Pacific Northwest faces second damaging atmospheric river in a week


Blue shows a predicted moisture plume reaching the Pacific Northwest over the weekend, the second of three atmospheric river events to hit the region within a week.
GIF: Earth Wind Map

The Pacific Northwest is still in shock catastrophic floods triggered by heavy rains last week. But residents must already prepare for the next disaster.

An atmospheric river prepares to deliver another double-digit dose of rain, picking up moisture from the depths of the tropics and sending it crashing into the coasts of British Columbia and Washington. The additional rainfall could result in some places having their wettest November on record and cause suffering in areas that are still picking up pieces from last week’s downpour.

Atmospheric river arrives wednesday

A plume of humidity lines up from Hawaii and heads northwest, making it a classic atmospheric Pineapple Express river. As the humidity approaches the coast, it will widen the gap between a high pressure system to the south and a low pressure system to the north. These will act as a Jug machine, which essentially launched a slingshot across the atmospheric river ashore in British Columbia on Wednesday night.

The first trickle of humidity will arrive just in time for Thanksgiving travelers to the U.S. side of the border. That trickle will intensify however, turning into a torrent on Thursday. The storm could produce up to 6 inches (15 centimeters) of precipitation for areas along the coast and cause wind gusts of up to 40 mph (64 km / h). The National Weather Service and its Canadian counterpart, Environment Canada, both issued various flood and wind warnings for this storm.

A magnified satellite view of total precipitable water, a measure of the humidity of the atmosphere, showing a tongue of moisture moving toward the Pacific Northwest from Hawaii.
GIF: CIMSS

It is the first of several atmospheric rivers

The first puff of moisture will be followed by a series of others. After the rain subsides on Friday, another system will sail ashore on Saturday and last until Sunday. Then rain could resume on Monday for what the NWS warns as “a potentially longer lasting third system” that keeps things soggy and underwater until Wednesday. Ultimately, up to 20 inches (51 centimeters) of rain could fall when this event occurs.

A rating created by a group of scientists from Center for Western Weather and Water Extremes and other institutions that mimic the scale of Hurricane Saffir Simpson show that this series of storms could peak as a Category 4 atmospheric river along the BC coast. The scale takes into account intensity and duration, and Category 4 indicates that this is a severe storm. Washington, for its part, will have “only” to face a category 2 atmospheric river.

Floods are a concern again

British Columbia was particularly affected by the Atmospheric River last week. At one point, all roads connecting Vancouver to the rest of Canada were closed because heavy rains triggered mudslides. This week’s series of storms could lead to a whole new wave of debris flows.

Even though the rainfall is not as intense as last week, three factors could increase the risk of flooding. First, the soils are still saturated from the storms of last week. It means more runoff. This summer’s forest fires add to the runoff problem. The monster that ravaged British Columbia destroyed the vegetation that holds the soil in place and turned slopes into real Slip ‘N Slides. This led to monster debris flows last week, and so could this storm. The soggy, loose soil also means winds could easily knock down standing trees and dead wood, adding another hazard.

The third factor is that the snowpack has increased in the mountains since the last storm. Yet, as Weather Network meteorologist Tyle Hamilton underlined in a video, the freezing level will increase during the second atmospheric river, which means that the rain will probably fall on the snow. This will add even more water to the soil, which is why it is essential to heed travel warnings, even if it means having to spend an extra day with your in-laws.

Overview: La Niña and climate change

It is the rainy season of the West. Even still, Jacob DeFlitch, a meteorologist in the Seattle NWS office, told Capital Weather Gang that “it’s quite extraordinary to have seen so many” atmospheric rivers so early in the season.

A few factors could contribute to the roaring start. The first is La Niña, which formed last month. The natural climatic phenomenon is marked by colder than normal waters in the eastern tropical Pacific, which in turn impact weather conditions around the world. This includes the Pacific Northwest, where La Niña increases the chances of wetter conditions.

Then there is climate change. For every 1.8 degree Fahrenheit (1 degree Celsius) increase, the atmosphere can contain 7% more water. This simple relationship even has a formula to describe it, nicknamed the Clausius Clapeyron equation. (It’s fun to hang out at the Thanksgiving dinner table.) This means atmospheric rivers are even more loaded with moisture than they were before. A to study published last month in Nature Climate Change, however, finds that industrial pollution acted as a shock absorber on atmospheric rivers and essentially led to a deadlock from 1920 to 2005.

This balance is starting to change, however, and will continue to do so over the next several decades, as climate change exerts greater control and leads to more intense atmospheric rivers. With more intense forest fire seasons Also at the rendezvous thanks to the rise in temperatures, this means that the more abundant precipitations will have more slopes to dislodge in unstoppable catastrophes.



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