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.