CAPE values during all four seasons of the year

[Jeff Duda]

One parameter you guys should pay more attention to, then, is the Bulk Richardson Number. The BRN is basically CAPE/(0-6 km shear), so it indicates the balance between shear and instability (much like the EHI). Research has shown that BRNs between about 10 and 50 are the best BRNs for supercells. BRNs that are too high (above 50 or 100 even more) indicates more buoyancy than shear necessary for supercells, and BRNs that are too low (around 10 or lower) indicate that shear is much greater than buoyancy. Yesterday's severe weather was definitely the latter case. I know the SPC site plots the denominator of the BRN expression, so you can just divide the CAPE by that to determine what the BRN in that area will be.

 

[Fred Gossage]

When dealing with low-instability events... as long as I have a couple hundred Joules of total CAPE... I'd rather look at sfc dewpoints, LCL heights, lapse rates, and 0-3 km CAPE when dealing with the thermodynamic side of things. If you only have 500-700 j/kg of total column CAPE, but you have 100-150 j/kg of it concentrated in the first few kilometers above the surface, you still have strong vertical stretching potential in the presence of the high low-level shear that is often found in these cool season setups.

 

[nickgrillo]

the March 12th, 2006 outbreak in Missouri was a good example of how you dont need a ton of instablity for tornadoes. Correct me if i'm wrong but I remember people talking about tornadic supercells where the surface temp was in the upper 30's or low 40's. Cold core events also are good examples too. There are many things that impact tornado development, instability is just one of them.

After modifying soundings invof of that morning supercell that moved ENE through KS/MO/IL, there was no CAPE whatsoever when using a surface or mean 100mb mixed layer parcels. The only CAPE I got, even after modification (using both observed soundings and RUC forecast soundings in the inflow sector) was <300j/kg and that was after lifting parcels WELL above the boundary layer. IIRC, a couple of the tornadoes that were reported from that storm were confirmed, which was extremely surprising from a meteorological standpoint. That storm was moving in a meager baroclinic zone well north of the main surface warm front which was further south into MO, and was ingesting elevated air parcels well above the BL. When I intercepted it near the MO/IL border, it was extremely cold and obviously elevated in nature, but showed weak cloud-base rotation -- and still being tornado-warned based on the continuous "reports" and rotation in the lower-level velocity scans. Now, the 0-1km SRH in the same environment was >500j/kg.

Now, the supercells which formed later in the day, and were surface-based in nature, had substantial surface-based CAPE. After modifying RUC soundings invof the "six-state" storm as it crossed into SPI, SB parcel CAPE was roughly ~2200j/kg, IIRC, the 0-1km SRH was around ~600j/kg.