Weather alerts are in effect across the Pacific north-west this week as a series of atmospheric rivers are forecast to deliver multiple rainfall events and heavy mountain snow from western British Columbia in Canada, to Washington and Oregon in the US.
More than 200mm (8ins) of rainfall is expected across the western half of Washington state and north-west Oregon by Friday, with between 100-150mm expected in cities such as Seattle and Portland. About 400mm are possible on the western side of the Cascades, while more than a foot of snow is expected above 1,800-2,100 metres.
The National Weather Service has issued a flood watch for north-west and central Washington state where urban and river flooding is likely. Sharp rises to river levels are possible on rivers that flow off the Cascades, with saturated soils increasing the potential for mudslides and land instability, too.
Atmospheric rivers are narrow bands of water vapour that form when warm, moist air over the tropics is drawn out of the tropics – typically into the mid-latitudes. As this moist air reaches land, water vapour is forced upwards by mountainous terrain, condensing the moisture into persistent rain or even snowfall. While atmospheric rivers play a natural role in the water cycle in the US, they are also associated with 80% of all flooding damage across the west coast, costing about $1bn (£750m) every year.
A significant amount of rainfall is also expected across east Iraq and north-west Iran this week, too. With winds from a southerly or south-westerly direction, spells of heavy rain will bring at least 50mm a day across the western side of the Zagros mountains through this week. Large cities such as Baghdad and Erbil in Iraq, as well as Kermanshah and Ahvaz in Iran, may experience cumulative rainfall totals adding up to about 50-150mm by the end of this week. Higher totals are possible locally.
For much of this region, this unsettled spell will offer a welcome relief. Iran, in particular, has been facing a severe six-year drought, with reservoirs supplying Tehran at their lowest levels in six decades. At least 19 dams are on the verge of drying out, with reservoirs filled to less than 5% capacity.
