A series of storms will bring a deluge of moisture into the Pacific Northwest later this week, sparking not only a threat for flooding rainfall, but also mountain snowfall, blustery conditions and travel disruptions across the region.
While some locations across Washington, Oregon, Idaho, Montana and Wyoming will already be experiencing a storm diving southeastward into the U.S. from western Canada into the start of the week, AccuWeather meteorologists warn there is much storminess to come as the week progresses.
Early-week pattern
The first storm impacting the Northwest through early week will continue tracking eastward and eventually through the northern Rocky Mountains into the early morning hours on Monday. Additional pockets of showers and periods of high-elevation snow showers will occur during this time.
There will be a lull in storm activity for most locations in the Pacific Northwest later Monday into Tuesday night, forecasters note. Most regions will observe mainly dry and calmer weather during this time as a northward bulge in the jet stream ushers in a zone of high pressure at the surface.
The early-week lull between storms will also provide temperatures near the historical average, or seasonal norm. Cities like Seattle and Portland, Oregon, are projected to have daytime highs in the lower to middle 60s approaching the midweek period.
An Atmospheric River to drench areas late week
AccuWeather meteorologists warn that the latter half of the week will feature a more tumultuous pattern across the West compared to the late weekend storm. Additional waves of rainfall, even times of visibility-reducing downpours, are on the docket with multiple storms expected between Wednesday and the upcoming weekend.
Later Wednesday, the next storm will advance into parts of southwest British Columbia, Canada, Washington and portions of northern Oregon with occasional showers and breezy conditions at the direct coastline.
A potent storm will follow in quick succession across the Northwest U.S. on Thursday, advancing first into western British Columbia as early as Wednesday evening with drenching rainfall and snow across the higher terrain. Wind gusts associated with this storm can range upwards of 50 mph over the ocean and the immediate coastal areas to 30-40 mph farther inland.
Merrill added that a series of ripples in the flow pattern will follow into the weekend and eventually spread south into Northern California.
While a portion of the interior Northwest U.S. is expected to observe between 1-2 inches of rain from the late-week storminess, coastal regions in Washington and Oregon are projected to pick up higher amounts on the order of 2-4 inches. Localized pockets of around 6 inches will not be off the table, particularly in zones along the coast that see repeated downpours from Thursday into the weekend.
A continued pattern of storms into the Northwest states will gradually help to aid the zones facing levels of severe and extreme drought. Area river levels in need of moisture can benefit from a pattern such as this. It will come at a cost; however, with travel disruptions likely and days of rain generating flooding from the Northwest coast into Northern California.
"The pattern setting up along the Northwest to Northern California coast is indicative of an Atmospheric River as the moisture can be traced back to Southeast Asia. This type of a pattern typically yields heavy rain and flooding with only very high-elevation snow in the Sierra given the warm air bleeding into the storm," added Merrill.
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In addition to the rain and wind, forecasters note that a heavy, wet snow will fall in the highest crests of the Cascades and Sierra, likely above pass level during the late-week period. The potential will remain for cooler conditions through the weekend to lower snow levels and impact heavier-traveled roadways and passes across the region.
"Atmospheric Rivers are most common from November to January in the Northwest into Northern California. They also usually dump heavy rain in a short time period; within 24 hours, several inches can trigger flooding and mudslides. The upcoming late-week and early weekend storm will likely be no exception to that rule."