Importance of checking local surface observations in computing low-level CAPE and CIN
Low-level thermodynamic parameters such as CIN, LFC height, and low-level CAPE are sensitive to the temperature and dewpoint of the near-surface lifted parcel used to compute them. It is therefore important to look at all available surface observations when assessing these parameters, particularly near surface lows and warm fronts where moisture pooling and/or moisture convergence may be taking place. Following is a good example on 6/11/01 in Minnesota.
The 19 UTC surface map shows a surface low evident in the pressure and
wind fields near AQP (Appleton) in southwest or west-central Minnesota, with a warm front
extending to the ESE using the temperature and wind fields:
Strong moisture convergence and pooling of low-level moisture is occurring near the low, with dewpoints in the 70s.
Using the 3 hr RUC-2 forecast profile valid at 20 UTC for Montevideo (located at MVE on the surface map) to generate profile comparisons using different nearby surface parcels, one can see the importance of the narrow N-S warm-moist axis across southwest Minnesota near the South Dakota border. First of all, using the relatively cool 79/70 F surface observation at Morris (MOX, located a few miles north of Appleton), a mean-layer mixed parcel yields large CIN (> 1590 J/kg) and a high LFC (> 2599 m) with only negligible 0-3 km CAPE, even though total CAPE, SRH, and vertical shear are all quite large. This environment appears "elevated" and unfavorable for tornadoes with the thick layer of near-surface CIN:
A few miles south at Appleton (AQP), the surface temperature and dewpoint observation is significantly higher (88/75 F). Comparing AQP to other observations nearby, it seems possible that the observation sensor at AQP may be overstating the case somewhat. But AQP is also near the center of the surface low in an area of strong moisture convergence, so it is quite possible that this location will have the largest dewpoint. To be conservative, we can adjust the AQP temperature and dew point downward to 85/73 F, which fits the surface pattern better while still reflecting the moisture convergence and pooling that appears centered in the Appleton area. Using this modified surface observation, the most unstable parcel (surface) now yields CIN that is considerably reduced (around 100 J/kg) from our earlier estimate near MOX, while total CAPE increases significantly and vertical shear remains large (0-1 km EHI is near 8.0 !! ):
While the CIN is still relatively large (see discussion about relatively large CIN associated with some significant tornadoes), the LFC height is nearly 1 km lower than the MOX estiamte, and some CAPE below 3 km has been introduced. This profile now appears more favorable for supercell tornadoes, particularly with the very large total CAPE, SRH and vertical shear.
Using the warmer but less moist observation at Olivia (OVL) to the southeast, a 90/68 F parcel yields less CIN (60-70 J/kg), but a higher LFC and a much higher LCL (near 1500 m AGL), all of which appear less favorable for tornadoes:
So, from the above NW-SE cross section of surface observations, it appears that there is a only a narrow area just east of the surface low near Appleton where the environment may be favorable for significant supercell tornadoes based on estimated low-level thermodynamic parameters.
Radar base reflectivity between 19 UTC and 20 UTC shows thunderstorms
developing and intensifying rapidly in the Appleton (AQP) area. The southernmost
storm became a large supercell that produced a damaging F3 tornado at Benson (marked
"B" on the images) around 20 UTC:
This was the only storm to produce a significant tornado that day. As this long-lived supercell moved into south central Minnesota where dewpoints were lower, it became more elevated in nature and ceased producing tornadoes, even though rotational signatures on radar remained strong.
This case emphasizes the importance of looking at all available surface observations across an area of interest when assessing low-level thermodynamic parameters, as well using common sense about how well an "enhanced" surface observation fits the surface map pattern.
back to operational low-level buoyancy parameter guidelines