The snow levels are on the rise (9,000ft) and rain is increasing as an atmospheric river takes aim at the NW. That means heavy rain and mild temperatures on top of the recent snow. Rain will be seen on all Cascades ski slopes. As uncle Jake would say, The snowpack is gonna take a whippin' with this one.
Currently (Thu am) we have a few inches to 1-3ft of snow on the slopes. Ski areas are snowcat packing some of it to hold up against the incoming rain. Yes, we will lose some snow, but expect consolidation of the shallow existing snowpack. Heavy rain on snow will melt some of the shallow snowpack (3-8). Deeper upper elevation snowpack will fare better, absorbing the rain, with only partial melt. The rain will hurt the pack and we have two warm storms to endure. The rain on snow does provide a stable snowpack foundation for the rest of the snow season hey, I am trying to put a positive spin on it.
This atmospheric river (AR) will produce 2-5 of rain, perhaps as much as 6 or 7 in spots, falling on the snowpack ouch! I would consider this AR as robust (AR scale 4) and may cause some significant runoffproblems. There will be ponding of water, major river rises (flooding) and possible landslides but right now, this is not a flood like the monster floods of Oct 2003, Nov 2006, Dec 2007 and Jan 2009 but deserves watching. However, if the AR stalls, all bets are off. There is always uncertainty in the future. Watch for updates.
After we get spanked by the atmospheric river (AR) today, tonight and Friday well see the snow level drop to about 5,000 ft for a day on Saturday, with partial clearing. We will see limited snowfall recovery with minor new snow but tapering off on Saturday. Then we will be blasted again on Sunday with higher snow levels (8,000 ft) caused by another new AR, pounding us Sunday into Monday. This will be on the heels of this current storm and could cause big problems, because there is not enough time for the runoff to fully drain.
The AR weakens and dissipates later on Monday. By late Monday and Tuesday, the snow level finally starts dropping with conviction. To 5,000 then 3,000 ft as the snow piles again, but snowfall will lessen. After this roller coaster snow level ride for the next week is done, well probably end up with about the same depth of snow in places as we started with this Thursday. Then a dry spell develops next week.
Whats in a name? The Chinook, Pineapple Express & Atmospheric rivers
An atmospheric river (AR) is long (1000 miles) and narrow (250 miles) corridor of moist water vapor, usually ahead of a weather front. An AR brings mild air with the water vapor from the subtropics. The result is a ton of rain as the water vapor slams into the mountains, rises and condenses into sheets of heavy rainfall. Youll see terrestrial rivers rise, swollen by heavy rain and some snowmelt on Friday. Remember, the flood peak is always driven by the heavy rain, not the snowmelt, but snowmelt does contribute. When we see heavy rain on snow, snowmelt is thought to contribute 10-20 % to the runoff into the rivers.
Our winter rain on snow resulting in snowmelt is often misunderstood by locals and outsiders. Especially people from the Eastern U.S. and Europe. John Muir said, Strange to say, the greatest floods occur in winter, when one would suppose all the wild waters would be muffled and chained in frost and snow. He was from Scotland and didnt know about U.S. West Coast weather and our winter rain on snow. Over the years he came to understand it much better.
Atmospheric rivers used to be called the Pineapple Express (1970s 2017) and before that the Chinook. The Chinook was used to describe NW storms with high snow levels and copious rainfall in the 1800s. Then a heist occurred. The Chinook name was stolen by East Coast meteorologists. The NW name, Chinook, was reappropriated to define a dry, warm downslope wind over the Rockies. I am still mildly upset about the obvious thievery.
The Chinook are the tribes who were originally based around Astoria, Oregon and SW Washington. The Chinook developed special cone like hats to shed the heavy rains of the NW, as they adapted to wet winters. The Chinook have many legends which described ARs with oral stories passed down through generations. The stories were often allegories describing complex family relationships, masking descriptions of how heavy rain falling on snow melted snow and caused floods.
Yes, there is a lot to a name and how it transitions over time. But with apologies to Shakespeare, an atmospheric river by any other name would be as soggy
The Grand Poobah of Powder
Larry Schick meteorologist