Another stormy week is ahead for the Northwest. While the East Coast sits in wait for any kind of interesting weather, the Pacific Northwest is going to be continuously slammed this week with 3 individual storm systems. The first, which passed through Monday, and the third, which will pass Thursday to Friday, will be similar: quick-hitting and average. The second storm, the one expected to hit today, will be particularly strong, and is expected to cause some problems for the region in the form of heavy snow, localized flooding, and high winds.
Like many Pacific storms, this one is driven by intense tropical moisture brought up to the Northwest by an atmospheric river (AR). Again and again, heavy rain and snow wouldn’t be possible without moisture from the “Pineapple Express,” a common AR that travels from near HI to the US west coast. The AR that will impact the region with today’s storm is relatively potent, meaning that it will transport more moisture than many other ARs.
When zoomed out, it’s easier to see this moisture’s origins, and the windy, filament shape of an AR. From the picture below, it is easy to see an area of enhanced moisture transport originating just north of HI, getting caught in flow from a strong marine low pressure cyclone.
It is this combination of intense moisture from the AR and the strong marine cyclone that will drive the storm. Marine cyclones are much less affected by friction than cyclones over land. As a result, low pressure centers become “secluded” instead of “occluded,” like we usually see on the East coast. These secluded lows develop a strong bent-back front that wraps around the cyclone and trails off to the south. Associated with these strong cold fronts are high winds and strong lift.
Wrapped up in the cyclone’s flow, the most intense moisture will hit the Pacific Northwest this afternoon, dropping 3-6 inches of rain along the coast and 1-3 inches further inland. Localized and urban flooding is likely with this storm, especially in areas near the coast where rainfall is higher. Strong wind gusts along the coast of near 60 mph could mean downed trees and possible power outages. Going into tonight, driving could be very dangerous due to flooded streets, low visibility from heavy rainfall, and unpowered street lights.
Snow is also expected for this storm, but only in high elevations above 3,000 feet in the Cascades. Here, over a foot of snow can be expected, meaning dangerous driving conditions and increased avalanche possibilities. Elevations of 2,000-2,500 feet can still expect lighter snow of 4-6″. Snow will spread east into the Northern Rockies by tomorrow.
Although this system will not become the end of the week storm for the East coast, a developing upper-level low over the Four Corners today will actually be. Make sure to follow the WeatherOptics team as we update you on all the latest with this potent storm as its travels to the East.