"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."
"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."
Atmospheric rivers will produce multiple rainfall events and heavy mountain snow from western British Columbia through Washington and Oregon, with more than 200mm expected across western Washington and north-west Oregon and 100-150mm in Seattle and Portland. About 400mm is possible on the western Cascades and over a foot of snow above 1,800-2,100 metres. Flood watches cover north-west and central Washington with likely urban and river flooding, rapid river rises, saturated soils and increased mudslide and land-instability risk. Atmospheric rivers form when warm tropical moisture is drawn poleward and forced upward by mountains, and they account for the majority of west-coast flood damage. Southerly winds will bring heavy rain to east Iraq and north-west Iran, with daily totals around 50mm across the Zagros and cumulative 50-150mm possible in major cities by week’s end.
Read at www.theguardian.com
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