Fall-like weather pattern through the weekend

It’s not often that we talk about deep low-pressure systems coming for the Pacific Northwest this late in the season, but here we are with an atmospheric river that is looking to do just that. We’ve had a small amount of rain today in the area, with 0.04 inches falling so far at the Tri-Cities Airport and 0.03 inches at my house here in Boardman.

Off and on rain showers are expected to continue this evening and overnight, but rain will become more widespread during the day tomorrow. Tomorrow isn’t the only day where we’re looking for quite a bit of rain, so before we dive into what to expect tomorrow let’s take a look at the forecast:

This atmospheric river is poised to funnel plenty of moisture into the Pacific Northwest. This has prompted the National Weather Service to issue a Hydrologic Outlook for the entirety of the area except for the Columbia Gorge. These alerts are issued when rainfall or snowmelt will cause rivers and streams to rise significantly.

Significant within bank raises can be expected, with some locations reaching bankfull stages this weekend. The greatest chance of bankfull conditions will occur on the Yakima, Naches, and Grande Ronde rivers. No rivers are forecast to reach flood stage at this time. However, minor flooding and ponding of water may occur along small streams, creeks, and ditches, particularly where snow, ice, or debris may clog drainages. Unpaved rural roads my become muddy and impassable. Rock or mud slides may occur in steep terrain.

NWS Pendleton

Areas west of the Cascade Mountains are expecting over an inch of rain expected tomorrow, with the precipitation falling as snow at elevations over 3,500 feet. This places snow down to White and Stevens Passes, but with Snoqualmie Pass lying at 3,022 feet, most of the precipitation there will be in the form of rain. As of writing (just before 5pm Wednesday), this next round is beginning to head ashore near the Oregon/California border, as seen by the following satellite image.

In our area we will be seeing rain, as overnight temperatures will remain at or above 40 in the lowlands. That said, we won’t be having as much rain as is expected west of the mountains. Rainfall values are looking to be between 0.10 and 0.25 inches for tomorrow throughout the Mid-Columbia. Indications right now are that winds will remain fairly light through the duration of this portion of the system.

We’ll see a break in the rain Friday as we will be in between systems. Some scattered showers are possible, especially in the morning, but rainfall amounts should remain light. Friday night the bigger storm begins to impact us with rain and increased winds. More recent model runs have backed off a little on the maximum wind gusts Saturday, but have still kept them near 35 mph at 10m (33 feet) above the ground.

That low-pressure system (marked as being as low as 972mb on the chart above) is expected to make landfall in mid- to northern Vancouver Island. As with everything in weather prediction, though, nothing is really set in stone until it happens. If this low were to remain offshore, rather than making landfall, our wind and rain will likely be lessened. On the flip side, if the low decides to make landfall on the south side of Vancouver Island, or even on the Olympic Peninsula, we can look forward to stronger wind and more rain.

Right now it looks like the low will bottom out with a minimum sea-level air pressure near 970mb (some models have it even lower). To give you an idea of its intensity, that’s similar to last year’s Hurricane Katia at its maximum strength before striking Mexico. Looking at it another way, that would be the air pressure during “normal conditions” at 1,200 feet above sea level (“normal” sea level pressure is 1013mb).

Rain should begin to let up Saturday night, though we could see some off and on showers lasting into Sunday. Breezy conditions are expected to continue through the day Sunday, letting up at the beginning of the work week.

We’ll continue to keep you updated on the latest with this incoming system here and on our Facebook pages, Tri-Cities Weather and WeatherTogether.