Nepal, Sept. 22 -- Nepal has not yet finished burying its dead from the Bhotekoshi disaster. The rescue teams are still out, the missing lists still open, and now a low-pressure system building over the Bay of Bengal is projected to bring heavy rain across the country from September 24 to 28. Meteorologists largely agree that enormous rain is coming. Where it will be heaviest, and on which of those five days, remains unsettled. Given how often heavy rain in this season has cost Nepal lives and property, that uncertainty is not a reason to relax.

Several forecast models point towards more than 400 millimetres of rain, with one outlier model suggesting a figure as high as 741 mm. To put it in context, the Department of Hydrology and Meteor...