LSI /CAP Strength ?

[Jon Miller]

In using the Lid Strength Index (LSI) on Earl's site , to evaluate the capping strength , what temp values should one be looking for throughout the chase season ?

ie:
http://wxcaster4.com/nam/CONUS_ETA212_ATMOS_LSI_09HR.gif

Thanks

Jon Miller
KT8NDO

 

[Bill Tabor]

Here is a link that discusses it briefly:
http://www.crh.noaa.gov/lmk/soo/docu/indices.php

Typically whether the cap will break has a lot to do with boundaries, forcing, heating, convergence, lift, and mixing and not just cap strength or inversion strength. The inversion or capping may be at different levels ~ 850mb to 700mb, and may be found on a sounding. Different temps are typically caps at different levels and vary by time of year. Sounds like the LSI tries to normalize this to some degree to make it an evaluative parameter, but it will still depend on the things I mentioned at first in overcoming a cap particularly in a more highly capped environment.

 

[Jason Persoff]

It sounds like you're asking two different things that are somewhat related:

1. LSI. As pointed out this is an indicator of "how capped" an area is with higher values, particularly +2 or greater, indicative of a stronger cap. But this is a mental exercise in some ways as it doesn't take into account large scale or small scale features that could break the cap. It only gives a clue as to whether or not convection will be likely (with many cells forming in areas of relatively uncapped air (<2)) or where it may be held off until something causes the cap to break (>=2).

2. Temperature. The best way to think of what temperatures you can consider as being capped is relative to the season, the geographic location, etc. Most of us talk about cap temps when temperatures around 700mb are in excess of 6 to 10 degrees over the southern Plains states in May/June. Usually, and I mean this very loosely, these cap temps are associated with convective temperatures >100 degrees or require outflow boundaries, fronts, or rapid movement of cooler temps aloft to help break the cap. As with all weather, these temps mean nothing in a vacuum: they are part of the broader view of the atmosphere that must incorporate other mesoscale phenomena and the evolving upper air situation that changes over the course of the day. But, yes, when I'm out chasing in May, when I see a 10 degree isotherm over the southern Plains, I get out the spf 30 and take a down day .