mlCape ?

Re: 0-3km mlCape - What range of values should we look for relative to possible tornadogenisis ? Thanks

Jon Miller
There's no connection, CAPE is simply the available energy to create a storm. For tornado prediction you need to look at something related to winds.

Straight off of the SPC's mesoanalysis page:

[FONT=Arial, Helvetica, sans-serif]3-km CAPE (J/kg) & Surface Vorticity
[FONT=Arial, Helvetica, sans-serif]CAPE in the lowest 3-km above ground level, and surface relative vorticity. Areas of large 0-3-km CAPE tend to favor strong low-level stretching, and can support tornado formation when co-located with significant vertical vorticity ne[/FONT][FONT=Arial, Helvetica, sans-serif]ar the ground. "[/FONT]
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You could look at the EHI. Given the EHI's suggested tornado threat you can work backwards and see what CAPEs are useful at given SRH's. Though with SRH you'll generally be settling for a ballpark figure. Frankly, all of it is ballpark figures anyway (get it? franks, ballparks)

0-3km MLCAPE gives a clue into the amount of low-level stretching, as the SPC description indicates. I don't think there's a spectacular correlation between 0-3km MLCAPE and tornadoes, but there's at least a loose correlation when the 0-3km layer is a relatively deep part of the updraft, such as is the case for low-toppped supercells. For example, Davies and Guyer have written, if I recall correctly, about the possible significance of 0-3km CAPE in tornadogenesis in cold-core environments (where there is a favorable-oriented low-level boundary). The same may be said for "landspout"-type tornadoes, which may be driven by the intense stretching of near-ground vertical vorticity (such as may be associated misocyclones), though I'm less familiar with landspouts.
0-3 km MLCAPE can be useful, but depends on in what framework you are using it. If you are wanting to use say the SPC mesoanlyses for determining current trends, then it could be quite useful - or not informative at all. Jon Davies has a nice demonstration of application of mesoanalysis products (including 0-3 MLCAPE):"0-3 km mixed layer CAPE"

Certainly, as Tim pointed out, the reliability of this parameter is often not good, but in general if 0-3 MLCAPE values are small/zero, the threat for tornadoes is not very high (particularly if the target area is 'sunny'). Forecasts of 0-3 km CAPE can be really bad, so use with considerable caution (as well as derived products, such as EHI). Since SPC mesoanlyses are only a 1 hr forecast, they can do quite well sometimes in areas with dense observations.

You could also interpret that if 0-3 km MLCAPE is very small (or zero), that there is probably substantial lifting needed to initiate a storm (perhaps useful when considering a weakly convergent surface boundary) and that very high 0-3 km MLCAPE probably indicates weak CINH - but again parameters should never be used in isolation but weighed with other considerations. Which parameters are most useful is generally tough to know beforehand - though following a forecast methodology such as suggested by Tim Vasquez is a good start by recognizing what the limiting factors are for a particular setup. If buoyancy is a limiting factor - finding the nose of 0-3 km MLCAPE might be the most informative parameter for that event.
Good day,

EHI is a good way to go, as the combination of BOTH CAPE and Helicity are reqired for tornadogenesis. EHI is given by the following equation...

EHI = (CAPE * HELICITY) / 160,000

A number of 2 or more is favorable for tornadoes. This is a ballpark figure.
EHI using 0-1km SRH as opposed to 0-3km SRH seems to be a more effective predictor for tornadoes:

Rasmussen said:
This formulation of EHI is substantially better at distinguishing between the TOR and SUP classes than is any parameter in RB98 (refer to Table 1 ). Only 25% of the SUP soundings had EHI0–1 > 0.5, whereas nearly 2/3 of the TOR soundings had values this large. The improvement of EHI0–1 over the conventional EHI can only be due to the improvement of the SRH0–1 over its 0–3-km counterpart. As shown in Table 1 , this version of the EHI is somewhat poorer in the SUP/ORD forecast discrimination than is the traditional EHI.

--> Rasmussen, E. N., 2003: Refined Supercell and Tornado Forecast Parameters. Wea. Forecasting 18, 530-535.

I'll refer you to the following publications:
Rasmussen, E. N., and D. O. Blanchard, 1998: A baseline climatology of sounding-derived supercell and tornado forecast parameters. Wea. Forecasting 13, 1148-1164

Re: cold-core setups:
Upcoming Guyer and Davies paper said:
This study found that 0-3 km SBCAPE was the best discriminator between C500L tornadoes and C500L null tornado cases (Fig. 3). The median 0-3 km SBCAPE was 210 J/kg for tornado cases, with 75% of events in excess of 160 J/kg. In contrast, null cases featured less buoyancy “concentrated” in the lowest 0-3 km layer, with a median of 132 J/kg and 75% of cases ≤175 J/kg 0-3km SBCAPE.
Guyer, J.L., and J.M. Davies, 2006: Enviroment Characteristics Associated with Tornado Events near Closed Cold Core 500 MB Lows. Preprints, 23nd Conf. Severe Local Storms, St. Louis MO.

Here are a few other papers that one may find interesting:
Thompson, R.L., R. Edwards, and C.M. Mead, 2004: An Update to the Supercell Composite and Significant Tornado Parameters. Preprints, 22nd Conf. Severe Local Storms, Hyannis MA.

Thompson, R.L., R. Edwards and J.A. Hart, 2002: An Assessment of Supercell and Tornado Forecast Parameters with RUC-2 Model Close Proximity Soundings. Preprints, 21st Conf. Severe Local Storms, San Antonio.

This one is also interesting: Broyles, C., N. Dipasquale, and R. Wynne: Synoptic and Mesoscale Characteristics Associated with Violent Tornado Episodes across Seperate Geographic Regions of the United States.
Unfortunately, sitting down in the morning and trying to forecast the afternoon's low-level thermodynamic parameters such as 3km CAPE and CINH can be tough. These numbers typically aren't very "large" to begin with... and they are highly sensitive to not only a plethora of atmospheric processes that we can't always anticipate, but also to the chosen lifted parcel (surface-based, mixed-layer, Tv-corrected, non-corrected, etc). This is important to keep in mind when evaluating forecast values off of various websites such as COD, UCAR, and Earl Barker's site.