Significant Atmospheric River Ahead

FastCast—Sunday, Apr. 9 to Wednesday, Apr. 12:

A significant atmospheric river is ahead for the Pacific Northwest, with steady rain impacting the area from Sunday morning through Monday night, with another system bringing rain on Tuesday. Through Tuesday, expect rain totals of 1-2 inches in the lowlands, with 0.25-1” in the rain shadow (Whidbey Island and the San Juans). Areas in the Cascades (under 6,500 feet), in the foothills, and from Chehalis southward will pick up 2-2.5”. The coast will get 2.5-4”, and isolated areas in the Coast Range and Olympics could get over 4”. In Eastern Washington, expect 0.4-1.25” of rain through Tuesday. Snow levels will rise to 6,500 feet by Sunday night, so all passes will receive rain, though snow levels will crash on Tuesday, dropping to 1,500 feet by Tuesday night. During this time, snow is expected at the passes (stay tuned in future blogs about this). Breezy conditions are expected around the region as well, with gusts of 25-35 mph in the lowlands and 35-40 mph on the coast. Expect highs in the mid 50s on Sunday dropping to the mid to upper 40s by Tuesday, with lows in the low 40s on Sunday, dropping to the mid to upper 30s by Tuesday. Standing water and ponding is expected on area roadways, with rises on area streams and rivers, and moderate flooding on the Skokomish River near Potlatch and Union. 

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The biggest rain event of 2023 is ahead for the Pacific Northwest, with a significant atmospheric river impacting the region from Sunday through late Monday, followed by more rain in a system on Tuesday, with some showers continuing through early Wednesday.

Below is the UW forecast for water vapor transport (how atmospheric rivers are measured) at 1 PM Sunday.

This atmospheric river will be aimed directly up Puget Sound, so there will be no rain shadow for the metro area. Expect a rain shadow from the Olympics to Southern BC, mainly west of I-5. 

Rain totals will be significant. Let’s start with the European model forecast through Tuesday night. This forecast includes the Tuesday system (post-atmospheric river).

In the metro area, expect 1-2” of rain, with 2-2.5” from Chehalis southward and in the Cascades/foothills. Notice the rain shadow north of the Olympics. Additionally, expect 2.5-4” on the coast and 0.4-1.25” in Eastern Washington.

Now for a shorter and higher resolution forecast, let’s take a look at the NAM forecast through early Tuesday morning.

Higher resolution forecasts tend to show higher rain amounts over terrain, since these forecasts can more accurately show terrain features. The NAM agrees with the European in giving 1-2” to the lowlands, though the rain shadow is more pronounced from Everett north and west. This forecast gives the coast and foothills 2-3”, with the Coast Range, Olympics, and Cascades getting 3-5”. Eastern Washington gets 0.3-1.25”, with some areas getting less, especially near the Cascades and in the Palouse.

We will look at one more forecast, the UW WRF ultra high resolution model, through 

The UW forecast shows by far the highest totals for the entire region, with 1.25-2.5” from Everett southward and quite a pronounced rain shadow to the north. The coast and mountains get significant totals of 3-5”, with isolated totals over 5”. This solution is a bit more unlikely than the others, but is still possible.

There is a slight risk that some of this rainfall may bring urban flooding at short notice (flash flooding), as seen in the Weather Prediction Center Excessive Rainfall Outlook for Sunday.

The marginal risk of excessive rainfall exists for the South Washington Cascades, the Olympics, and the coast & Coast Range from Florence, OR northward.

Finally, due to this atmospheric river bringing more mild subtropical air, snow levels will skyrocket to 6,500 feet by Sunday night, remaining from 4,500-6,500 feet through Monday night. Then, as a colder system arrives on Tuesday, snow levels will drop rapidly to 1,500 feet by Tuesday night. Snow isn’t expected at the passes until Tuesday.

Remember that there is a significant risk of urban flooding, standing water, ponding, and rises in small streams & creeks. Rivers will rise, with bankfull stage possible on the Snoqualmie River and moderate flood stage on the Skokomish River near Potlatch. 

Stay safe in all this rain & stay tuned for the next blog by Monday night!

Weak Atmospheric River, Big Changes to Extended Forecast

FastCast—Wednesday, Apr. 5 to Sunday, Apr. 9:

Another calm day is ahead on Wednesday, with a chilly morning (low to mid 30s in the lowlands), and a mostly cloudy day. Rain will move in after dark as a weak atmospheric river parallels the Washington and Oregon coasts. Rain will continue through midday Friday, becoming showery from late Thursday night through midday Friday. Expect rain totals in the lowlands of 0.6-1.3”, most from Seattle southward. On the coast and in the Olympics, expect 1-3 inches. The Cascades will also receive 0.8-1.5”, and snow levels will hover at 4,000-5,000 feet. Eastern Washington will also pick up 0.25-0.5” of rain. Another system, essentially a mini atmospheric river, will impact the area from Saturday through Sunday. This system will bring widespread rain to the region, but rainfall amounts and snow levels aren’t certain yet. Through Sunday, expect highs in the low to mid 50s and lows in the low to mid 40s. 

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A weak atmospheric river is ahead for the Pacific Northwest, impacting the region from late Wednesday through midday Friday. The UW forecast below shows the integrated water vapor transport (IVT) values at 2 PM Thursday.

This graphic essentially shows the amount of water vapor in the atmosphere. The atmospheric river is clearly visible…the long trail of red paralleling the WA & OR coasts. 

