The Most Impressive Rain Shadow in the Nation!

When air approaches a mountain barrier, it is forced to rise and, in doing so, cools.  As the air cools, it has less ability to "hold" water vapor, and eventually the air becomes saturated, producing clouds and precipitation.   

On the lee side of the barrier, the air sinks and warms by compression, causing the relative humidity to drop, causing clouds to evaporate and ending any chance of precipitation

This dry area on the downstream side of terrain is called a rain shadow.

 Washington State is "blessed" with world-class rain shadows...one on the downslope (eastern) side of the Olympics and the other to the east of the Cascades.

According to all the AI queries I made, Washington has the most profound rain shadows around (see sample below).

This week was a great example.   

On Monday, there was a potent atmospheric river of massive amounts of water vapor  approaching Washington from the west.

As a result, the western side of local terrain experienced heavy precipitation, but the mountains produced a profound rain shadow downstream (see the precipitation totals over the past 72 h below).

3.5 inches on the windward (western) slopes of the Olympics and 0.04 inches on the lee (northeast) side of the barrier.  Wow.

That's roughly 85 times more on one side than the other.  Mama Mia!  That is a spicy rain shadow.

Here is a close-in look at the rain shadow action.    No wonder some folks like living in Port Townsend and Sequim! 

On Tuesday morning, the rain shadow was very apparent in the visible satellite image:

And the weather radars showed the profound impacts on rainfall.  Very nice rain shadow.

Some precipitation is coming in on Friday, but most of the precipitation and rain shadow action during the next few days will be over British Columbia:

Be proud.... one online source event even provided us with a medal!

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You are invited to attend a very interesting weather lecture for the general public on the UW campus. Open to the general public!

Title: What I Have Learned from Flying Through the Clouds

Speaker: Armin Sorooshian, The University of Arizona

Date: Thursday, October 8, 2026

Time: 7:00pm - 8:30pm

Location: Kane Hall 220

To register, use this link:

https://forms.gle/S5xWHUWhrqzexKtH9

Abstract

What is it like to fly through clouds in the name of science?

And why should we even fly expensive aircraft filled with scientific instruments in and out of clouds?

Aside from their natural beauty and ability to invoke our imagination, clouds play a critical role in regulating Earth's climate, influencing precipitation, and affecting air quality. A necessary ingredient to make clouds is particles (e.g., dust, smoke).

This talk will describe (with a lot of visuals) what it is like to spend hundreds of hours in the back of various types of airplanes flying in and out of clouds. Stories and examples will highlight the importance of this type of work for reasons ranging from public health to visibility and climate change. 

A Significant Atmospheric River Hits the Pacific Northwest

The first significant atmospheric river of autumn has arrived.  

The satellite imagery tonight is impressive.

For example, consider the water vapor image from the NOAA GOES-West satellite.  A magnificent moisture plume from the Central Pacific into our region.

Here is another.

The forecast of the transport of water vapor by the European Center model for tonight at 10 PM is shown below.  The atmospheric river is moving huge amounts of water vapor into the Pacific Northwest, with the center of the plume directed to the northern Olympics and southern Vancouver Island.

The plume of moisture will be forced to rise by our regional terrain, producing substantial windward precipitation.

Below is the prediction of the total precipitation through 5 AM Wednesday (from the European Center forecast).  Southern BC and northern WA get hit hard.

The UW, ultra-high-resolution forecast for the same period shows the details (below).

As much as 3-5 inches on the western slopes of the Olympics and central/northern Cascades. Substantial amounts reach the eastern slopes of the Cascades.  Nice rain shadow over central Puget Sound.  

 

I bet I can get away with biking to the UW tomorrow!

If you had refrained from watering your garden... go ahead.  Probably time to find your umbrella as well. September will end up with normal precipitation for most of the region.

________________________

Attend the very interesting weather lecture for the general public on the UW campus. Open to the general public!

Title: What I Have Learned from Flying Through the Clouds

Speaker: Armin Sorooshian, The University of Arizona

Date: Thursday, October 8, 2026

Time: 7:00pm - 8:30pm

Location: Kane Hall 220

To register use this link:

https://forms.gle/S5xWHUWhrqzexKtH9

Abstract

What is it like to fly through clouds in the name of science?

And why should we even fly expensive aircraft filled with scientific instruments in and out of clouds?

Aside from their natural beauty and ability to invoke our imagination, clouds play a critical role in regulating Earth's climate, influencing precipitation, and affecting air quality. A necessary ingredient to make clouds is particles (e.g., dust, smoke). It turns out that aircraft are necessary to really dig into how particles and clouds interact.

This talk will describe (with a lot of visuals) what it is like spending hundreds of hours in the back of various types of airplanes flying in and out of clouds to learn about these complex interactions. Stories and examples will highlight the importance of this type of work for reasons ranging from public health, visibility, and climate change. 

How Unusual Was this Summer's Weather?

Now that we have entered fall, it is time to evaluate whether our summer weather was unusual.

Are summers getting warmer or drier in Washington State?

