Major Pattern Change Ahead for the Pacific Northwest

FastCast--Friday, Sep. 22 to Monday, Sep. 25:

A major weather pattern change is ahead for the Pacific Northwest, beginning this weekend. First, we will have our final "nice" day for quite awhile on Friday, with sunny skies and lowland highs in the mid to upper 70s and crisp morning lows in the mid 40s to low 50s. Conditions will change over the weekend, with clouds arriving and much cooler temperatures expected. On Saturday, expect cloudy conditions and highs in the mid to upper 60s. Rain is expected to arrive in the afternoon as a weak system moves through, with 0.1-0.3" possible in the lowlands, with isolated higher amounts possible. A much more robust system, likely our first storm of the season, will move through from Sunday evening through Monday. Models still have some disagreement on the strength of this system and how much rain it will bring, but a good consensus for the lowlands is an additional 0.4-1" by Monday night, the largest rain amounts we've seen since early May. Highs from Saturday to Monday will likely be in the mid to upper 60s, with lows in the upper 40s to mid 50s. More systems (and more rain) are likely through next week, but details remain uncertain, so stay tuned!

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Continue reading the full blog below!

A major pattern change is likely for the Pacific Northwest this weekend, with wet and stormy conditions expected into next week (and beyond). Let's take a look at the forecast!

Below is the European Ensemble (EPS) upper-air forecast for late Sunday night.

Notice the large trough off the Pacific Northwest. Troughing will be dominant through the next 7+ days, bringing storms into the region. 

Let's compare this to the GFS Ensemble (GEFS) for the exact same time (late Sunday night).

The GEFS has a nearly identical forecast, showing that there is very good model agreement in a pattern change.

But, before we get there...the last warm & sunny day for quite awhile is ahead on Friday. Below is the European model forecast for Friday's highs.

Expect lowland highs in the mid to upper 70s, summer's last stand (ironically on the first day of astronomical fall). The coast will reach the mid 60s to low 70s (except in the mid to upper 50s at the beaches), the Willamette Valley will reach the upper 70s, and Eastern Washington will reach the upper 60s to mid 70s.

Clouds will increase overnight into Saturday, with some rain from a weak system arriving Saturday afternoon. Then, a more significant system will begin impacting the area Sunday evening, continuing through Monday. We'll take a look at the rain forecast through Sunday night on the EPS and GFS forecasts.

First, the European EPS forecast through 11 PM Sunday.

The European EPS forecast shows 0.2-0.4" in the lowlands by Sunday night, with 0.4-0.6" on the coast and 0.5-0.75" in the mountains. Also, notice the swath of rain in Eastern Oregon and SE Washington, which is from the previous system (Wednesday-Thursday of this week).

We'll compare this to the GFS forecast for total rain by 8 PM Sunday (3 hours earlier than the EPS).

Notice the GFS shows significantly more precipitation by Sunday night, with 0.4-0.6" in the lowlands and 0.4-1.2" on the coast, plus 0.5-1.4" in the mountains. This forecast also shows the typical rainshadow northeast of the Olympics. 

This shows that there is still disagreement in the timing of the second, more powerful system (Sunday night to Monday). The European EPS shows the heaviest rain not moving in until early Monday morning, while the GFS shows it moving in Sunday evening. Stay tuned for more updates on this.

Now, let's take a look at the extended forecast and what is likely ahead with this pattern change. First, we'll look at Seattle's rain chances, then temperatures in Seattle and Spokane.

Below is the European EPS forecast with all 50 ensemble members showing total rain through the next 15 days.

The ensemble mean (bar at bottom) and all the members agree with rain onset over the weekend, getting heavier midweek. The ensemble mean for the next 2 weeks is around 2.5". These totals will be higher in the mountains and on the coast, and likely far less in Eastern Washington.

Let's compare this to the GEFS forecast, which only has 30 ensemble members, showing total rain through the next 10 days (less than the previous forecast).

The GEFS shows around 2" of rain through the next 10 days (around October 1), which is nearly the same as the European EPS forecast. Both forecasts show moderate confidence in light rain on Saturday, with much higher confidence in the late Sunday-Monday system.

Now, let's take a look at temperatures in Seattle and Spokane. First, the Seattle forecast from the European EPS model.

This forecast shows highs in the upper 70s on Friday, followed by a clear decrease to the 60s for the remainder of the forecast. Notice a warmer day on Monday (highs in the upper 60s), with warmer lows from Monday to Wednesday. This is likely due to the subtropical component of the incoming systems, since there is an atmospheric river potential.

Next, let's look at the same forecast for Spokane, seen below.

This forecast shows Spokane's highest temperature on Monday, then a sharp decline over the remainder of next week, reflected both in highs and lows.

To round out the blog, we'll take a look at the NWS Climate Prediction Center 6-10 day forecasts for September 27 to October 1, showing what to expect late next week.

First, the temperature outlook, seen below.

This forecast shows a 50-70% probability of below average temperatures for the Northwest.

Next, we'll look at the precipitation outlook.

This forecast shows a 50-70% probability of above average precipitation for the Northwest.

So, the bottom line is that a significant pattern change is likely for the Northwest, with much more rain and cooler temperatures expected. Stay tuned over the coming days as details get clearer and forecasts get more refined!