Satellite-positioning antennas measure the water vapour above them, and the minute-to-minute fluctuations of that measurement, usually smoothed away, describe how the moisture is arranged. We read them at eleven Brazilian antennas during two floods with little or no radar coverage, in Rio Grande do Sul in 2024 and at Recife in 2022, against six-hourly rain gauges. Over a season they separate wet from dry periods 6 to 12 hours ahead, with an area under the receiver operating characteristic curve of 0.80 to 0.87. In both floods the antennas rose into their top 5% hours before the worst rain.