The Pacific Northwest is still reeling from catastrophic flooding unleashed by heavy rains last week. The residents need to prepare for the next calamity.
An atmospheric river is setting up to deliver another double-digit dose of rain, sending it crashing into British Columbia and Washington. The added rain could cause some locations to have their wettest November ever recorded.
The Atmospheric River arrives Wednesday.
A classic Pineapple Express atmospheric river is forming from Hawaii and heading toward the Northwest. The gap between a high-pressure system to the south and a low-pressure system to the north will be shot as the coast is near. Those will act like a jugs machine, slingshotting the atmospheric river in British Columbia on Wednesday evening.
Thanksgiving travelers on the U.S. side of the border will be the first to experience the first trickle of precipitation. The trickle will intensify on Thursday. Up to 6 inches (15 centimeters) of rain could be delivered for areas along the coast and winds could reach 40 mph (64 kph). The National Weather Service and Environment Canada have both issued warnings for the storm.
This is the first of many.
The first blast will be followed by many others. After rain ends on Friday, another system will come in on Saturday and Sunday. The NWS warns that there could be a third system that keeps things soggy and underwater until Wednesday. Up to 20 inches (51 centimeters) of rain could fall when this event is over.
A group of scientists at the Center for Western Weather and Water Extremes and other institutions created a rating that mimics the Saffir Simpson Hurricane scale, which shows that this series of storms could peak as a Category 4 atmospheric river along the British Columbia coast. The scale factors in intensity and duration show that this is a serious storm. Washington will only have to deal with a Category 2 atmospheric river.
Flooding is a concern again.
British Columbia was hit hard by the atmospheric river. At one point, every road leading to the rest of Canada was closed because of mudslides. A new wave of debris flows could come from this week's storms.
There are three factors that could increase the risk of flooding. The soils are still saturated from last week's storms. That means more rain. The summer's wildfires are adding to the problem. The vegetation that held soil in place was destroyed by the monster blazes that ripped through British Columbia. It could do the same thing with this storm as it did last week. The loose soil means that winds could easily topple trees and cause other problems.
The third factor is that the mountains have more snow since the last storm. Tyle Hamilton pointed out in a video that the freezing level will rise during the second atmospheric river, meaning that rain will fall on snow. It is important to heed travel warnings even if it means spending an extra day with your in-laws.
Climate change and La Nia are the bigger picture.
The West is wet. Jacob DeFlitch, a meteorologist at the NWS Seattle office, told Capital Weather Gang that it is fairly extraordinary to have seen so many atmospheric rivers so early in the season.
There are a few factors that could be contributing to the start. La Nia formed last month. The natural climate phenomenon is marked by cooler-than-normal waters in the eastern tropical Pacific, which in turn, impact weather patterns around the world. La Nia increases the odds of wet conditions in the Pacific Northwest.
Then there is climate change. The atmosphere can hold more water if the temperature goes up by 1 degree Celsius. The Clausius Clapeyron Equation is a formula to describe the relationship. That is a great one to play at the Thanksgiving dinner table. The atmospheric rivers are more moist than they used to be. A study published last month in Nature Climate Change found that industrial pollution has acted as a damper on atmospheric rivers and led to a stalemate from 1920 to 2005.
Climate change will lead to more intense atmospheric rivers and a shift in the balance of that balance. With more intense wildfire seasons also on tap, the heavier rain will cause more slopes to fall into disasters.
