Skew-T Stability Indices — CAPE, CIN, LI, K-Index, SWEAT, BRN Explained

Skew-T Stability Indices — CAPE, CIN, LI, K-Index, SWEAT, BRN ExplainedA field guide to the stability indices in the BloomWX hourly Skew-T sidebar. CAPE, CIN, Lifted Index, Showalter, K-Index, Total Totals, SWEAT, PWAT, BRN, LCL, LFC, EL — what each one measures, the operational thresholds Skywarn / EMA training uses, and how forecasters read several together to assess severe weather.The numbers in the sidebar next to every Skew-T are computed from the model's vertical profile of temperature, dewpoint, and wind. Each one summarises the air mass's potential to produce thunderstorms in a different way — a single number you can scan at a glance instead of mentally integrating the whole sounding.No single index is the "right" answer. Forecasters read several together because each catches a different failure mode (e.g. CAPE can be high while a strong cap suppresses storms; K-Index captures moisture but misses shear). Below: what each one actually measures and the rough thresholds that flag a meaningful threat.Energy / instability

CAPE (J/kg) — Convective Available Potential Energy. The integrated buoyant energy a rising air parcel has between its LFC and EL. The "fuel" for updrafts. 0–500 = weak / unlikely to support thunder; 500–1500 = moderate; 1500–2500 = strong; 2500+ = very strong; 4000+ = extreme. Severe storms typically need 1500+; tornadoes typically 1000+ when shear is also present.

CIN (J/kg) — Convective INhibition. The negative-buoyancy "cap" a parcel must overcome before reaching the LFC. Always reported as negative. 0 to -25 = no real cap; -25 to -100 = moderate (afternoon heating usually breaks it); -100 to -200 = strong cap; below -200 = capped (storms unlikely).

LI (Lifted Index) — Difference in °C between an environmental parcel lifted from the surface to 500 mb and the actual 500 mb temperature. Negative = unstable. 0 to +3 = stable; 0 to -3 = marginally unstable; -3 to -6 = strongly unstable, severe possible; below -6 = extreme.

Showalter — Same idea as LI but lifts from 850 mb instead of the surface. Less affected by surface heating. 0 to -3 = unstable; below -3 = severe weather possible; below -6 = strongly favorable.

Composite "ingredients" indices

K-Index — Mid-level moisture + lapse-rate composite (uses 850, 700, and 500 mb). Designed for non-severe air-mass thunderstorm potential. Below 20 = unlikely; 20–25 = isolated; 25–30 = scattered; 30–35 = numerous; 35+ = high coverage of thunderstorms with heavy rain.

Total Totals — Combines low-level moisture (850 mb dewpoint) with mid-level temperature lapse (850 to 500 mb). Below 44 = isolated thunder; 44–50 = scattered storms with thunder; 50–55 = severe possible; 55+ = severe likely, isolated tornadoes possible.

SWEAT — Severe WEAther Threat index. Multi-ingredient composite of moisture, instability, wind speed at 850 and 500 mb, AND directional shear. Below 250 = limited threat; 250–300 = severe possible; 300–400 = severe likely; 400+ = tornado potential.

BRN (Bulk Richardson) — Ratio of CAPE to 0–6 km bulk shear. Tells you what storm MODE the environment favors. Below 10 = too much shear / not enough CAPE (splitting supercells); 10–45 = SUPERCELL window (the goldilocks zone); 45–100 = multicell possible; above 100 = pulse / single-cell.

Moisture

PWAT (Precipitable Water) — Total liquid-equivalent depth of all water vapor in the column above the station. Below 12 mm (~0.5") = very dry; 12–25 mm (0.5–1.0") = average; 25–40 mm (1.0–1.6") = moist; 40–50 mm (1.6–2.0") = very moist; 50+ mm (2.0"+) = tropical (training-storm flash flood territory). Compare to your local climatological max.

Parcel-trajectory altitudes

LCL pressure — Lifting Condensation Level. Pressure (mb) where a rising surface parcel saturates and forms a cloud base. LCL ≤ 1500 m AGL (often ≤ 800 mb) is associated with tornadic supercells; LCL > 2000 m typically suppresses tornadoes.

LCL temp — Temperature (°C) at the LCL. Used in derived indices and as a quick sanity check on the moisture profile.

LFC — Level of Free Convection. Pressure (mb) where a lifted parcel becomes positively buoyant on its own. The "altitude where storms break free of the cap." Lower LFC pressure = harder to trigger.

EL — Equilibrium Level. Pressure (mb) where the parcel becomes neutrally buoyant again at the top of the storm. Marks the anvil base. Lower pressure value (higher altitude) = taller, stronger storm.

How to read them together1. Is there fuel? CAPE > 1000 and LI < -3 = unstable air mass. CAPE near zero with positive LI = no fuel, stop here. 2. Is the cap holding? CIN below -100 with no front, dryline, or strong heating = capped, storms unlikely. CIN 0 to -50 with afternoon insolation = cap will break. 3. Is there shear? Look at BRN. 10–45 = supercell window. Total Totals + SWEAT pair captures the ingredients composite — if both are elevated AND CAPE is moderate, severe storms are favored. 4. Will tornadoes be tornadoes? Low LCL (≤ 1000 m AGL) increases tornado probability when a supercell forms. High LCL = mostly damaging wind / hail. 5. Is it a flood setup? PWAT well above local normal + slow storm motion (look at the wind profile) = training storms = flash flood watch.Important caveat — model spreadThese values are computed from the SELECTED model's profile (RAP or HRRR by default in BloomWX). Each model uses different vertical resolution, microphysics, and boundary-layer parameterization, so a CAPE of 2500 J/kg in HRRR might show as 1800 J/kg in RAP for the same county. That model spread IS the forecast uncertainty — flip between RAP and HRRR with the picker and look for agreement. Big disagreement = the boundary layer is doing something the coarser model can't resolve.Where else to find them

SPC Mesoanalysis: spc.noaa.gov/exper/mesoanalysis — double-click any point on the map for a popup sounding with these same indices, computed from RAP analysis.

University of Wyoming Sounding Archive: weather.uwyo.edu/upperair/sounding.shtml — observed (balloon) soundings with all of these indices computed and tabulated.

Thresholds above are the operational ranges Skywarn / EMA training materials use. They're starting points, not absolutes — your local WFO has region-specific climatology that adjusts them.Part of the BloomWX learn library — beginner-friendly explainers covering every surface of the BloomWX weather dashboard. Open BloomWX to see live data for any U.S. county.