beragampengetahuan Weather Blog: Valley Drainage Season – Beragampengetahuan

The high-resolution visible satellite this morning around 7 AM was striking.   Few clouds overhead at higher elevations, but lots of sinuous clouds in river and mountain valleys.

As I will explain, this is a very typical situation this time of the year.

During the late summer and early fall, nights are getting much longer, but the extensive cloudiness of winter has not yet begun.   Thus, the ground is able to radiate infrared radiation to space and to effectively cool.   The cold ground cools the air near the surface.  Cool air is denser and thus is heavier than warm air and tends to sink down the slopes of valleys (see figure below).

Cool air pools at lower elevations of the valley, and then slides down the valley towards lower elevations.

If the air has sufficient moisture, the cooling along the valley slopes can cool the air sufficiently so that the moisture condenses into small water droplets–thus producing shallow valley fog.

This is why many of Washington’s river valleys filled with fog and low clouds this morning. 

Let’s take a look in more detail!

First, consider the Chehalis River Valley (below).   The valley was filled with fog and amazingly a “fog jet” pushed into Willapa Bay and then appeared to contribute to a cloud bank along the shore.

Near Seattle, the Green and White River Valleys had fog.

And the great Columbia River had a wide valley fog area.

But fog is not the only thing that can drain into valleys.    So can smoke!

There are some smoldering, dying fires at higher elevations in the Olympics, and their smoke has descended into some river valleys draining to lower elevations.  This morning’s visible satellite image showed some of that valley smoke action! (see below).

Finally, you might ask whether modern weather prediction models can predict such valley fog.

It is a very, very difficult forecast:  you need very high resolution and the ability to handle the complex physics near the surface.

Most weather prediction models do not have the resolution to deal with such valley fog, but the UW uber-high resolution WRF forecast system (with a grid spacing of 1.3 km) has a chance.

Below is the prediction of low-level cloud at 5 AM this morning from the UW WRF model.   We got a good piece of the low-level clouds in the Chehalis Valle–and even the coastal clouds!– but underplayed the clouds in the Columbia River Valley.

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Announcement:  Portland American Meteorological Society Meeting on Saturday, September 30.

When: Saturday, September 30th 2023 @
10AM
Where: OMSI (Oregon Museum of Science and Industry) in
Portland. Main auditorium. 1945 S.E. Water Ave. in Portland.
Driving
directions to OMSI: http://tinyurl.com/6rrz8em
Meeting:
This meeting is free and open to all ages of the general
public. 
Overnight Accommodations: For overnight
accommodations in Portland, please see: http://tinyurl.com/7boqrsf 
Subject:
The Portland office of the National Weather Service and Dr. beragampengetahuan (U of W)
will reconstruct and dissect the record-setting February 2023 Portland snowstorm
in this technical meeting. Maui wildfires will also be discussed.