Monsoons shift away from normal watersheds
Pakistan's monsoon rains normally emanate from moisture swept in over India from the Bay of Bengal. In typical years, the rains open up in the east, centered on Punjab province, roughly near the cities of Lahore and Faisalabad. Experts say the rains then migrate northwest, dissipating by the time they reach the capital, Islamabad, and ending in scattered rains before dying out in Afghanistan.
Isolated flooding incidents occur every year, but Punjab is normally capable of absorbing the monsoon rains. The densely populated province is home to four major rivers that eventually drain into the Indus River, the nation's largest. Punjab is also home to an intricate network of irrigation and water management systems designed for crop use, energy production and flood control.
But for the past few decades, PMD officials have noticed that the center of Pakistan's monsoon has been gradually shifting to the northwest, away from the nation's watershed in Punjab. Whereas flat Punjab is home to long, winding river systems capable of absorbing enormous quantities of water, Khyber Pakhtunkhwa and Federally Administered Tribal Areas on the Afghan border generate relatively shorter and narrow rivers cascading from the mountains that cover roughly half the region.
"Over the past 25 to 30 years, there is a latitudinal redistribution," Khan explained. "Previously, in the past, our flooding was in these river systems [in Punjab], but ... the rainfall has shifted. Its main focus has changed from the eastern parts to the western parts."
PMD believes that climate change is to blame for this northwesterly migration of the monsoons.
The same trend occurred again this year, only farther northwest than ever before, to lands with few major rivers to absorb rains but plenty of vertical surface area to collect water and sweep it downstream. From Lahore and Faisalabad in the east, this year the center and start of Pakistan's monsoon season became instead Nowshera and Charsadda.
And like the fabled "perfect storm" of North Atlantic lore, those monsoon rains eventually collided into two other atmospheric anomalies happening at the same time, creating a perfect storm of their own.
As the monsoons were headed for Pakistan's northwest, from July 25 to Aug. 5 a portion of the jet stream was forced farther south than usual for this time of year by a system of blocking air that mysteriously developed over western China. This buckling of the jet stream dragged with it a wave of low pressure from the west, a system PMD calls the "westerly wave."
This westerly wave low-pressure area collects moisture from the Mediterranean Sea and is responsible for the snows that fall in the Karakoram Mountains. But it usually only comes that far south in the winter months -- in the summer, the normal pattern is for the westerly wave to track north of Afghanistan and miss Pakistan completely.
But because of the blockage of the jet stream's normal course, the westerly wave followed its winter trajectory in late July and early August instead, meeting the monsoon system at Khyber Pakhtunkhwa.
"Subtropical highs that normally redistribute heat in this region, they were shifted northward," said PMD scientist Muhammad Hanif, describing the system that developed over western China that disturbed the jet stream. "This type of interaction is not very usual."
A climate change-related mystery
The cause of this blocking system in western China remains a mystery. Pakistan's Ministry of Environment suspects climate change is to blame, through record high surface temperatures on land. PMD is investigating that, but is also investigating the possibility that the La Niña phenomenon in the Pacific Ocean caused the disturbance. But all agree that the excessive amount of moisture pumped into the atmosphere is a result of high water surface temperatures in the Mediterranean and Bay of Bengal.
That added fuel, and the constrained movement of air caused by the jet stream's dip created a column of rain clouds towering 40,000 feet above Khyber Pakhtunkhwa, much higher than normal monsoon storm clouds, which rarely top 10,000 feet in height. There the clouds stayed for a week, dumping up to 12 feet of excess moisture before traveling to the far west of China, inundating communities there and killing some 1,200 people in landslides.
It was a weather event that the residents of Nowshera, most now either living in camps or in the wrecked hulls of what is left of their homes, say they will never forget.
"These terrible memories will go with me to my grave," camp resident Zunaira said.
"When rain came on Thursday night, we all panicked, it was really very scary," recalled Shamsa, a 16-year-old girl put out of her home and into one of tent cities now ringing the town. "I thought we all were going to drown in floodwaters and die that night, and we had to put up staying on the rooftop of a clinic for two days. But Allah sent the Pakistan Army, who came in their rescue boats and rescued us."
And if this "superflood" event struck at the worst possible place, it also struck at the worst possible time.
Record high temperatures in Pakistan's far north were already producing higher amounts of snowmelt and glacial meltwater runoff from the Karakoram Range and into the Indus River System. Thus, the Indus and other rivers were already swollen with water in the north by the time the supercell of rains from east and west merged above them.