outflow boundary

[David Williams]

Hello all,
I have heard a lot of talk lately about outflow boundaries and storms developing near them. Would someone please enlighten me about what outflow boundaries are and how they relate to the storm? Thanks!

 

[Joey Ketcham]

An outflow boundary is basically air that comes from within the storm's downdraft and spreads out away from the storm. You can generally see outflow boundaries on radar:

You will often hear chasers refer to a storm as being "outflow dominant" which simply means that the tornadic threat has decreased and more than likely you'll just experience hail and winds with the storm.

Outflow boundaries though can also act as a lift and cause new storms to fire.

 

[Paul Knightley]

Studies have shown that storms which interact with boundaries often become stronger, especially just on the cool side. This is because the backed flow can enhance effective shear, and increase the SREH the storm ingests. This can also give the storm a better chance of tornadogenesis - the high RH on the cool side lowers LCLs, which can help too.

The best example I've seen in the flesh was on May 26 2008 - an HP supercell was approaching Pratt, KS, from the WSW. An outflow boundary was moving WNW from earlier storms over N OK. The storm had been tornado-warned for quite a while, but didn't produce. I'd noted the boundary and so wanted to be looking down the notch as the storm moved just east of Pratt. We were in that position at that time, and the storm produced two tornadoes in quick succession, generally only visible from a fairly limited area.

 

[Dean Baron]

Outflow boundaries can also create better shear, locally increasing the tornado threat. A lot of times outflow boundaries help give that extra boost to break the cap as well. They can be seen on base reflectivity, satellite, and supposedly velocity but I've never seen one on velocity.

 

[Darren Addy]

It is amazing how far those outflow boundaries can travel. Ryan McGinnis and I were sitting in Broken Bow, NE a couple of years ago on a day that threatened to be a blue sky bust for the area. Big storms had fired much earlier several hours away in the northern Nebraska panhandle & SW South Dakota. We watched a large outflow boundary move SE toward us. Late in the afternoon another line appeared on radar, which we surmised was the now retreating dryline. The two were nearly parallel to each other and closing. We figured that something interesting would happen when they met, and the effect was visible in the sky almost instantly. The cap was broken and within about 1/2 hour there was a line of storms from South Dakota to Kansas and a tornado was produced in western Custer County.

 

[Marc Remillard]

It happened a couple days ago in Indiana. A small supercell we chased was moving from SW to NE and slowly started to turn right as it was intensifying. An outflow boundary from W to E was in the path of this storm. As the storm turned right, it went directly (or almost) on the boundary. The storm became stronger and cycled 3 times before dying. There is an unconfirmed tornado report with this storm.

 

[Skip Talbot]

You can also detect them on surface obs. Look for a station where the wind barb backs (turns counterclockwise), whereas other stations remain constant. This is a little harder to pick up on days where the surface winds are mediocre (in random directions or with no speed). A lot of outflow boundaries fall in between surface stations and can be missed completely, which makes radar a better tool for picking them out. It is helpful to know when you see it happen on a surface chart, however.

 

[EricGermann]

What in the outflow boundary gives the radar it's return ?

 

[Jeff Snyder]

What in the outflow boundary gives the radar it's return ?

Typically, it's from something called Bragg scattering. Along OFBs (or other baroclinic boundaries) much of the time, there are significant gradients in the refractive index; it is this gradient in refractive index that "returns" some of the radar's energy. There is some contribution from bugs and other biological scatterers (and dust, I suppose) that tend to be located along OFBs and fronts as well.

 

[Greg McLaughlin]

I love it when people bring up outflow boundaries. I find outflow boundaries magical. I can list off the top of my head 25+ tornado events in which outflow boundaries played a major role in tornado development.

Anytime I do a forecast on a chase I always keep an eye out for an outflow boundary and do my very best to analyze them properly. Outflow boundaries left over from morning convection are often seen on satellite and surface obs in the morning hours, but then appear to wash out in the afternoon. Just because this happens doesn't mean the boundary is gone. Often times a boundary will show up again on satellite when strong mixing is occuring and cumulus develop along the boundary due to moisture convergence. Just this past week on June 16, 2009 a supercell exploded late in the day near Wichita, KS along an outflow boundary. The boundary didn't show up at all on surface or satellite, but became quite apparent on ICT base reflectivity less than an hour before the storm exploded along it. The previous day Justin and I witnessed two tornadoes with a storm that exploded along an outflow boundary in the same area.

One thing I really look for with outlfow boundaries are ones that are oriented to the right of the mean environmental flow. Often times boundaries oriented in such a manner help a storm to deviate once the storm interacts with it. Justin and I have had great success with supercells that form to the south of a boundary then anchor on the boundary and deviate. Often times the storm will slow down greatly enhancing streamwise vorticity while the deviant motion enhances SRH.

Another great thing about deviant storms along outflow boundaries is the storm's ability to cycle new mesos away from the precip core. This is especially true in high cape environments even if the mid-level flow is somewhat weak (<40kts). Some of my greatest tornado intercepts have been along outflow boundaries.

