Buoyancy/Instability Acronyms in SPC Forecasts

Steven Weiss

SPC Science and Operations Officer (SOO)

The SPC uses nomenclature for Convective Available Potential Energy (CAPE) and Lifted Index (LI) that identifies the lifted parcel being used in the calculation. This is done because the magnitude of CAPE or LI can be very sensitive to the parcel choice, and users of SPC forecast products should have knowledge of what parcel is used in the calculation of CAPE and LI. Even in cases of strong instability, the values calculated from a sounding can vary considerably depending on the parcel that is lifted (e.g., there can be a large difference between using the most unstable parcel and the 100 mb AGL mean layer parcel).

The acronyms are listed and defined on the SPC website at:


and appear as subheadings under the terms "CAPE" and "LI".

The acronyms for "CAPE" and "LI" values used in SPC product discussions include:

1. For Surface Based lifted parcels....SBCAPE and SBLI.

2. For Most Unstable lifted parcels....MUCAPE and MULI (typically the MU parcel in the lowest 300 mb AGL).

3. For Mean Layer lifted parcels....MLCAPE and MLLI. (typically the mean parcel in the lowest 100 mb layer AGL)

4. For parcels used in operational model forecast gridded fields, CAPE or LI will be preceded by the acronym of the model being used, such as NAM MUCAPE, etc. This is necessary because operational models such as the NAM, GFS, and RUC may utilize different lifted parcel definitions than those used in the SPC sounding analysis program called N-SHARP.

Note the first three parcel types are used on current diagnostic data as well as with model forecast data available in the SPC N-AWIPS workstations.

Current Diagnostic Fields:

5. For current surface-based, mean-layer, and most unstable instability values produced by the hourly SPC Mesoscale Analysis fields (see: http://www.spc.noaa.gov/exper/mesoanalysis), which are derived using real-time surface observations coupled with hourly RUC model data above the surface, SBCAPE, SBLI, MLCAPE, MLLI, MUCAPE, and MULI will be used without any modifiers. For current diagnostic values of instability, it shall be assumed that the SPC Mesoscale Analysis or current RUC sounding modified by representative surface conditions is the source of these values.

Model Forecast Fields

6. For forecast instability values based directly on model grids or model soundings without any modifications, the nomenclature in 1 through 3 will be used preceded by the model acronym, for example, NAM SBCAPE, NAM SBLI, GFS MLCAPE, GFS MUCAPE, etc.

7. For forecast instability values using model soundings that are subjectively adjusted for expected surface conditions, the nomenclature in 6 will be used, but any adjustments made will be mentioned in the text of the discussion.

The above usage procedures should cover most cases. However, if an SPC forecaster wishes to use a different parcel method for assessing instability in a particular situation (for example, the 700 mb parcel may be chosen when the most unstable parcel is located above the lowest 300 mb of a sounding), this information will be spelled out specifically in the text of the discussion (e.g. ..."MUCAPE based on lifting a parcel from 700 mb").

Further, for all LI values, 500 mb will be considered the level of lifted parcel-environment comparison by default. If an SPC forecaster uses a different level of comparison (e.g., 300 mb) for the LI, that information will be included in the text discussion.

If you have any questions please contact Steve Weiss by email: steven.j.weiss@noaa.gov or phone: 405-325-2073.

For more information, see:

Blanchard, D.O., 1998: Assessing the vertical distribution of convective available potential energy. Wea. Forecasting, 13, 870–877.

Bothwell, P.D., J.A. Hart and R.L. Thompson, 2002: An integrated three-dimensional objective analysis scheme in use at the Storm Prediction Center. Preprints, 21st Conf. Severe Local Storms, San Antonio, Amer. Meteor. Soc., J117-J120.

Bunkers, M.J. and B.A. Klimowski, 2002: The importance of parcel choice and the measure of vertical wind shear in evaluating the convective environment. Preprints, 21st Conf. on Severe Local Storms, San Antonio, Amer. Meteor. Soc., 379-382.

Craven, J.P., R.E. Jewell, and H.E. Brooks, 2002: Comparison between observed convective cloud-base heights and lifting condensation level for two different lifted parcels. Wea. Forecasting, 17, 885–890.

Doswell, C.A. III and E.N. Rasmussen, 1994: The effect of neglecting the virtual temperature correction on CAPE calculations. Wea. Forecasting, 9, 625–629.