Washington Flooding to Finish January

Yesterday, my parents drove down all the way from Seattle to celebrate my 27th birthday with me in Portland. I knew that the Willamette was running high, so we decided to go visit Willamette Falls in Oregon City. In my opinion, Willamette Falls is one of the most under-appreciated natural phenomena in the world – it is the second-largest waterfall in North America (after Niagara Falls), yet it is an afterthought compared to some of the taller but far less massive waterfalls along the Columbia River Gorge, such as Multnomah and Bridal Veil Falls. While Willamette Falls is only 42 feet tall, it is 1,500 feet (5 football fields) wide – making it the 17th widest waterfall in the world. And when we saw it yesterday, it did not disappoint!

It has been a very rainy month for the entire Pacific Northwest. But while the sheer amount of rain has been impressive, most the real kicker this month has been the nearly incessant rain. Portland has seen measurable rainfall an incredible 25/28 days this month, but this has “only” totaled 6.94 inches, compared to an average of 4.48 inches for this date. As a result of this fairly evenly-distributed rainfall, river flooding hasn’t been a major threat. Our cool/wet/snowy weather the first half of the month was amazing for Cascade snowfall but kept river levels relatively low, and the mild/wet storms the latter half of this month have generally been quick-hitters, keeping river levels high but preventing them from reaching flood stage. The only exception was some minor flooding last Thursday/Friday for a few rivers flowing off the Olympics and North/Central Washington Cascades, but this flooding was short-lived and minor.

However, more significant flooding is likely for Friday-Sunday, primarily over the Olympics, Northern Washington Cascades, and any rivers flowing off Vancouver Island or the BC Coast Range. A strong atmospheric river will stall over NW Washington and Vancouver Island from Thursday to Saturday, and with antecedent river levels so high, I wouldn’t be surprised if some rivers, particularly those over the North Cascades, saw moderate or even major flooding. The vast majority of precipitation will be to the north of Portland, so this does NOT look like a flood producer for the rivers of NW Oregon or those flowing off the Southern WA Cascades.

One of my favorite charts for looking at atmospheric rivers is the integrated vapor transport (IVT). The IVT measures the amount of water vapor being advected horizontally in the atmosphere and it is a function of both the moisture content of the atmosphere and the velocity at which this moisture is moving. The IVT is particularly useful when considering the effects of terrain, with high IVT values corresponding to strong, moist flow that drop torrential amounts of rainfall on windward slopes.

The IVT chart above shows the bulk of moisture headed north into Vancouver Island, and indeed, Vancouver Island is expected to see the highest precipitation over the next several days. However, if the atmospheric river were to head just a little further south, all of that super heavy precipitation would fall over the North Cascades and Olympics, sending rivers to major flood stage.

The UW WRF model (below) is going for 3-8 inches over the North Cascades and Olympics… easily enough to cause widespread minor/moderate flooding for rivers there.

Snow will be confined above 6,000 feet for the BC Coast Range and Northern Washington Cascades and 8,000 – 9,000 feet for the Oregon Cascades, but spots above the snow level will see obscene amounts of snow. The top of Whistler/Blackcomb may very well see 5-6 feet of new snow in 48 hours, while the bottom sees 3-4 inches of rain!

Unfortunately for those below the snow level, the warm, moist, air associated with atmospheric rivers is extraordinarily efficient at melting snowpack – far more so than dry air of the same temperature. This is because dry air absorbs moisture from the snowpack, and this change of phase from ice to water vapor (sublimation) requires energy and dramatically lowers the temperature of the air, slowing the melting process. Sublimation can’t occur when the airmass is already saturated, and as a result, warm/moist air cools much less when it moves over ice, as it only has to melt ice to liquid water and this takes far less energy than turning it to water vapor.

To summarize, we’ll end a very soggy January on a mild note, with NW Washington seeing heavy rain and flooding as a strong atmospheric river plows into the area. Thankfully for snow lovers, we’ll cool and dry significantly after Super Bowl Sunday as a strong cold front comes through, and the latter half of next week looks rainy with near-average temps, with snow continuing to pile up at the resorts.

Thanks for reading, and have a great week!
Charlie