Here are a few dates that have payed off for me:

June 1, 1999
June 3, 2001
May 1, 2003
May 24, 2008
May 26, 2008
June 15, 2009

Also....two of the most significant tornado outbreaks of the 1990's were impacted by outflow boundaries:

March 13, 1990
April, 26, 1991


One more note,

This past February I was attending the spotter class the NWS Tulsa hosts every year. They did a presentation on the Pitcher, OK EF4 tornado on May 10, 2008. They had noted how the storm exhibited an intense low-level mesocyclone on radar, while the rotation in the mid levels was rather weak compared to many supercells which produce such strong tornadoes. This caught my curiosity, so I looked into that event in more detail. What I found wasn't surprising. The Pitcher storm had developed just north of the Oklahoma/Kansas border and was generally moving in an easterly direction when it deviated suddenly right before it hit Pticher. The storm continued a southeast movement into southwest Missouri all the while producing a long track violent tornado. What caused the storm to deviate so suddenly and produce a violent tornado when a strong mid-level meso hadn't been present? I bet if you have been reading this far, you know the answer. Yes, an outflow boundary. A careful inspection of visible satellite reveals an area of convection (probably elevated in nature) which had persisted over eastern Kansas and western Missouri had put down an outflow boundary oriented from the northwest to southeast across far southeast Kansas and far northeast Oklahoma. The storm deviated and produced the tornado right as it ran into the boundary. Its resonable to assume this boundary could have had a direct impact on the storm's deviant motion and sudden tornadogensis despite having a relatively weak mid-level mesocyclone. I threw this case out there as an example of how relatively new outflow boundaries can enhance an existing supercell and lead to sudden tornadogensis.

Outflow boundaries can produce tornadic activity when the overall environment wouldn't support tornadoes i.e. May 24, 2008. Sometimes outflow boundaries enhance tornadic activity in an already favorable environment i.e. April 26, 1991.

While not all outflow boundaries lead to tornadic activity Justin and I have made a rule for ourselves to never ignore them. And trust me, I have busted on many occasion playing an outflow boundary.

 

[Connor McCrorey]

Kinda off-topic, but it's related. If anyone wants to know what an outflow boundary looks like on radar, check out KUEX radar right now. That big circle moving away from the storms is an OFB (and a pretty cool looking one too!).

 

[Joe beier]

Kinda off-topic, but it's related. If anyone wants to know what an outflow boundary looks like on radar, check out KUEX radar right now. That big circle moving away from the storms is an OFB (and a pretty cool looking one too!).

I noticed this earlier but you beat me to the punch

 

[Brian Stertz]

I have seen some good things come from outflow boundaries but also a lot of bad. The key is to have the airmass recover behind the departing "bubble high" . This has to trend to a progressive nature of the outflow producing storms to move away and allow for the airmass to modify...all the while keeping good backed sfc winds. Too much outflow continuing over a given area will send periods of outflow and deepen the cold pools. That is when you see the ugly arcus show up...ugly meaning end of the show for good tornadic situations in the near term. Things can get very complicated with colliding outflow boundaries...DL supercell interactions....and other crazy stuff that can cause very localized shear zones....and this not meaning gustnadoes either. Good write up Greg.

 

[Greg McLaughlin]

I have seen some good things come from outflow boundaries but also a lot of bad. The key is to have the airmass recover behind the departing "bubble high" . This has to trend to a progressive nature of the outflow producing storms to move away and allow for the airmass to modify...all the while keeping good backed sfc winds. Too much outflow continuing over a given area will send periods of outflow and deepen the cold pools. That is when you see the ugly arcus show up...ugly meaning end of the show for good tornadic situations in the near term. Things can get very complicated with colliding outflow boundaries...DL supercell interactions....and other crazy stuff that can cause very localized shear zones....and this not meaning gustnadoes either. Good write up Greg.

Excellent point Brian. I have seen many days when the airmass never modified behind the boundary and new convection developing along the boundary quickly became undercut. In fact June 9th in southern Kansas is an excellent example.

 

[Eric Duncan]

Yes. I would have to say 80% of my tornadoes have been in direct relation to outflow bundries. Take the pig farm tornado in Oklahom back in 2008. That cell i was on rode an outflow boundry all day, and produced numerous tornadoes. I would however have to say that iv noticed some cells have not produced once intersecting outflow boundries due to perhaps the orientation of the boundry. Dont get me wrong, iv seen a supercell literally completly change its path once it intersected the boundry, 110 degrees to follow the outflow boundry and produce numerous tornadoes. On the other hand, iv seen a NESW oriented boundry intersect a NE moving cell, and have nothing happen. No increased SREH, and minmal surface backing was increased. Iv seen more cells go tor when there riding the SOUTH side of the boundry then the NORTH side of it. Its interesting how these mesoscale fetures all interact with eachother.