What exactly is surface-based instability?

[yoda]

I saw this used on a AFD by LWX Sterling. What does this mean? Could anybody clear this up for me? Also, how can you tell instability on weather maps, if you can? Thanks for all your help!

[yoda]

*bump*

I still do not understand. Could someone please clear this up for me?

[Allexpert Mike]

From SPC-

"Instability - The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for severe weather - the greater the instability, the greater the potential for severe thunderstorms. "

Basically the measure of the instability tells us how unstable the atmosphere is. Stronger the instability the greater potential for severe weather. Theres a lot that goes into determing how strong the instability but just by looking at a CAPE plot tells you how much. But several other different areas also contribute to this,,,,,,LLJ,,,,,Diurnal heating,,,TT,,,

SBCAPE=surface based cape ,,,,what your forecaster was referring to.

There is different kinds of instability (potential,,,convective,,,conditional,,,etc.)that are present during severe weather but what were discussing is thermodynamic instability.

[yoda]

ok thanks! but how does surface-based instability occur? I saw that on the website, but I still do not quite understand how it differntiates from convective instability. How is SBCAPE measured? I know you probably won't have all the answers, Allexpert Mike, but your input is greatly appreciated!

[ColdFront77]

It sure is greatly appreciated. Even things we know about meteorology, is nice to be reinforced as we continue to discuss weather in general along with the current potential for activity and the activity happening at this moment.

[Allexpert Mike]

ok thanks! but how does surface-based instability occur? I saw that on the website, but I still do not quite understand how it differntiates from convective instability. How is SBCAPE measured? I know you probably won't have all the answers, Allexpert Mike, but your input is greatly appreciated!

Its a measure of CAPE is the troposhphere.......Its measures in j/kg. A scale of CAPE profiles go a little like this.....

Less than 1000j/kg- rather weak
1000-2500j/kg- moderate
2500+- strong

[yoda]

i see thanks! BUT if SPC says that CAPE will be AOA 500 j/kg, how will the damaging winds and tornado threat occur? Will it be because of the low-level shear and trough? see the above SPC outlook for more details...
THANKS!!! you are really helping me out here and helping me understand!! THANKS A LOT ALLEXPERT MIKE!

[ColdFront77]

BRN (Bulk Richardson's Number) Shear

While BRN can be useful in assessing While BRN can be useful in assessing the potential for supercell and middle-level mesocyclone development, it is less suited to assess low-level mesocyclone and tornado potential. Conversely, BRN shear may be more useful in differentiating between those supercells that will and those that will not produce tornadoes, although BRN shear still cannot be used independently for this purpose as storm-scale interactions are crucial for tornado development.

BRN shear = 25 to 100 Associated with tornadic supercells (assuming supercells form on a given day).


Storm-Relative 500 mb Winds

Middle-level (represented well by the 500 mb level) storm-relative (S-R) winds also may be useful to help differentiate between tornadic and non-tornadic supercells within the overall environment, assuming supercells will form on a given day. Middle-level S-R winds are important in order to create a balance between the low-level storm inflow along the forward front flank baroclinic zone and the low-level outflow associated with the rear flank downdraft. If very weak S-R winds are present, a large amount of precipitation tends to wrap around the mesocyclone leading to the generation of excessive rain-cooled outflow along the low-level rear flank of the storm. This cool air then undercuts the middle-level mesocyclone and disrupts the low-level circulation. On the other hand, if middle-level S-R winds are very strong, then the middle-level flow may remove too much precipitation downwind from the mesocyclone, inhibiting the development of enough rain-cooled outflow (i.e., downdraft) along the rear flank to help focus convergence and generate baroclinic vorticity. Between these two extremes exists a balance where the rear flank downdraft is pronounced but balanced by significant low-level S-R flow into the system. S-R winds at 500 mb can be calculated from subtracting the observed or forecasted storm motion from the observed or forecasted 500 mb wind speeds.

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500 mb S-R winds = 16 kts (8 m/s)   Lower limit for tornadic supercells.
 500 mb S-R winds = 40 kts (20 m/s)  Approximate upper limit for tornadic supercells.


Storm-Relative Helicity

Storm-relative (S-R) helicity (Hs-r) is an estimate of a thunderstorm's potential to acquire a rotating updraft given an environmental vertical wind shear profile, assuming thunderstorms are able to develop.

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Hs-r = 150:         The approximate threshold for supercell development.
Hs-r = 150 to 299:  Weak tornadoes (F0 and F1) possible.
Hs-r = 300 to 449:  Strong tornadoes (F2 and F3) possible.
Hs-r over 450:      Violent tornadoes (F4 and F5) possible

[Allexpert Mike]

i see thanks! BUT if SPC says that CAPE will be AOA 500 j/kg, how will the damaging winds and tornado threat occur? Will it be because of the low-level shear and trough? see the above SPC outlook for more details...
THANKS!!! you are really helping me out here and helping me understand!! THANKS A LOT ALLEXPERT MIKE!

Read coldfront's topic on FL threat he just started,,,I posted in there why damaging winds will be main threat with the weak CAPE in place.

[yoda]

Alright. Thanks for ALL your help. It is GREATLY APPRECIATED!!