Some months back, I wrote a review of F5 Data, a powerful weather forecasting tool that aggregates a remarkably exhaustive array of atmospheric data–including over 160 different maps and a number of proprietary indices–for both professional and non-professional use. Designed by storm chaser and meteorologist Andrew Revering, F5 Data truly is a Swiss Army Knife for storm chasers, and thanks to Andy’s dedication to his product, it just keeps getting better and better.
My own effectiveness in using this potent tool continues to grow in tandem with my development as an amateur forecaster. Today I encountered a phenomenon that has puzzled me before, and this time I decided to ask Andy about it on his Convective Development forum. His insights were so helpful that, with his permission, I thought I’d share the thread with those of you who are fellow storm chasers. If you, like me, have struggled with the whole issue of CINH and of figuring out whether and where capping is likely to be a problem, then I hope you’ll find this material as informative as I did.
With that little introduction, here is the thread from Andy’s forum, beginning with…
My Question
SBCINH vs MLCINH
I’m looking at the latest GFS run (6Z) for Saturday at 21Z and see a number of parameters suggesting a hot spot around and west of Topeka. But when I factor in convective inhibition, I get either a highly capped environment or an uncapped environment depending on whether I go by MLCINH or SBCINH. I note that the model sounding for that hour and for 0Z shows minimal capping, which seems to favor the surface-based parameter.
From what I’ve seen, SBCINH often paints a much more conservative picture of inhibition, while MLCINH will show major capping in the same general area. How can I get the best use out of these two options when they often paint a very different picture?
Andy’s Answer
This is a great question, and very well worded… I guess I should expect that from a wordsmith!
SB *anything* is calculated using a surface-based parcel. ML *anything* is calculated using a mixed layer parcel. It is done by mixing the lowest 100mb temperature and lowest 100mb dew points and using those values as if those values were the surface conditions, and then raising from those values.
This is why when you look at a sounding it looks to favor the SB CIN because the parcel trace on those soundings is always raised from the surface. If you were to ‘average’ or mix the lowest 100mb temperatures by simply finding the section of the temperature line that is 100mb thick at the bottom of the sounding, and find the middle of that line (average value) and see what that temperature is, and then go to the surface and find where that temperature would be on the sounding at the surface, and raise the parcel from there (after doing the same thing with the dewpoint temperature) then you will have the ML Parcel trace and would then have MLCIN and MLCAPE to look at in the sounding.
A drastic difference in capping from SBCIN to MLCIN indicates that there is a drastic difference in values just above the surface that are causing this inconsistency. So when the parcel is mixed it washes out the uncapped air you get from the surface value.
We have different ways of looking at these values with different parcel traces because quite frankly, we never know where this parcel is going to be raised from. The same idea is why we have Lifted Index and Showalter Index. ITs the same index, but Showalter uses the values at 850mb and pretends thats the surface, while Lifted Index uses the surface as the surface.
We just never know where the parcel is going to raise from.
It seems to be consensus that ML-anything is typically the favored parcel trace. This means smaller CAPE and bigger CIN usually.
I have stuck strictly to my APRWX CAP index for years now because it considers both of these, as well as the temperature at 850mb, 700mb, and temperatures at heights from the surface up to 3000m, cap strength/lid strength index, as well as some other things when looking at capping. It seems to perform very well.
To summarize though… capping is a bear. If anything is out of line, you’ll easily get capped. So what I do is look at every capping parameter I can, and if *anything* is suggesting it being capped, then plan for it to be capped during that time period.
Now to confuse the situation even more, keep in mind that capping only means that you won’t get a storm to take in parcels from the suggested parcel trace location… IE.. from the surface. You can be well capped and have elevated storms above the cap. However for them to be severe you tend to need ‘other’ parameters in place, such as very moist air at 850mb (say 12c dew), some strong winds at that level, etc. to feed the storm.
Another map that is neat to look at for capping is the LFC-LCL depth. You may be capped, but want to be in position where the cap is ‘weakest’ and may have the best chances at breaking… with this map you get into your area of interest and then look at this map and find where the LFC-LCL depth is ‘smallest’.
For a capped severe situation, this usually means high values with a donut hole of smaller values in the middle. This is a great indication that the cap would break most easily in the middle of that donut.
This map (in a different, but similar form) can be seen on the SPC Mesoanalysis web site as LFC-LCL Relative Humidity. Its the same idea, but on their map you want high humidity values for weakening cap indication.
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So there you have it–Andy’s manifesto on capping. It’s a gnarly subject but an important one, the difference between explosive convection and a blue-sky bust. There’s a lot more to it than looking at a single parameter on the SPC’s Mesoanalysis Graphics site. If nothing else, this discussion has brought me a step or two closer to knowing how to use the ever-increasing kinds of forecasting tools that are available.
