Record Flooding Strikes The Blue Mountains

This post was co-produced with fellow WeatherTogether blogger Mark Ingalls of Kennewick, WA.

From Wednesday, 2/5 – Saturday 2/8, the Blue Mountains of Southeast Washington and Northeast Oregon witnessed a historic, unprecedented period of heavy rain and flooding. Like almost all of out wintertime river flooding events, this event associated with an atmospheric river – a narrow filament of subtropical moisture that extends into the mid-latitudes. But while most atmospheric rivers that affect the Pacific Northwest arrive from the SW, this atmospheric river arrived from the WNW as it was directed around a powerful ridge of high pressure in the NE Pacific. The WNW flow allowed for extraordinary orographic (terrain-induced) enhancement on windward slopes that were perpendicular to this flow. Many rivers flowing off the Central Washington Cascades saw moderate or major flooding, and some flowing off the Blue Mountains of SE Washington and NE Oregon, like the Umatilla and Grande Ronde, reached all-time highs.

In this post, we’ll start with the precipitation climatology of the Pacific Northwest, with particular attention to the orientation of mountain ranges against our prevailing southwesterly flow. We’ll then move on to a discussion of what role terrain plays in the case of atmospheric rivers (and this one in particular), and then we’ll finally dig down into the upper-level pattern and surface impacts of this atmospheric river in particular.

Precipitation Climatology in the Pacific Northwest

Precipitation in the Pacific Northwest is highly influenced by our terrain. Because our storms typically arrive from the SW, SW-facing windward slopes tend to get the highest amount of precipitation. The Olympic Mountains are the wettest location in the Pacific Northwest, as they are a tall (albeit small!) mountain range with SW exposure and are located directly off the ocean. Even though the Blue Mountains are approximately the same height as the Olympics, they receive much less precipitation due to their inland location and NW-facing exposure. The map below shows the distribution of precipitation in the Pacific NW – the Olympics have widespread 120-180 inch annual totals with some spots seeing over 200 inches, while the Blue Mountains are closer to 40-50 inches.

Don’t take our (or the map’s word for it), look at the difference in flora between Umatilla National Forecast (in the Blue Mountains, 40-50 inches of rain per year) and the Hoh Rainforest (in the Olympics, 120-160 inches of rain per year). I don’t think I need to tell you which one is which. 🙂

Atmospheric Rivers and “Orographic Enhancement”

A notable feature of many atmospheric rivers is that they produce far more rain over terrain than in the lowlands, and that there is generally very pronounced rainshadowing on the lee sides of mountain ranges. This is due to the strong, moist, laminar (non-turbulent) flow associated with atmospheric rivers – mountains are very effective at “squeezing” out moisture from these atmospheric rivers on the windward slopes of terrain. Lee slopes see strong subsidence and rainshadowing during atmospheric rivers, and, if the atmospheric river is strong enough, they can even see blue skies! It may at first seem counter-intuitive that the strongest atmospheric rivers that cause the most destructive flooding are also the ones that cause the sunniest and warmest weather on lee slopes, but it makes sense; stronger uplift on windward slopes = more rain, and stronger subsidence on leeward slopes = more sunshine.

As the image below shows, skies over Central Washington were completely blue on Wednesday during this atmospheric river. Further south, you can see mountain waves (another terrain-induced feature we won’t go into here) that are perpendicular to the upper-level flow. Mountain waves are associated with strong, stable, laminar flow and are commonly found in the lee of mountain ranges during atmospheric river events. If you look carefully, you can also see partial clearing over the Willamette Valley and even Southern Puget Sound – an area that the Olympic Rainshadow rarely visits.

The radar during this time shows the how dramatic these orographic effects were. You can see a dramatic rainshadow south of Sea-Tac, much more intense precipitation over the Cascades, and most notably, extraordinary precipitation over the Blue Mountains… it’s not often you see yellows there that aren’t associated with a convective shower/thunderstorm!

