Along the Eastern Seaboard, a nor'easter is gathering itself for a weekend arrival — visible enough now for meteorologists to speak of it with confidence, yet still too distant to reveal its full intentions. By Wednesday, the storm's broad shape had emerged from the Atlantic's uncertainty, offering residents a reason to prepare without yet giving them the precision to plan. It is the ancient bargain of weather forecasting: we can see the storm coming long before we can know what it will ask of us.