Multiscale analysis of three consecutive years of anomalous flooding in Pakistan

A multiscale investigation into three years of anomalous floods in Pakistan provides insight into their formation, unifying meteorological characteristics, mesoscale storm structures and predictability. Striking similarities between all three floods exist, from planetary and large-scale synoptic conditions down to the mesoscale storm structures, and these patterns were generally well-captured with the European Centre for Medium-Range Weather Forecasts (ECMWF) Ensemble Prediction System (EPS). Atmospheric blocking events associated with high geopotential heights and surface temperatures over Eastern Europe were present during all three floods. Quasi-stationary synoptic conditions over the Tibetan plateau allowed for the formation of anomalous easterly midlevel flow across central India into Pakistan that advected deep tropospheric moisture from the Bay of Bengal into Pakistan, enabling flooding in the region. The Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar observations show that the flood-producing storms exhibited climatologically unusual structures during all three floods in Pakistan. These departures from the climatology consisted of westward-propagating precipitating systems with embedded wide convective cores, rarely seen in this region, that likely occurred when convection was organized upscale by the easterly midlevel jet across the subcontinent. Similar mesoscale structures in storms in other regions of the world contribute significantly to climatological precipitation and produce flash floods because of their combination of longevity and intensity. Predictability analysis using the ECMWF EPS system shows the ability to accurately forecast the conditions favouring storms of this type and hence floods in Pakistan over a week in advance with significant confidence.