Significant Atmospheric River to End January

FastCast—Friday, Jan. 26 to Monday, Jan. 29:

The end of January will be punctuated by a significant atmospheric river will bring 2-3” of rain to the lowlands through late Monday. The heaviest rain will fall from late Friday to early Sunday, with standing water and ponding expected across the region. Additionally, moist subtropical flow due to the atmospheric river will bring very warm temperatures for January, with highs from Saturday to Monday reaching the upper 50s to low 60s, with isolated areas reaching the mid 60s on Sunday and Monday. Friday will only be in the upper 40s to low 50s. Expect lows in the mid to upper 40s. Another potential cause for concern is massive rises in snow levels in the Cascades and Olympics. On Friday, snow levels will hover around 4,000 feet, but by Saturday, they will have skyrocketed to 7,000-8,000 feet. It appears likely that snow levels will remain at 6,000+ feet through the middle of next week. During this time, the Cascades will receive 3-5” of rain, with 4-6” in the Olympics.

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Continue reading the full blog below!

A significant atmospheric river is expected to impact the Pacific Northwest over the next few days, with the heaviest rain impacting the region from late Friday night through early Sunday (longer in BC). This atmospheric river will bring heavy rain, potential urban and river flooding, and abnormally warm temperatures to the region.

Below is the European model ensemble forecast showing precipitable water late Friday night.

Precipitable water is a measure of the amount of moisture in the atmosphere, which shows atmospheric rivers. Note the huge area of moisture that extends from Hawaii to the Pacific Northwest, which is where the term “Pineapple Express” originates from. 

This atmospheric river will impact Western Washington from Friday to early Sunday, with areas of showers continuing through Monday and Tuesday. Let’s take a look at the total rain forecasts.

First, the forecast for total rain through late Monday night from the European model.

This forecast shows 1.5-2.75” of rain across the lowlands through Monday night, except 0.8-1.5” in a rain shadow to the NE of the Olympic Peninsula. On the coast, expect 3.5-4.5”, with the Cascades getting 3-6” and the Olympics getting 4-6”. Areas from Shelton northward along the Kitsap Peninsula will get 2.5-3.5”. Even Eastern Washington will be getting rain with this system, with 0.2-0.5” expected.

Now, let’s compare this to the GFS forecast, also showing total rain through Monday night.

This forecast is generally less rainy than the European, with more rain for areas Portland southward. However, it shows 1-2” of rain for the lowlands, except 0.2-0.8” in a very notable rain shadow NE of the Olympic Peninsula. The coast gets 2-3”, with 3-6” in the mountains. Eastern Washington gets 0.2-0.5”, except up to 0.75” from Grand Coulee to Spokane.

Another impact of this atmospheric river will be abnormally warm temperatures due to strong S-SW flow. Below is the European model forecast for highs on Monday.

This forecast shows highs west of the Cascades reaching the upper 50s to low 60s. Even the mountains themselves will reach the mid 30s to low 40s, with most of Eastern Washington reaching the low 40s to low 50s. 

Finally, let’s take a look at the NWS Advanced Hydrologic Prediction Service forecast for potential river flooding.

As of the writing of this blog, forecasts aren’t showing this atmospheric river as a widespread river flooding event. The Skokomish River in Mason County will likely reach moderate flood stage, with the Snoqualmie/Snohomish, White, Nooksack, Chehalis, Cowlitz, and Grays Rivers reaching bankfull stage. Even if a river doesn’t reach bankfull, every river will rise due to this storm. Stay tuned for updates as this forecast can change. 

Since this is an atmospheric river situation, and some uncertainty remains as to rain totals and their impacts, I will have another update on Friday night. Stay tuned!