Using a multitude of data sources, the synoptic-dynamic development and predictability of the event is investigated in detail for the four-day period 1200 UTC 25 July 2015 to 1200 UTC 29 July 2015 during which the major portion of the rainfall was observed. A slowly moving upper-level subtropical trough and the associated surface low in the northern Gulf of Tonkin promoted steady moisture convergence and long-lasting convection over northeastern Vietnam. The humidity stemmed from a moisture transport band emanating from an unseasonal tropical storm in the Bay of Bengal. Analyses of the ECWMF ensemble forecasts clearly showed a sudden emergence of predictability of the extreme event at lead times of three days that was associated with the correct forecasts of the intensity and location of the subtropical trough in the 51 ensemble members. Thus, the Quang Ninh event is a good example in which predictability of tropical convection arises from large-scale synoptic forcing; in the present case it was due to a tropical–extratropical interaction that has not been documented before for the region and season.