Millimetre-wave satellite communication systems relying on uplink power control (ULPC) require a short-term predictor of likely variations of rain attenuation. Its function is to minimise the control errors over the response time of the ULPC system. Large errors will require a larger backoff for the satellite transponder and they would result in larger intermodulation products and interference levels. Presented is an experimental study of a short-term predictor of rain attenuation relying on on-line measurement of rain fade slope. Although best performance can be obtained using the last three samples of fade slope, the results indicate that a predictor assuming that rain attenuation does not change over the prediction interval is appropriate for practical purposes owing to its reasonable performance and simple implementation.