Heavy rains and historic flooding over Pakistan in late July 2010: Synoptic conditions and physical mechanisms

Widespread extremely heavy rains (> 250 mm) and historic flooding occurred in the Indus River basin throughout Pakistan during 27-31 July 2010. The heaviest rains fell on 29-30 July, where many stations in the northern provinces and tribal areas received over well over 150% of their climatological July precipitation in less than 48 hours. Noteworthy rainfall totals in northern Pakistan (with the July monthly rainfall climatology in parentheses) include 346 mm (265.6 mm) at Garhi Dopatta during 27-30 July, 333 mm (46.1 mm) at Peshawar during 29-30 July, and 338 mm (93.4 mm) at Cherat during 29-30 July. Preliminary reports have indicated that the high-impact flooding has affected nearly 17 million people, resulted in over 1500 fatalities, and left over 4 million people without homes. The aim of this presentation is two-fold. First, we will examine the evolution and forecastability of the large-scale antecedent conditions that contributed to a favorable synoptic-scale environment for heavy rain. Second, we will investigate the subsynoptic-scale processes that helped organize an extended outbreak of convection over Pakistan during 27-31 July.

Analysis of the antecedent large-scale conditions indicate that a high-latitude blocking anticyclone over western Asia and eastern Europe, responsible for producing record heat over Russia, helped anchor an anomalously deep upper-level trough over central Russia just northwest of Pakistan throughout much of June and July. During 20-31 July, downstream development and diabatic outflow from a widespread outbreak of convection over the Bay of Bengal contributed to intense ridge building over the Tibetan Plateau. This anomalous ridge (+3 to 4 sigma relative to the long-term climatology), combined with the persistent anomalous trough (ƒ{1 to 2 sigma) over central Russia, resulted in an intense upper-level jet and jet-entrance-region over northern Pakistan that persisted throughout the last week of July. Within this synoptic-scale envelope favorable for deep quasi-geostrophic (QG) ascent, anomalously strong low-level southeasterly flow over northern India just south of the Himalayas likely helped to advect deep tropical moisture (precipitable water values > 65 mm) into Pakistan where deep upslope flow was prevalent. The anomalous southeasterly flow occurred in conjunction with the passage of a monsoon low-pressure center over the southern part of India and Pakistan during 27-31 July, and was likely further intensified in response to deep easterly flow on the equatorward side of the Tibetan anticyclone and by QG processes associated with the upper-level jet entrance region just north of Pakistan. Ensemble forecasts from the Global Forecast System/Ensemble Kalman Filter (GFS EnKF) will be used to assess the model performance for this extreme rain event. In particular, we will examine the ability of the ensemble to accurately predict (i) the ridge building episode over the Tibetan Plateau and attendant upper-level jet intensification, and (ii) the development of enhanced low-level southeasterly flow over northern India and attendant moisture flux into Pakistan.