Detecting outflow boundaries

[Paul Redmond]

I am a little unsure as to how to detect outflow boundaries on upper air charts. I know that they can pinpoint the eastern most advance of a dryline and can be quite usefull in predicting convection. Any pointers or suggestions would be appreciated.

Thanks in advance.;)

[Jeff Snyder]

I am a little unsure as to how to detect outflow boundaries on upper air charts. I know that they can pinpoint the eastern most advance of a dryline and can be quite usefull in predicting convection. Any pointers or suggestions would be appreciated.

Thanks in advance.;)

Outflow boundaries are low-level features; as such, you cannot find them on upper-air charts. Your best bet for locating any possible OFB is to examine surface observations and radar and satellite data. OFBs, nearly by definition, mark the leading edge of surface cold pools produced by thunderstorms (and just showers too, I suppose), so you should be able to pick them out by looking at surface temperatures, dewpoints, and winds. For example, you'll often see lower Ts, lower Tds, and more backed surface winds in the relatively cool air behind the OFB. You may also be able to corroborate your suspicion of the OFB location by looking at radar and visible satellite data, as OFBs may be marked by a radar "fine-line" or a difference in the character of low-level clouds across the OFB.

Since the surface thermodynamic conditions are usually different across the OFB, there tends to be enhanced near-surface vorticity along the OFB (the baroclinic generation term of the vorticity generation / destruction equation). In terms of forecasting for tornadoes, supercells, etc, we often look for the "cool side" of the OFB to "cook" during the day. In other words, many chasers and mets closely monitor how much insolation is occurring on the cool side of the boundary. Sometimes, strong surface heating can lead to significant destabilization on the cool side of the OFB, leading to very favorable shear-instability conditions. Note that enhanced, baroclinically-generated vorticity may still be present even if the temperature or theta-e gradient across the OFB is removed by the afternoon! The presence of a buoyancy gradient or baroclinity means that vorticity is being generated, so the removal of said gradient merely means that vorticity is no longer being generated, even though vorticity may still be very significant since it has been accumulating through the preceeding hours. Eventually, friction / diffusion / viscous forces will "spin down" this vorticity, but it's important to remember nonetheless.

[rdale]

http://rammb.cira.colostate.edu/visit/nssl/icu92/ic921.html

[Mike Peregrine]

Residual OFB's can be cool in the summertime (or at least it seems to me that they become more of a player then) and can reinforce other boundaries ... there are times when SRH is enhanced thanks to outflow boundaries.

You could easily see an OFB associated with the gust front (http://img.photobucket.com/albums/v143/mikeperegrine/BowEcho5-1-08b-1.png) on the bow echo that came over KC a couple weeks back (look in front of the anticyclonic - lower half of the bow). I bet that gust front would have looked AMAZING if it happened during daylight!

If you're looking for an OFB several hours or even a day later, then as Jeff says, you'll have to seek out some surface plots or satellite imagery. Sometimes the temperature variations are pretty slight - -