Puyallup, Nisqually, Chehalis rivers among those under threat of flooding. Here’s when and where
Heavy rain over the last few days throughout Western Washington has meteorologists concerned about minor flooding along several rivers in the south Puget Sound region.
As of late Monday night, over 2.5 inches of rain had fallen in the Seattle-Tacoma area since Friday, while Olympia has seen 3.77 inches. The deluge of rain results from a phenomenon called the “Pineapple Express,” which is when subtropical moisture, typically from Hawai’i, is pulled toward the northwest United States, resulting in heavy rainfall.
The National Weather Service in Seattle has issued a flood watch for most of northwest and west central Washington until 4 a.m. on Thursday.
Where will the flooding occur?
Weather Service meteorologist Kirby Cook told McClatchy News that there are currently four areas of concern for river flooding:
The Puyallup River near Orting
The Carbon River near Fairfax
The Nisqually River near the Billy Frank Jr. Nisqually National Wildlife Refuge
The Chehalis River at Grand Mound, and the Black River
Cook said that all four locations are in danger of flooding that overflows river banks and can pool on roads. On the Chehalis River, the weather service says access to the Chehalis Reservation could be cut off, and some residences along the Chehalis and Black rivers in the Independence Valley could flood.
“It might be early tomorrow morning, but definitely late this evening into early Wednesday morning,” Cook said on the timing of the flooding in an interview on Dec. 5.
But even though most of the flooding may be minor, it can still pose a danger to motorists if flowing fast enough. According to the Snohomish County Department of Emergency Management, the following depths of fast-moving water can cause the following problems:
Six inches: Can knock over and carry away an adult
12 inches: Can carry away a small car
18-24 inches: Can carry away most large SUVs, vans and trucks
“We’re seeing a lot of reports of (water) ponding on roadways and other heavy rainfall impacts,” Cook said. “If people see ponding on roadways and need to drive through that, they need to be careful of hydroplaning and things like that.”
Why is flooding a possibility?
Flooding is a concern for meteorologists with the current storm because of the “Pineapple Express” factor.
A typical December storm in Washington state will pull cold moisture from the Gulf of Alaska, meaning even if it’s raining at lower elevations, precipitation will fall as snow in the mountains, sticking to surfaces.
But the Pineapple Express — also known as an atmospheric river — is pulling in warmer moisture from Hawai’i, drastically raising the altitude at which snow can fall.
“Any precipitation that’s falling, which is quite often heavy associated with these (systems), because of the deep moisture, it’s falling as rain in the mountains and not snow,” Cook said. “And that rain is able to run off and affect rivers. And so this is why we get a lot of flow in the rivers. It goes right into the rivers and can cause flooding.”
This story was originally published December 5, 2023 at 11:55 AM with the headline "Puyallup, Nisqually, Chehalis rivers among those under threat of flooding. Here’s when and where."
