Outflow Boundaries??

[Steve Polley]

I am wondering how you can determine where outflow boundaries are setting up. I do know that they are leftover from convective systems usually. The storms Saturday 5-24-08 near Enid,OK from what I understand were near one of these boundaries. I busted in NE that day and would have went south if I knew how to better identify these.

 

[Chris Bowman]

Steve,
If the boundary is close enough to a radar you can usually see a line of low reflectivity as it kicks up clouds, or anything that can reflect energy back to the radar. If it's a strong enough outflow boundary but not necessarily close to a radar you can usually see it on a satellite image/loop. Finally, there will be some minor differences in the surface flow if there are enough observation points to capture what is going on at the surface.

 

[Steve Polley]

I had seen that before on radar but did not know how to pick them out using metars, etc.

Thanks

 

[Jeff Snyder]

Let's look at some images of the OFB in OK on 5-24-08 . The first image is the reflectivity image KVNX on 1955 UTC:

In the above, notice a couple of radar "fine lines", primarily the NW-SE oriented line of low reflectivity that extends from SW KS towards the southern supercell. The fineline that extends SSE from the southern supercell is likely the demarcation between southwesterly flow across western OK and southerly flow nearer I35 in central and southern OK.

The above is the visible satellite image valid about the same time as the radar image. Note the line of cumulus along and to the east of the OFB. Note also how the character of the clouds changes across the OFB to the east of the supercells. Nearer I40 in central OK, the cumulus are relatively cellular; to the north of the OFB east of the supercells, there are cloud streets / boundary layer rolls that indicate low-level stability. There is some boundary layer roll structure S of the OFB in central and southern OK, but notice how such rolls are oriented in the NNW-SSE fashion, while the rolls N of the OFB are nearly E-W oriented.

Finally, let's look at the mesonet and ASOS/AWOS observations valid at that time. The easiest thing to look for is the change in wind direction across the OFB, but you may also note that temperatures behind the OFB are cooler than ahead of the OFB. The color shading is relatively humidity, from which we can see that the dewpoint depression (RH) is lower (higher) behind the OFB. The southern supercell (the primary supercell that became the tornado machine) tracked right along the southern edge of the outflow (i.e. it tracked along the outflow boundary).

 

[Darrin Rasberry]

Could anyone post a pic of an outflow boundary as seen in the sky? I'm pretty sure there's been one over Iowa from the mass that developed Sunday evening.

 

[Steve Polley]

Ive know that these boundaries were a focusing mechanism for storm initiation, but I was struggling with how to identify them other than using radar images. I will be watching these much closer next time. Thanks for the explanation.

 

[Chris Foltz]

Steve,

They are much, much easier to pick out from surface obs when there is a dense mesoscale network in place such as in OK or west TX. Unfortunately, most states do not have a sufficient number of surface observation stations to allow these outflow boundaries to be detected with any certainty. Also, many AWOS/ASOS stations only provide hourly obs unless there is a significant enough change in conditions to warrant the issuance of a special observation or "SPECI". IMO, radar and visible satellite are the best ways to locate outflow boundaries in the absence of a mesonet.

 

[Steve Polley]

I can see that as making it more difficult. Here in Missouri, the lack of ASOS/AWOS stations is a huge problem IMO. Nebraska could be a little better as well. Iowa and Oklahoma are fortunate in these regards.

 

[Chris Foltz]

Steve,

While certainly not as dense as OK, west TX or IA, there are several weather stations scattered throughout the state which provide real-time or near real-time data. The closest to you is up in Albany in Gentry County. Here's a link to those stations.

http://agebb.missouri.edu/weather/stations/

You can also go here...

http://www-frd.fsl.noaa.gov/mesonet/

to see a plotted version of all mesonet data ingested into the MADIS network. Just click and drag over an area you are interested in and the map will zoom to that level. You can then place your cursor over each icon to see the weather data for that site. Hope this helps a bit.