Brief Spring Atmospheric River Bringing a Wet Weekend

 FastCast--Saturday, May 4 to Thursday, May 9:

A fast-moving atmospheric river will bring a wet weekend to the Pacific Northwest. With the majority of the atmospheric river being aimed at Oregon, Washington's rain totals will be heaviest the further south you go. In the lowlands, generally expect 0.75-1.25" from Seattle southward, and 0.4-0.8" from Seattle northward. Expect rain at times from early Saturday through Sunday, becoming more showery on Monday. Some showers may linger into Tuesday. Conditions will change on Wednesday, turning partly sunny. Mostly sunny skies are likely by Thursday. Regarding temperatures, the weekend will be chilly around the lowlands, with highs in the low to mid 50s. However, Saturday's highs north of Everett will likely reach the mid 60s. Temperatures will be in the mid to upper 50s on Monday and Tuesday, before reaching the low 60s on Wednesday. By Thursday, a warmup is likely to begin, with highs reaching the upper 60s to low 70s. Expect lows through Thursday to remain in the low to mid 40s. Even warmer conditions, likely the warmest of 2024 so far, are possible for Mother's Day weekend, so stay tuned!

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A brief spring atmospheric river is impacting the Pacific Northwest, especially Western Oregon, with significant precipitation this weekend. Let's take a look at the forecast!

We'll start with the European model forecast for precipitable water late Friday night, showing the location of the heavier concentrations of water vapor (aka...the atmospheric river).

Notice how the highest concentrations of moisture are directed at Western Oregon, with some elevated moisture content making its way into Western Washington.

Now, let's look at rain totals. First, we'll take a look at the European model forecast for total rain through Monday night.

From Skagit County southward, this forecast shows 0.5-1" of rain, with areas from Olympia to Portland and on the coast getting 1.25-2", and the Willamette Valley and Oregon Coast being soaked by 2-2.5"! Whatcom and San Juan Counties will likely receive 0.3-0.5". Even Eastern Washington will get in on the rain, with most of the region getting 0.4-0.8", except 0.75-1.25" along and south of I-82.

We'll compare this to the NWS NBM high-resolution model forecast, seen below, also showing total rain through Monday night.

This forecast shows a similar picture for the lowlands and coast, with the Willamette Valley getting 1.4-2", and the Oregon Coast getting 1.5-2.5". Eastern Washington's highest amounts are confined to areas south of I-90 and near the Idaho border, with totals of 0.5-1.2", highest near Walla Walla and the Tri-Cities.

Temperatures will be on the cool side due to all the rain and cloud cover. Below is the NWS NBM forecast for highs on Saturday.

As rain and clouds will move from south to north, notice that highs increase as you move north through the lowlands. Areas from Everett to Tacoma will reach the mid to upper 50s, with areas north of Everett in the mid to upper 60s. From Tacoma south, expect highs in the low to mid 50s, except in the low 50s on the coast and in the Willamette Valley. Removed from the rain on Saturday, Eastern Washington will reach the upper 60s to mid 70s.

The entire region will be cool and cloudy on Sunday, as seen in the NWS NBM forecast below.

On Sunday, expect highs west of the Cascades to reach the low to mid 50s, with Eastern Washington reaching the low 50s near Spokane to low 60s near the east slopes of the Cascades. However, the Okanogan River Valley may reach the upper 60s to low 70s.

Here's the NWS NBM forecast for Monday, as the atmospheric river's associated rain begins to move out of the region.

Temperatures on Monday will remain on the cooler side, with highs in the low to mid 50s west of the Cascades, and in the upper 50s to mid 60s in Eastern Washington.

However, a significant change is possible as we move through next week. Below is the European model forecast for Thursday.

Notice the large ridge of high pressure located just north of Washington. This ridge, which will build from Wednesday onward, will set the stage for a warm end to the week and Mother's Day weekend, so stay tuned! There is a potential for the warmest conditions of 2024 so far!