If you checked the WRAL Weather app Sunday afternoon and evening, you may have noticed a ring surrounding some of the storms.
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It's called outflow, and it's exactly as it sounds. It's the air flowing out from the storms.
What is outflow?
As mentioned above, outflow is just the air that flows out from a thunderstorm. You'll often feel the air cool a few degrees ahead of the storm when this outflow/gust front is present.
If you were to conceptualize this outflow boundary as a perfect circle, then you could split it up into two regions: inside the circle and outside the circle.
Inside, storms are cooling the atmosphere. This cuts off any further storm development, so these storms rain themselves out.
On the outer rim of the outflow boundary, the storms have essentially created a new front/boundary for air to rise. This means new storms can develop along that outer rim.
Did that happen Sunday?
The above section is just the conceptual part. What happened Sunday is almost a perfect real-world example of how these concepts come into play.
At 1:40 p.m., there were several storms parts of Wake, Franklin, Johnston and Vance Counties. You can see, however, a narrow green line on the perimeter of these storms.
That's your outflow.
Why does it look like a narrow, green line?
As the storm is breathing out, it's kicking up dust, dirt, bugs, etc. A different "channel" of radar, called correlation coefficient, shows these non-meteorological phenomenon.
By 2:40 p.m., almost every storm that was within the outflow was gone.
Below is a video of the radar loop, showing the concept play out in real time.
These boundaries could provide enough rising motion for more storms to develop Monday afternoon.
Have questions about the weather and how it works?
Send me an email with the subject line 'Ask the Meteorologist:' to cmichaels@pugongyingbo.com.