Surface melt forces summertime ice-flow accelerations on glaciers and ice sheets. Here, we show that large meltwater-forced accelerations also occur in winter in Greenland. We document supraglacial lakes (SGLs) draining in cascades at unusually high elevation, causing an expansive flow acceleration over a ~5200 km² region during winter. The 3-component interferometric surface velocity field and decomposition modeling reveals the underlying flood propagation with unprecedented detail as it traveled over 160 km from the drainage site to the margin, providing novel constraints on subglacial water pathways, drainage morphology, and links with basal sliding. The triggering SGLs continuously grew over 40 years and suddenly released decades of stored meltwater into regions of the bed never previously forced, demonstrating surface melt can impact dynamics well beyond its production. We show these events are common and thus their cumulative impact on dynamics should be further evaluated.