How much rain is expected with this atmospheric river? Let’s take a look at the European model’s forecast below, through Friday night.

In general, expect 0.6-1.3” of rain in the lowlands, most from Seattle southward and on the Kitsap Peninsula. The coast and Olympics will receive 1-3”, and the Cascades will receive 0.8-1.5”, with snow levels at 4,000-5,000 feet, likely bringing rain to Snoqualmie Pass and rain/snow mix (or all rain) to Stevens and White Passes.

One note before we take a look at big changes to the extended forecast. Below is the forecast for low temperatures on Wednesday morning, per the European model.

Expect chilly lows in the low to mid 30s across the lowlands, with outlying areas dropping into the upper 20s. Eastern Washington will drop into the low to mid 20s, except in the upper 20s to mid 30s in the Columbia Basin.

Now…big changes in the extended forecast. If you remember my previous blog, I talked about the NWS Climate Prediction Center (CPC) temperature outlook for April 8-12. That outlook showed a significant warmup ahead for the Northwest. This was due to predictions of a large high pressure ridge setting up over the Northwest, which would deflect weather systems away and bring warmer weather.

However, in the past few days, forecasts have significantly changed, pushing the high pressure ridge further east toward the Rockies and beyond. This has cooled and dampened the forecasts considerably for the Northwest, since we’re now wide open for systems to arrive.

Let’s take a look at the CPC temperature outlook for April 10-14.

This is a huge change from the previous outlook. Now, we’re expecting a 40-70% probability of below average temperatures, a total pendulum shift from the previous blog. You can see the area of above average temperatures on the edge of this map, mainly centered over the Great Plains and parts of the Southern Rockies.

Now for the CPC precipitation outlook for April 10-14. As you’d expect, chillier temperatures will bring some more rain.

This outlook puts Washington in the bullseye of above average precipitation through mid-April, with a 40-60% probability of precipitation being above average. While dismal, this is actually good news, since the snowpack has been below normal for the past few months. With a drier summer ahead, we need all the snowpack possible to help suppress wildfires. Cooler and wetter conditions will help with snowpack and solidify our already strong reservoir capacity.

Stay tuned as active weather continues!

Chilly & Showery, Upcoming Atmospheric River

FastCast—Monday, Apr. 3 to Friday, Apr. 7:

A mixed bag of weather is ahead this week in Western Washington. Expect residual showers to continue on Monday, especially in the morning and evening. Some showers will carry into Tuesday morning. Then, expect a partly to mostly sunny Tuesday afternoon. Wednesday will be mostly cloudy across the region, with rain spreading inland from the coast in the evening. An atmospheric river will impact the area from late Wednesday through Thursday, likely bringing 1-2” of rain for the lowlands and 2-4” on the coast (more details in the coming days). Expect the atmospheric river to move out of the area by midday Friday. In the mountains, expect 2-4” of snow at the passes through Tuesday. Snow levels in the atmospheric river will start out around 2,500 feet on Wednesday night, but will steadily increase to 5,500 feet by Friday. In the lowlands, expect this week’s highs to be in the low 50s, except the mid 50s on Friday. Lows will increase from the mid 30s to mid 40s through the week. Some isolated areas may drop below freezing in the mornings during the first half of the week.

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After a showery weekend, with small hail, brief snow (in isolated spots), and even a few thunderstorms, showers will taper off through Tuesday. Generally, expect 0.1-0.4” of rain in showers across the lowlands & coast through Tuesday.

Temperatures will be chilly to start the week, as seen in the European model’s forecast for Monday’s highs.

Expect highs in the lowlands and on the coast to be in the upper 40s to low 50s, a trend that will continue through most of the week. In Eastern Washington, highs will be in the low to mid 40s, except in the low to mid 50s in the Lower Columbia Basin.

Low temperatures will be on the cold side early this week, shown in the European model’s forecast for Wednesday morning’s lows.

Similar lows are expected on Monday morning as well. Expect lows in the low to mid 30s, warmest near the water. Some outlying areas may drop into the upper 20s. Eastern Washington will be quite cold, with lows in the low 20s near the Cascades, and in the mid 20s to low 30s elsewhere.

Now for the real action…the atmospheric river expected to impact the region from Wednesday night through midday Friday. The atmospheric river is seen below on the UW model.

As of now, the SW to NE orientation of this atmospheric river ensures that the Seattle area likely won’t be rainshadowed. 

What kind of rainfall amounts could we be looking at? Let’s take a look at the European model’s forecast below, showing total rain through next Saturday.

(Keep in mind, this forecast does include rain that will fall from Sunday to Tuesday). With that noted, we can reasonably expect 1-2” of rain in the lowlands from late Wednesday to midday Friday, except 0.4-1” from Oak Harbor to the Canadian border, due to a rain shadow. The coast will likely pick up 2-4”, and Eastern Washington will get 0.5-1.4”. The Willamette Valley and SW WA could get up to 2.5” as well.

With snow levels expected to rise to near 5,500 feet, mountain precipitation isn’t certain yet. Be sure to stay tuned to the blog for more updates on the atmospheric river!

Let’s end with some great news! The outlook below from the NWS Climate Prediction Center shows temperature probabilities from April 8-12.

This is a huge improvement from the consistent below average temperatures we have been experiencing! Across WA, there is a 40-80% probability of above average temperatures! 

What does this mean for the lowlands? Let’s just say that mid 60s are likely, and 70s aren’t out of the question! Stay tuned…details are still coming into focus!