Atmospheric River Imagery

We confess that we get a significant amount of joy in blogging about atmospheric rivers just because of how colorful some of the images we use to describe them are. The above chart shows the Total Precipitable Water (TPW, the amount of liquid water if all the water vapor in a column of the atmosphere were condensed), and you can see the atmospheric river moving north from Hawaii and curving east due to the anticyclonic (clockwise in the Northern Hemisphere) circulation around this ridge.

The model chart offers another view and explicitly shows this ridge of high pressure in the Eastern Pacific. The placement of this ridge was the key feature in altering the trajectory of this atmospheric river and causing historic flooding for the Blue Mountains.

Atmospheric rivers are amazing beasts; there are typically 4-5 atmospheric rivers in each hemisphere at any given time and they are responsible for almost all of the meridional water vapor transport even though they typically cover less than 10% of the Earth’s surface at any given latitude (Newell and Zhu, 1998). If you looking for more info about atmospheric river, I highly suggest reading Charlie’s blog here.

Impacts: 

With strong WNW flow, the Blue Mountains and Cascades took the brunt of this precipitation, with the Olympics getting a relatively small punch.

Precipitation amounts were highest in the Central Cascades. The Lynn Lake Snotel site at 3,900 feet in King County was the precipitation “winner” from this event, with over 20 inches between the 4th and the 8th and, as the image above shows, 17.20 inches in the 70-hour period between 6am Tue 2/4 and 4am Fri 2/7. Compare this to the Tacoma Narrows Airport site less than 40 miles to the west, which saw extraordinary rainshadowing from the Olympics and only managed 0.24″ in the 72-hour period between 6am Tuesday and 6am Friday!

West of the Cascades, the Snohomish, Pilchuck, Puyallup, Nisqually, Cowlitz, Carbon, White, Cedar, Green, and Snoqualmie Rivers all reached flood stage. Issaquah “Creek” also saw moderate flooding; you’d be hard-pressed to call it a “creek” at the time though!

Wild flooding on Issaquah Creek right now. Water coming up into Creekside apartment complex as they furiously sandbag. One woman told me she had a foot of water in her apartment. @KING5Seattle pic.twitter.com/ZY3dqgKlvN

— Michael Crowe (@MichaelReports) February 6, 2020

Fellow WeatherTogether blogger Brie Hawkins of Little Bear Creek Weather took some incredible footage of Snoqualmie Falls flowing at 20,000 cubic ft/sec. on Thursday.

Though the Blue Mountains saw approximately half as much precipitation as the Cascades, the rivers flowing off the Blues are less acclimated to heavy rain than those off the Western Cascades and it does not take as much precipitation to cause them to spill their banks. Some spots in the Blues saw 10 inches – roughly 20-25% of their seasonal total – in 4 days – and this combined with significant snowmelt caused major flooding across the region with significant impacts to local communities.

Before we get into impacts, there are a lot of repeat names here as they shared by different geographic features. Hopefully this list will clear up any ambiguity for those who don’t live in this area:

WALLA WALLA	UMATILLA	YAKIMA
Walla Walla (city) – about an hour east of the Tri-Cities. Walla Walla County – In Washington on the WA/OR state line east of Benton County. Walla Walla River – Flows from the Blue Mountains in Umatilla Co. to the Columbia River at Wallula.	Umatilla (city) – about 20 minutes south of the Tri-Cities Umatilla County – In Oregon on the WA/OR state line directly south of Walla Walla County. Umatilla River – Flows from the Blue Mountains in Umatilla County to the Columbia River at Umatilla.	Yakima (city) – a little over an hour northwest of the Tri-Cities. Yakima County – In Washington bordering Benton County to the west. Yakima River – Flows from near Snoqualmie Pass to the Columbia River at Richland.

Most streams/creeks in NE Oregon/SE Washington topped their banks, and three of the largest rivers in the region – the Grande Ronde, Umatilla, and Walla Walla, saw major flooding, with the Grande Ronde and Umatilla setting all-time records (black line in the graphs below) and the Walla Walla coming three inches short of record stage at the Touchet flood gauge.

Early on February 6, flooding was forecast for the Pendleton and Walla Walla areas. The flooding far exceeded expectations in Pendleton, where a levee downtown was breached sending water from the Umatilla River into the city’s center. The impacts from this were well documented by the East Oregonian because the breach was right next to their building. Unfortunately, one flood death was recorded in the Pendleton area.

