Balkans floods: why did it rain so much?
In recent days, pictures have emerged from the Balkans, showing what has been described as the worst flooding in living memory.
Three months’ worth of rain fell in the space of just a few days, causing severe flooding that is estimated to have put one-third of Bosnia under water and killed dozens of people.
Mile of mile of landscape is flooded and there are still concerns that problems will persist in some areas, as rivers continue to rise in response to rainfall that fell days ago.
The Balkans is no stranger to heavy downpours of rain – especially at this time of year when thunderstorms are common as summer heat builds. But why did it rain so much?
As is always the case with severe weather events, there are a number of factors that came together to produce the perfect storm.
Slow-moving low pressure
The cause of the heavy rainfall was an area of low pressure that formed over northern Italy on 13 May and then drifted across the Balkans the following day.
However, rather than move through, the area of low pressure got stuck in roughly the same place for three days.
The Met Office pressure charts below from 13-16 May (ordered top to bottom) show this well, with the low pressure labelled ‘L’ moving very little during this time.
As a result, the Balkans simply got day after day of rain, with no respite.
A good feed of warm, moist air
Another factor that made the rainfall so heavy, was the constant supply of warm, moisture-laden air from the Mediterranean.
Wind flows around an area of low pressure in an anti-clockwise direction in the northern hemisphere, so the low pressure scooped up lots of warm, moist air from over the Mediterranean Sea.
Warmth and moisture act as a source of energy for areas of low pressure, so in effect, this added fuel to the fire and enhanced the rainfall.
Orographic enhancement of rainfall
The orography, the height of the hills and mountains in the Balkans, will also have enhanced the intensity of the rainfall.
Clouds, and subsequently rain, form when air is forced to rise, cool and condense. This process happens due to low pressure, but the rate at which it occurs increases significantly when mountains are present.
On 15-16 May, this process would have been in play over the Balkans, as a strengthening north easterly wind shoved warm, moisture-laden air up the sides of the steep mountains that make up a significant proportion of the landscape.
This increase in the rate of air being lifted up into the lower atmosphere would have not only increased rainfall intensity, but released potential instability in the air, creating embedded thunderstorms.
The is illustrated by the map below showing an abundance of lightning strikes recorded over the region during 13-15 May.
Thankfully, high pressure is going to build across the area during the next few days, which means that there’ll be no more than a few showers.
This respite in prolonged rainfall will allow river levels to gradually lower and aid efforts to search for those who are still missing or stranded.
Images: Met Office, Nasa, Lightningmap.org