Ample Cascade Snowpack, A Stubborn La Nina, And The Week’s Forecast

La Nina years are known for bringing hefty snows to the Cascades. I still remember the La Nina year of 2007-2008, when I went skiing in the Alpental backcountry on Memorial Day with my mom. We had to skirt around waterfalls & avoid a few bare patches, but overall there was still a ton of snow on the mountain. A particularly poignant memory was the roar of numerous small waterfalls echoing through Alpental Valley… I’ll never forget that day in the mountains.

Not all La Nina years bring epic snowpacks. Some, like the 2000-2001 winter, can be downright awful. But the 2020-2021 winter has fit the “typical” La Nina pattern of above-average Cascade snowpack. As the graphics from the Western Regional Climate Center (WRCC) below show, temperatures and precipitation have been marginally below and above-average, respectively, for the Washington Cascades, but this magic combination has resulted in some of the highest Washington Cascade snowpack in years.

Snowpack in the Washington Cascades is approximately 120-140% of average, with the highest snowpack in the Central Cascades and the lowest snowpack further north towards Mt. Baker where precipitation has been slightly less.

Scott Sistek of KOMO News wrote an excellent blog on the April 1 snowpack at Snoqualmie Pass, which is at its third highest value of the century after 2012 and 2008, both of which were moderate/strong La Nina years. Though the timing varies between regions/elevations (southern, lower-elevation regions peak earlier than northern, higher-elevation ones), April 1 is near the peak average snowpack of the winter for many locations and is often used as a reference point when comparing historical snowpacks.

The graphic from Mote et al. (2018) below shows the observed decrease in snowpack at various SNOTEL stations across the Western US from 1955 – 2016. Mote found that 90% of stations exhibited a decline in 4/1 snowpack, with the West as a whole seeing a 15-30% reduction in 4/1 snowpack over this time frame. In a warming climate where Cascade snowpack has decreased significantly since the mid 20th century (Mote et al., 2018) and will continue decreasing at an accelerating rate in the decades to come, we skiers need to make the most of these Cascade snowpacks while they are still around!

La Nina/Blob Update

Although it has weakened substantially, we are still officially in a weak La Nina with sea-surface-temperatures (SSTs) in the tropical Pacific some 0.5 – 1 degrees C below-average. Additionally, the atmospheric circulation over the tropical Pacific is consistent with La Nina, with stronger-than-average trade winds and enhanced convection over the Maritime Continent. Closer to home, the SSTs off the West Coast are cooler-than-average and there is no sign of the dreaded “Blob.” Sea-surface-temperatures directly off our coast are well-correlated with our temperatures, particularly overnight lows in the summer. This is primarily because these masses of water are associated with higher atmospheric dewpoints, and water vapor is a very potent greenhouse gas.

The warm, sterile water associated with these Blobs is disastrous for marine life; the “mega-Blob” of 2014-2015 resulted in massive dieoffs of Cassin’s Auklets along the Oregon Coast and was disastrous for salmon runs throughout the Pacific NW. As an avid salmon fisherman, I am very happy to see not even a trace of the Blob off our coast!

This Week’s Forecast

A weak ridge of high pressure will build over the Pacific NW Monday and Tuesday, giving low 60s to Portland on Monday and mid/upper 60s on Tuesday under mostly sunny skies both days. As we often see in the spring and summer, we’ll likely see a layer of stratus clouds fill into Puget Sound & the Willamette Valley Monday night and burn off a few hours after sunrise Tuesday morning.

A weak cold front will bring rain & mountain snow to BC/NW Washington on Wednesday, with Central/Southern Washington seeing lighter rain Wednesday night and NW Oregon seeing a few showers Thursday morning.

We’ll finish the workweek on a dry note, but another weak system may bring a few clouds and a shower on Saturday. Sunday through next week looks slightly drier-than-average with near-average temperatures.

Have a great week and be sure to enjoy all the flowers that are currently out! The cherry blossoms are in full bloom along the waterfront, the tulips in my neighborhood are bursting with color, and even the maple trees across the street are displaying their showy green buds. After a long, dark, and isolated winter, it truly feels like summer is knocking on the door.

As an additional note, Portland General Electric is now hiring a meteorologist with experience in numerical weather prediction to join our team! I’m currently the only other meteorologist at PGE and we’d work closely together and be under the same manager. Feel free to apply (local PacNW meteorology knowledge is a huge plus!) and show to friends/colleagues who you think may be a good fit! More details can be found here.