NORTH SOUND, DEC. 9: Are you ready for a lot more rain? Brace yourself, since an atmospheric river sent a surge of rain into the North Sound and the mountains on Monday, Dec. 8. This weather system sagged south toward Northwest Oregon Tuesday for a brief break in the rain and then is expected to pivot north again Tuesday night and Wednesday for another surge of rain.
An atmospheric river is a moisture stream from tropical or subtropical sources transported into mid-latitude storm systems, producing copious amounts of rainfall with relatively high snow levels. This feature is actually quite common around the globe and helps produce the bulk of mid-latitude annual rainfall, including the Pacific Northwest.
How Much Rain?
Forecast rainfall amounts through Wednesday, Dec. 10, are indicative of a soggy atmospheric river. The Cascades will receive the greatest amount of rain, ranging from 6 to 12 inches. The Olympics will also retain its rain forest reputation with 5 to 10 inches expected.
The North Sound will benefit from the Olympic Mountain rain shadow, reducing rain amounts from about a half to one and a half inches of rain through Wednesday.
Atmospheric river events involve warm, moist air, resulting in elevated mountain snow levels. On Monday, the snow level rose as high as 7,000 feet. With the system sagging south into Northwest Oregon Tuesday, snow levels have dropped close to 4,000 feet.
On Tuesday night and Wednesday, however, as the stream of warm moisture pivots back north, snow levels are forecast to rise again, this time close to 6,000 feet. With so little current mountain snow, the heavy amounts of rainfall are quite likely to run straight off into rivers and streams.
Impacts: Flooding
The last heavy rainfall event of this kind was back in November 2022. The most serious flooding was in the North Sound, primarily in Whatcom and Skagit Counties.
This atmospheric rain event has created more widespread flooding in the North Sound and much of Western Washington. Over two dozen rivers from near the Canadian border to the Columbia River have exceeded flood stage with over a dozen reaching major flood levels closing roads and flooding homes and farmlands.
The most serious flooding in the North Sound involves rivers flowing out of the Cascades, including the Skagit, Stillaguamish, Skykomish, Snoqualmie, and Snohomish rivers. Monitor the latest river and flood conditions by monitoring the National Weather Service website at weather.gov/seattle/ on the rivers and lakes page.
If you live in or have just moved into a flood-prone or low-lying area, there are steps to take in advance of flooding or during flooding. Some resources include the Washington State Emergency Management Division flood preparedness webpage, FEMA’s flood readiness webpage, and your local county emergency management websites.
Those who commute through these low-lying and flood-prone areas may also find roads closed due to high water. Follow the detours. Remember, turn around, don’t drown.
In addition, many other roadways may have standing water. When driving, slow down, avoid hydroplaning, and leave plenty of distance between your vehicle and the vehicle in front of you.
Impacts: Wind
Winds can be another impactful factor this week. The next surge of rain with this weather system is forecast to be accompanied by more strong southerly winds, likely to pick up with gusts up to 30 to 45 mph Tuesday night into Wednesday morning. Local downed trees or limbs, and power outages are possible.
Impacts: King Tides
This month’s King Tides period peaked over this past weekend, yet high tides remain relatively high, and minor tidal overflow flooding remains possible in low-lying areas into Wednesday.
Impacts: Landslides
The calendar year has been a dry one. Through this past weekend, Everett Paine Field had received less than 40% of its annual rainfall. This period of rainfall, though, will moisten upper soil layers while deeper soil layers remain dry. This scenario makes it easier for the upper-heavy wet soils on steep slopes to slide and makes these regions more prone to landslides. Several landslides have already occurred.
What’s Ahead
Rainfall from this atmospheric river is expected to taper off starting Wednesday night. Rain remains in the forecast, heading into the coming week, but not fueled by sub-tropical moisture. Yet, it will take a period of time for rivers to recede back below flood stage.
The latest weather outlook as the calendar approaches the Christmas holiday continues to reflect warmer and wetter than average conditions, not good news for those who want snow on mountain slopes.
As always during flood season, remain vigilant and prepared in case of significant flooding, not only for your home or business, but also for your commute if it takes you through river basins.
