levels, MLAs were told yesterday.
Rivers Agency chief executive David Porter told a Stormont committee a combination of "pretty shaky" predictions and the fact that it can take 40 days to lower the lough by just a foot left him with an ongoing problem.
The sluice gates used to drain the lough have been fully open since November, but the agency has been criticised for failing to do more to bring down water levels ahead of winter storms.
Giving evidence to the regional development committee, Mr Porter explained the problems facing the body. "Our difficulty is in the level of confidence in forecasts," he said. "We get good information on a three-day basis, and we might place some emphasis on a five-day weather forecast, but after that our level of confidence greatly diminishes."
Mr Porter also told how once the level of the lough came down, the agency was powerless to reverse the situation.
"If you draw down the lough excessively in anticipation of weather and don't get it, you are sitting with a (low level) lough and there's no way you can actually reverse that," he said.
"In order to draw it down by 300 millimetres, we need to release 165 million tonnes of water. In order to do that, on top of what we would be doing in our normal day-to-day flow, we need between 25 and 38 days. That is the nub of the problem.
"In order to reduce the lough by a foot, we need between 25 and 38 days' notice, and after about five days our level of confidence in the weather forecast is pretty shaky.
"Hopefully that describes the dilemma that we are in. We can't anticipate to the extent that people believe we can."