Outflow Boundaries

The Storm Prediction Center (SPC) has issued a Marginal Risk today for my area. I won’t go chasing, but I might try for some lightning photography tonight, if the conditions are appropriate.

Instead, I will post about an outflow boundary I saw on radar back on 5/7/15.

The severe storms that had formed across northern Texas were producing a large pocket of rain-cooled air. This sinking air fell and spread out along the surface of the Earth, producing a “mini” cold front out ahead of the storms. This mini cold front is called an “outflow boundary” as it is a direct result of outflow from the storm.

Often, outflow boundaries become the focus for new storm development. They act just like a cold front, (as they are just cold air advecting along the surface), which acts as a lifting mechanism. Furthermore, because this front is now interfering with the parent storm’s updraft, the parent storm may weaken, and new storms may from along the outflow boundary. This is how storms can “outrun” a synoptic boundary, and how the same area can be threatened multiple days in a row, even though several lines of storms have passed through the area.

On radar, we can see an outflow boundary as an area of light purple reflections moving southeast away from the western cluster of storms. This outflow boundary is moving towards the radar site at the bottom center of the screen. Storms are triggering just behind the outflow boundary. You may have to click on the image to get it to animate.

The following images were taken on 5/9/15 of a different storm. There is a base reflectivity image on the left and a storm relative velocity image on the right.

If you look carefully, you can see a darker line of purple reflections to the southeast of the radar site in the reflectivity image. It matches with the interface between red and green signatures on the storm relative velocity.

On a storm relative velocity chart, the red signatures are particles moving away from the radar site; these are being blown by the outflow winds. Green signatures are particles moving toward the radar site. If we think about that, we can see that the area of the outflow boundary is an area of convergence. The warm inflow is moving towards the radar site, and, being less dense, is being undercut by the denser, cooler outflow moving away from the radar site.

Thank you for reading my post.

The Severe Weather Outlook is from the Storm Prediction Center website.
The satellite imagery is from the NOAA Satellite and Information Service.
The surface observation and upper level charts are from Unisys Weather.

About highplainschasing

This blog is about my tales in storm chasing. My name is Seth Price and I am an instrumentation instructor at New Mexico Tech. My amateur radio call sign is N3MRA.
This entry was posted in Commentary, Radar Imagery and tagged , , , , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.