Flooding does not mean the Pacific Northwest drought is over
Heavy rain on snow in the Blue Mountains led to flooding on the Walla Walla and Umatilla Rivers plus their watersheds over the weekend. A few other waterways throughout the Pacific Northwest also experienced flooding.
Spring floods are common on Pacific Northwest rivers as mountain snow melts. It gets accelerated when we have heavy rain at mid-elevation locations, such as what occurred in the recent atmospheric river. Despite this rain and flooding, drought conditions continue to exist in Washington, Oregon, and Southern British Columbia.
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The juxtaposition of flooding during a drought is difficult to communicate. Droughts are long-term weather events that last for months to years whereas most floods (especially on smaller river systems like the Walla Walla) last for a few days to a few weeks. That said, larger rivers like the Mississippi can experience flood events lasting much longer.
A lot of water is coming out of the mountains. There is some benefit here as new soil gets added to farms on area floodplains and it is true that a modest amount of new water enters the groundwater system that nearby wells pull from.
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Flooding like we saw in the Pacific Northwest over the weekend does not represent enough water to end the regional drought, however. Most of the floodwater will make it from the Walla Walla and Umatilla Rivers to the Pacific Ocean within a week, at which point it is lost to the system. It becomes unusable to Columbia Basin farms much earlier than that.
Many reservoirs are full, such as the large ones on the Upper Yakima River. Those have been pretty full since around Christmas and while it is nice to see, these reservoirs filled with rain and not snow.
The real driver behind Pacific Northwest water supply for most irrigated farms is mountain snowpack. This year’s snowpack remains quite abysmal despite recent gains in the Washington Cascades. Last week’s storms brought the snow-water equivalent in the Pacific Northwest from 58% of average to 66% of average over the course of three days.
These gains are a big deal but the fact remains that the region is still far short of what is needed to maintain sufficient water supply (especially in arid regions) through the summer. Full reservoirs in the late-Winter and early-Spring are great, but water is needed most during the summer months which are very dry and, east of the Cascades, quite hot.
Without sufficient snowpack to keep water flowing into the reservoirs through June and into July resource managers will have a difficult time providing plentiful water into the summer months.
The ideal for Columbia Basin and Yakima Valley water is to have a heavy winter snowpack that melts slowly. This trickles water into the system well into the summer so that reservoirs are not beginning to be lowered until July or even later. All indications are that this year will not look like that.
High pressure parked near California will lead to warm temperatures in Washington and Oregon, melting a lot of the snow that just fell. We may not even be able to hold onto last week’s snow until April, let alone June or July. Meanwhile the storm track is aimed at British Columbia so northern parts of the province should see some decent gains.
We will have good years in the future, but gradually warming temperatures are slowly decreasing the amount of snow that falls in the mountains. Precipitation itself is not decreasing, it’s just that more water is falling as rain instead of snow in mid-elevation locations. Mid-elevation snow is key because despite having a lot of rugged terrain very few summits in Washington and Oregon are actually high elevation.
Using Stampede Pass as an example, 2025-26 snowpack is about half of average for this time of year (a value that doubled last week). Total precipitation is actually above average, though, which is why reservoirs are full. Stampede Pass is up near Snoqualmie Pass.
It looks like El Niño will develop later this year. El Niño is an indicator that drought conditions will continue into next winter as the dominant flow jet stream pattern is one of split flow with moisture being aimed at Southeast Alaska and California while the Pacific Northwest remains (relatively) dry in between. It would be nice to see this pattern not be the case, but I wouldn’t hold my breath.
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