Our first respectable storm of the season drenched Western Washington and Oregon with heavy rain on Wednesday and Thursday, bringing an inch of rain to many places in the lowlands and several inches in the mountains. Over the past 24 hours, the rain has primarily been concentrated over SW Washington and NW Oregon as a juicy cold front/atmospheric river has slowly sagged southward, bringing nearly an inch of rain to the Portland metro area and over 4 inches to the Sheep Canyon SNOTEL site just west of Mt. St. Helens. By the way, I’m gonna send an email to NOAA tomorrow asking them if they can include 48 and 72-hour precip totals on their interactive hazards map, as that would be awesome for keeping track of storm totals.
Credit: National Weather Service
The southwestern slopes of the Olympics received the most rain from this storm, with the notoriously rainy US Climate Reference Network station 4 miles NE of Lake Quinalt picking up 6.87 inches of rain in the last two days. But with antecedent stream levels being relatively low, flooding did not occur on any rivers. The Skokomish at Potlatch came mighty close though…
Credit: National Weather Service Advanced Hydrologic Prediction Service
Friday will feature scattered showers as cool air aloft filters in behind the cold front that came through this afternoon, with snow levels falling to 4,500 feet or so. There is a snow advisory in effect until 5 PM Friday for the SW Washington/NW Oregon Cascades for 3-7 inches at the passes and 8-15 inches at higher elevations.
I’m really keen on an even stronger system slated for this weekend. A powerful ~970 hPa low in the Gulf of Alaska should drive a strong warm front into the area Saturday morning, followed by a narrow but intense atmospheric river that will pummel Washington later Saturday and drift down to Portland early Sunday morning. Flood watches are already in effect for Western Washington, and hydrologic outlooks are in effect for Western Oregon (I suspect these will be upgraded to flood watches soon).
Recent models have been ratcheting up the rainfall totals, and the latest runs now have up to 2 1/2 inches of rain falling between Seattle and Corvallis between 5 AM Saturday and 5 AM Sunday. The mountains will see much more, with the potential for 24-hour totals of over 10 inches along the volcanoes.
Credit: University of Washington Atmospheric Sciences
The heaviest rain for Portland should occur Sunday morning as the atmospheric river sags southward. Urban flooding is a concern due to leaves clogging storm drains, but there is also the potential for flash flooding and debris flows by the Eagle Creek Fire burn scar.
Finally, the coast will get battered by high winds with this system, and a steep north-south pressure gradient will set up across the Willamette Valley as well, potentially giving advisory-level winds there. The graphic below from the 4km WRF-GFS shows how dramatically winds increase once you are in the storm’s “warm sector” (behind the warm front and ahead of the cold front).
It’s getting late here, I’ve gotta hit the hay. I’ll have another update for you tomorrow!
Thanks for reading,
Charlie