Starting with the overall view for the entire state, below is the July 1-Sept 26 precipitation difference from normal.  

A complex pattern.  The western slope of the Cascades (where there was little fire) was drier than normal, while the eastern slopes (with more fire than normal) were WETTER than normal.  

The majority of the state was near normal for temperature (light green or yellow below), with a few areas a bit above normal 

In short, not an exceptional summer period for temperature and precipitation.

What about the long-term trends in summer temperatures?   

Average daily temperatures have risen over the past 70 years at Hoquiam, Olympia, Yakima, and Spokane by about 2-3.5 F over the period.

In contrast, the trend in daily high (max) temperatures has been very small west of the Cascades, and about 4F at Yakima and 1°F at Spokane.

Finally, for precipitation, there is modest summer drying in the west and a small downward trend east of the Cascades.   But there is very little precipitation during the summer in any case.

Fortunately, there is no overall downward trend in annual precipitation, so the region should be ok with its water supply.

The bottom line of this blog.  

This summer was not that unusual for temperature and precipitation in Washington State (but there was an unusual lightning event in July).  

The region is slowly warming, although high temperature extremes are hardly changing.  Annual precipitation is steady, but our dry summers are getting slightly drier.  

Tens of Thousands Have Lost Power as a Strong Cyclone Makes Landfall

As the models predicted, a strong storm with powerful winds is moving across our region, and nearly 100,000 customers are without power as I write this blog.

The radar imagery has been stunning.  At 7:30 AM, a well-defined low center (red arrow) was on the coast near the Columbia River.

And by 10 AM, the low was moving inland

Maximum winds so far today (through 10 AM) have been substantial over northern Oregon and southern Washington, with many locations getting to 50 mph.   Up to 54 mph in Olympia and several reaching 70 mph.

Early-season strong winds inevitably mean lots of power outages, with trees being untested and still with lots of leaves.

Right now, at 11:30 AM, about 71,000 customers have lost power in Washington, with more in Oregon, and the event is not over.

The latest European Center forecast suggests the strong winds, and thus power outages, will remain south of downtown Seattle.

Finally, the winds last night at Mt. Rainier's slope around 10,000 ft (Camp Muir), with gusts up to 101 mph:

This is the result of the strong upper-level low associated with this storm (see map at 700 hPa pressure, about 10,000 ft, for this morning).  Not a good time for an ascent. 😁

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Link

Pacific Windstorm Will Hit The Pacific Northwest Late Thursday and Friday

Autumn began yesterday at 5 PM and the atmosphere has taken notice.

A significant midlatitude cyclone will cross the northern Oregon coast late Thursday and Friday morning, bringing strong winds and localized power outages.

Consider the UW WRF model forecast for Friday morning at 5 AM (sea level pressure and maximum wind gusts are shown below). 

Wow.  A very intense, but small, low-pressure center will make landfall on the northern Oregon Coast, with gusts up to 70 knots.   

 

Plotting the maximum wind gusts over time for this event (below) indicates the northern Oregon  Coast will get a direct hit with winds reaching 50 knots. 

This early in the season, this means power outages.

Other weather prediction systems are doing the same thing.   The excellent European Center model has a similar maximum wind swath. (The graphic below is in mph).  Seattle only gets some modest gusts.

The simulated satellite imagery for Thursday evening indicates an impressive system (the arrow points to the low center).  Looks like midwinter!

Precipitation totals from this system will be substantial, with the mountains and the coast getting several inches (see below).  Even eastern Washington gets substantial rain.  

All good for terminating the wildfire season and providing agriculture with a nice wetting rain.    Reservoirs will gain valuable water.

I am concerned about the lack of coastal wind warnings by the National Weather Service offices in Seattle and Portland (see their latest forecast graphics below).

They really need to have some kind of warning out for the northern Oregon coast.  

I could comment on issues with National Weather Service forecasts, but I will leave that to another time.

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Finally, someone emailed me about a good book on Northwest Weather.  Well, I do know one 😉

Link

Peak Bird Migration Reveals Gaps In Northwest Weather Radar

We are now at the peak of the bird migration season, and weather radar does a good job of tracking our feathered friends. 

The massive bird swarms over us now are a powerful tool for determining gaps in weather radar coverage.

Let's examine the regional radar imagery in more detail.

As some background, here are some weather radar-based bird migration statistics (from the Cornell BirdCast website).  Birds like to fly from sunset to sunrise, with a peak around midnight/

And we are nearly at the peak time of the year for the southward migration. 

At 5 PM yesterday, the combined radar image showed was just some local ground clutter.

The situation was very different at 9 PM, with the birds in flight (below).  The birds don't like to fly offshore. 

But over land, there are clearly major radar gaps, such as the southern  Oregon Coast, larger areas of north-central Washington, and much of eastern Washington.

Similar story around 3 AM today.

And this morning at 8 AM, the bird signal is gone, with just some ground returns.

The current weather radar locations are shown below with blue dots.   

 Suggested locations for new radars are shown by the red circles. Additional radars would be very helpful for weather prediction and other uses (e.g., wildfire management, flood warnings, and more).