The wettest day of the year is possible for Seattle as a soaking and stormy pattern resumes, almost right on cue for the typical wet season.
Published Sep 25, 2023 1:24 PM EDT | Updated Sep 26, 2023 5:57 AM EDT
An uptick in storms moving in from the Pacific Ocean in recent days has already marked an end to the summer dry season in parts of the northwestern United States and British Columbia. AccuWeather meteorologists say potent storms this week will bring bouts of significant rain, gusty winds and rough seas to the region.
From June 1 to Aug. 31, a stretch known as meteorological summer, many areas of the coastal Northwest pick up 2-3 inches of rain, according to historical averages.
The period from mid-September to November typically marks an increase in rainfall for the region. Almost right on cue, rain began to fall more regularly in many coastal areas last week compared to the prior few months.
The historical average rainfall in Seattle is 1.61 inches in September, but the monthly historical average jumps to 3.91 inches in October and 6.31 inches in November. As of Tuesday morning, Sept. 26, Seattle has received 2.28 inches of rain, with more to drench the area through the week due to a potent storm.
"Before Monday, Seattle's wettest day in 2023 was on May 5 when 0.70 of an inch of rain was recorded," AccuWeather Senior Meteorologist Reneé Duff said. As AccuWeather accurately predicted, the Emerald City picked up more than that on Monday with 0.82 of an inch, making it the wettest day this year.
Since record-keeping began at Seattle-Tacoma International Airport in 1945, there has never been a year without a daily rain amount of an inch or more, according to the National Weather Service (NWS).
"A series of Pacific storms will deliver an extended period of mostly beneficial rainfall to the coastal Northwest and much of British Columbia this week, especially through Thursday," AccuWeather Senior Meteorologist Brett Anderson said.
Downpours will extend as far south as coastal areas of Northern California and as far to the north as the entire coast of British Columbia.
A general 1-2 inches of rain will fall through Wednesday with pockets of 2-4 inches of rain west of the Cascades. Rain totaling 4-8 inches is likely to pour down on the Olympics in northwestern Washington and on the interior of Vancouver Island and much of the west-facing slopes of the British Columbia mainland.
Where the heaviest rain falls, ponding can occur on area roads.
The more humid air, even where winds increase, will slightly lower the wildfire risk from Northern California to British Columbia. Forecasters still urge caution with open flames and power equipment as little to no rain may fall in some areas east of the Cascades.
East of the Cascade Range, less than 0.50 of an inch of rain is likely to fall as the mountains will tend to effectively screen out the Pacific moisture, Anderson said.
"In burn scar areas, there will be the risk of flash flooding and perhaps some debris flows that could lead to detours on exposed roadways," Anderson added. Rocks may be knocked loose by the sudden rainfall, potentially falling on secondary roads through the mountains and hilly areas.
As the potent storm continues to spin and pivot offshore of British Columbia, the strongest winds will be directed toward coastal areas.
"The main impacts from the wind will be along the coast with rough surf with the potential for some tree damage and loss of power for some locations," Anderson said. "The strongest winds will be directed into Vancouver Island, where gusts of 60-80 mph are most likely."
Some gusts of 30-40 mph are likely along the Interstate 5 corridor between Seattle and Portland, Oregon, and gusts of 40-60 mph will be common along the immediate coasts of Oregon and Washington.
The strong winds generated by the storm for many days over the Pacific have stirred large waves in the region. A heavy surf advisory was in effect for portions of the Oregon and Northern California coasts. According to the NWS, breakers will average 20-25 feet, and some could exceed 35 feet. A gale warning was in effect for much of the Northwest coast.
The combination of rain, gusty winds and low clouds will lead to airline delays and the potential for flight cancellations.
The storm lasting through Tuesday will be the most potent of the series, but additional smaller, less-intense storms will pivot in from the Pacific and bring smaller pockets of drenching rain and locally gusty winds into Friday.
The storm systems will also reinforce cool air already in the region.
"Temperatures much of this week will average anywhere from 5-15 degrees below the historical average across the Northwest," Anderson said.
As the stormy pattern eases along the Northwest coast, chilly air will take root over the interior West with areas of low-elevation rain and mountain snow likely to ramp up.
Temperatures tend to trend downward by a degree every two to three days this time of the year. In Seattle, the historical average high trends downward through the 60s F this week. In Portland, Oregon, the historical average trends downward through the middle and lower 70s F. In Vancouver, British Columbia, the average temperature trends downward through the lower 60s F (17 to 16 C).
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