Friday, October 25, 2013

Persistent ridges deliver abundant fog and stratus

Impacts of a persistent ridge

A strong ridge of high pressure continues to sit over the Pacific Northwest as of this afternoon (10/24). The ridge has essentially remained stationary all week and consequently we have been subject to rather stagnant weather conditions.  

500 mb map and IR satellite 10 am 10/24

One of the weather conditions associated with ridges this time of year is widespread fog. The fog has typically been forming late at night and eroding over most locations by midday. Here's a look at what the fog looked like at 10 am on Tuesday (10/22).
Visible satellite 10/22 10 am
And here's how it looked 48 hours later. Although the images look similar the coverage has expanded westward into the Okanogan  Valley. 
Visible satellite 10/24 10 am

This is a typical occurrence with persistent ridges. Each day the fog has been burning off a little slower as well. While most of the Inland Northwest has seen the fog erode by midday. One persistent are of low clouds and fog has remained fixed over the Kootenai River Valley from Bonner Ferry (north of Sandpoint) and north into British Columbia. 

Below is a loop of how the fog eroded on 10/24. This erosion process has been consistent each day with most locations seeing widespread sunshine by early afternoon. Again note the clouds remain fixed over the Bonners Ferry and extreme north Idaho through the entire day. This erosion process takes place as solar heating works its way through the low clouds and fog until it can effectively break up the inversion. 

Visible satellite loop courtesy of CIMMS University of Wisconsin

Another process that can break up fog in the valleys has to do with a typical valley and valley slope heating. On a typical day the sides of a valley will heat up before the valleys below. This leads to a phenomena termed upslope flow. As the winds begin to blow up both sides of the valley it creates a void in the center of the of the valley. Consequently a circulation forms with a sinking motion developing over the center of the valley. This often times will lead to a clearing in the center of a cloud/fog choked valley. The image below shows a couple good examples of that.

polar orbiting visible satellite imagery 10/24

The fog pattern will stick with us through Saturday or Saturday night. After that a big change in the weather is expected care of a strong Canadian cold front.  

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