Identifying Outflow Boundaries

[David Williams]

Good-day,
I was hoping that someone could explain to me how I would use current data to identify the location of an outflow boundary (what data would I use and what would I be looking for).

Thanks,
David

 

[Mike Johnston]

Although the passage of an outflow boundary can be associated with a wind shift, temperature drop, and pressure increase, I've found it difficult to identify the location through conventional readings from surface stations. Probably the most practical way is through monitoring radar. Often, an outflow boundary will show as weak radar signal of a thin, arcing line. Sometimes, an outflow boundary may be a hundred miles or more from the originating thunderstorm from earlier in the day or even the previous day. The radar signatures seem to be more prominent when the boundary itself is relatively close to the radar site. Also, it helps to loop the radar to track the boundary's movement and verify it as an outflow boundary. The boundary will often have a motion "independent" of the prevailing winds/expected storm motion vectors for the day. In this respect, it's similar to watching a sea breeze or lake breeze boundary develop during daytime heating. I understand from some sources that the radar signature itself may be attributable to flying insects that get caught up right along the boundary.

 

[Jim Parsons]

Satellite loop is another good way of spotting them...

 

[Andrew Stoller]

http://www.youtube.com/watch?v=FNq4upbr6is

Many times it's helpful to loop a radar for 24 hours to spot outflow boundaries to foecast for a chase day. Many synpiotically less than ideal days turn out to be spectacular because of the presence of an outflow boundary.

 

[Jeff House]

The radar discussion of OFBs above is spot on so I'll add to VISible satellite and surface charts. Other methods for spotting OFBs are fine lines (of clouds) on VIS, that you know are not associated with synoptic fronts. Also sometimes low clouds linger on the cool side while the warm side of the OFB is more sunny. When the low clouds on the cool side start to retreat from the OFB, storms may develop shortly. Keep in mind where morning rain fell. One can make a first guess that an OFB was laid down and/or drifted to the south and southwest.

Finding OFBs on surface charts is not as straight forward as finding synoptic fronts. You might still get a slight wind shift, but often not noticeable against random slight wind shifts. Look for more backed (SE not SW) winds on the cool side. Temps and dewpoints can offer assistance, especially after mid-morning. Early it can be somewhat uniform. Temps may warm faster on the south/west side of the OFB. If it is sunny on both sides temps may be close to even. Dewpoints can offer clues. I've seen it go both ways. Dews can be higher on the cool side due to the rain. Dews can be higher on the warm side due to moisture advection. Situations are different, but one can usually discern the OFB with the combination of methods above. Good luck!

 

[Jeff Duda]

http://www.youtube.com/watch?v=FNq4upbr6is

Many times it's helpful to loop a radar for 24 hours to spot outflow boundaries to foecast for a chase day. Many synpiotically less than ideal days turn out to be spectacular because of the presence of an outflow boundary.

I've got another example:
http://www.meteor.iastate.edu/~jdduda/perfectOFB.html

 

[Mark Blue]

This is a very good question and one I have often wondered about. Often times you can see them on GRLX products and if you do it would be wise to follow it, especially if you know it will eventually intersect another known boundary already depicted on the current surface map.

I checked out Haby Hints and there were two examples of boundaries there that I thought I'd share. They are as follows:

Outflow Boundary #1
Outflow Boundary #2

Another classic example that was shared with me was the June 3rd, 1980, tornadic event that took place in Grand Island, NE. There is a thread I created in the Historic sub-forum and Tim Vasquez was kind enough to post satellite images prior to and after initiation had occurred. Those satellite images can be found here.

We have an ample amount of experienced meteorologists here, so hopefully they can chime in and share their knowledge in this arena. One has already chimed in (Jeff Duda) but I can think of a couple more who I think could shed some serious light on the subject. Jeff S, Patrick M, and Rob D would be excellent candidates to chime in on this subject matter.

 

[Todd Renshaw]

Here's my contribution:

Watch video >

Reminds me of ripples in a pond expanding and overlapping. This was from a month ago here in Michigan.

 

[David Williams]

Excellent information! Thanks everyone!