Some footage of flooding Thursday night behind the East Oregonian. pic.twitter.com/yQduSK8iJ5

— East Oregonian (@EastOregonian) February 7, 2020

On February 7, Mark went to the Walla Walla Valley to help maintain individual weather stations in AgWeatherNet’s weather station network and witnessed the remarkable flooding over the Valley first-hand.

Walla Walla River at Touchet, but this time looking upstream. #wawx #wallawalla #flood #weather @NWSPendleton pic.twitter.com/ceVbCrj65q

— Mark Ingalls (@markaingalls) February 8, 2020

The Touchet and Walla Walla Rivers completely filled their flood plains in many places, forcing the closure of multiple stretches of US 12 in Walla Walla and Columbia Counties. Despite reaching record levels, Walla Walla was largely spared by Mill Creek. The engineering projects through the city itself held their ground remarkably well despite the muddy water surging through the channel. College Park to the west doesn’t have as extensive of a system of levees and man-made channels, which was clearly evident in the flooding and bridge washouts.

The Walla Walla River at Touchet came to within three inches of breaking its record. While doing so, it overtopped a few bridges and washed out others. At one stop, Mark saw that the river had also washed out underneath railroad tracks. This was only a small area so the lines themselves were left dangling over the bubbling water. Upstream, about a quarter of Milton-Freewater was evacuated and the Oregon National Guard was out conducting rescues by helicopter for both Washington and Oregon residents between Walla Walla and Pendleton.

Meanwhile, the extreme flood was moving down the Umatilla River. West of Stanfield, the water reached up to Interstate 84 causing extensive damage. As of the evening of February 13, the freeway is still down to one lane in each direction as ODOT crews work to repair damage to the westbound lanes.

CONT: The I-84 closures are expected to remain in place for a significant amount of time. Once the water has receded, we’ll need to inspect the highway to assess the amount of damage. This photo shows how scouring damage is already impacting the interstate at MP 187 WB. 2/2 #orwx pic.twitter.com/AW2T1MDHSr

— OregonDOT (@OregonDOT) February 7, 2020

Since the Umatilla River doesn’t flow through Hermiston, the city itself fared pretty well. West of town, however, roads were washed out and a park was severely damaged. One driver attempted to drive through a closed and flooded road and discovered why meteorologists and transportation officials frequently urge people to not do that:

Two days later the water level had dropped enough to see the extent of the damage:

Flood waters often obscure road damage, even in water that is seemingly clear. If you come across a flooded road, turn around, don’t drown! Most flood deaths occur in vehicles.

Interestingly, while the Walla Walla and Umatilla floods were similar to another massive flood in 1996, the Yakima River’s flooding was much more modest. This is partially because the system that brought the 1996 event was preceded by a prolonged cold period with lowland snow. This froze the ground, making it so that water could not seem in and instead flowed almost directly into area rivers. For more on that, see Mark’s writeup on AgWeatherNet.

Finally, after the event was over Umatilla County sent out a notice to residents living in some parts of the Blue Mountains. This read, in part, “[residents] are advised to assess their resources and determine whether they are able to shelter in place for several weeks.” The people this was sent to were not threatened by flooding themselves but were cut off from the outside world because of flooding. Oregon National Guard airlifted dozens of people out of these areas and repairs are underway on those roads.

We could call this a forgotten impact of flooding. The people who were evacuated were those whose homes were directly threatened by flooding, not those who were threatened with being cut off. This drives home two important points:

First, you need to be aware of your surroundings and what routes you have as an option to you if you need to leave. In some events, like flooding or wildfires, you may not be evacuated due to being threatened but knowing your surroundings puts you in a better situation to self-evacuate if you deem it safe and reasonable to do so. Knowing your surroundings can also help you gauge whether you should self-evacuate.

Second, it is important to have food, water, and other resources stored in case of emergency. The example here, of course, is because of flooding but the usefulness of storage like this can go far beyond natural disasters. Nobody ever regretted being prepared!