Thunderstorms are rare this summer, but there will be some thunderstorms in the Cascades and eastern Washington today.
There were actually quite a few thunderstorms yesterday in southwest Oregon (see below, each dot is a lightning bolt) and the activity is moving north as I write this.
So what’s going on? Let’s start with the satellite image visible around 7am this morning (below). I have a red arrow pointing towards Seattle.
Wow. A very impressive cloud band stretching north of the Oregon/Washington border from the ocean to eastern Oregon. What a change.
This band even has some lightning in it (see the 6am lightning below)
Radar (below) indicates some precipitation, although very little falls to the ground.
Why is there a cloud/rain band there?
The key point is that the persistent upper level ridge of high pressure (high pressure) has moved inland and a high pressure trough is approaching (see upper level map – 500 hPa/about 18,000 ft pressure) below). The height (i.e. pressure) is shown by the solid lines and the shading shows the relative humidity at that level, with white being the highest.
A trough is moving north along the Oregon coast. The approach of the trough causes an upward movement that increases the relative humidity ahead of the trough and releases instability in the atmosphere. Such instability can cause thunderstorms.
Some amateur weather forecasting sites claim that “monsoon moisture” is moving up from the southwest, but that’s not true in this situation.
In Seattle, you can see some clouds and turbulence starting to form overhead, as illustrated by this image from Seattle PanoCam around 7am (see below). It’s not dramatic at the time, but you can see something is happening up there.
So what happens this afternoon?
As the low pressure trough approaches and the ground warms, unrest, convection and thunderstorm activity will increase, especially in the Cascades and eastern Washington.
Here is the radar reflectance forecast (which represents precipitation) at 5pm today from NOAA’s HRRR model. Some significant thunderstorms over the Cascades!
Simulated radar imagery from the UW WRF model two hours later from the UW WRF model shows significant activity in the skies over eastern Washington.
Low-lying areas of western Washington will likely be spared rain and lightning.
Of course, another concern is wildfires caused by lightning strikes, which is something the DNR and other agencies will have to watch out for.
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