Convective Watch — Thunderstorm Scoring
How PlaneWX assesses thunderstorm risk along your route using multi-model atmospheric analysis, composite scoring, and horizon-scaled penalties.
The Big Picture
Before every flight, PlaneWX samples 9–15 points along your route and pulls atmospheric data from up to three weather models (HRRR, GFS, and ECMWF). At each point, it measures four atmospheric properties that together determine whether thunderstorms are likely, and if so, how strong they’d be.
These measurements are combined into a single Threat Score (0–100) that appears on your Convective Watch card. The higher the score, the greater the thunderstorm risk along your route.
0–15
None
No thunderstorm risk
15–35
Isolated
Scattered pop-ups possible
35–60
Scattered
Multi-cell storms likely
60–100
Widespread
Organized severe weather
What makes this different? Most weather apps show CAPE or a single instability number. PlaneWX combines four atmospheric indices, weights them by importance, checks whether the models agree, and factors in atmospheric caps that might suppress storms. The result is a score that reflects true risk — not just raw energy.
Reading the Convective Watch Card
When conditions support thunderstorms along your route, a Convective Watch card appears in your briefing. Here’s what each metric means:
Threat Score (0–100)
The composite score combining all four atmospheric indices, model agreement, and suppression factors. This is the single best number for gauging overall thunderstorm risk along your route.
Instability / CAPE (J/kg)
The maximum CAPE (Convective Available Potential Energy) found along your route — the atmosphere’s “fuel tank” for storm updrafts. Higher means stronger potential storms.
Affected Segments
How many of the sampled route segments show thunderstorm potential. “2 / 15” means 2 of 15 points along your route have conditions that support storms — the threat is localized, not route-wide.
Storm Probability / K-Index
Predicts the probability of air-mass (pop-up) thunderstorms based on temperature and moisture through the atmosphere. Above 35 is high probability.
Stability / Lifted Index
Measures whether air parcels will rise or sink. Negative values mean the atmosphere is unstable. Below −4 indicates severe weather potential.
Where the Data Comes From
PlaneWX uses three independent data sources. No single weather product captures the full convective picture, so combining them gives you the most reliable assessment.
Numerical Weather Models
HRRR (3 km resolution, US-only), GFS (25 km, global), and ECMWF (9 km, global) provide atmospheric profiles at multiple pressure levels. PlaneWX extracts CAPE, K-Index, Lifted Index, and CIN from each model at each sample point.
WPC Discussions (CONUS only)
For US flights, the Weather Prediction Center’s short-range and extended discussions are analyzed for convective language. This adds regional context that models alone may miss. WPC data is geographically filtered so you only see language relevant to your route — not weather in distant regions.
TAF Convective Scanning
Terminal Aerodrome Forecasts at departure, arrival, and en-route waypoint airports are checked for thunderstorm indicators including TS, TSRA, VCTS, CB, TCU, and PROB30/40 convective groups. En-route TAF scanning catches convective weather forecasts that only appear at airports along your corridor, not just at your endpoints.
METAR Convective Scanning
Real-time METARs at departure, arrival, nearby (30 NM radius), and en-route corridor airports are scanned for live convective indicators. This catches storms that are happening now, which models may not have predicted.
Potential Max Tops (Equilibrium Level)
When convective conditions are detected, PlaneWX computes the Equilibrium Level (EL) — the theoretical maximum altitude a thunderstorm updraft can reach based on the atmospheric sounding. This is displayed as “Potential Max Tops” alongside your cruise altitude to give you immediate vertical situational awareness.
Important: The EL represents a theoretical maximum, not a guarantee. Actual storm tops can exceed the EL due to overshooting, and significant turbulence and hail can exist thousands of feet above the visible cloud top. PlaneWX never suppresses convective warnings based on altitude — a pilot at FL450 still needs to know about a CB below them because storms can grow at 6,000 ft/min, and hail can be ejected into clear air downstream.
How Potential Max Tops Are Calculated
The Lifted Parcel Method with Most Unstable (MU) parcel fallback.
How Potential Max Tops Are Calculated
PlaneWX uses the Lifted Parcel Method — the same approach used by the National Weather Service. A parcel of air is virtually “lifted” from the surface through the atmospheric profile:
- Below the LCL (cloud base), the parcel cools at the dry adiabatic rate
- Above the LCL, the parcel cools at the moist adiabatic rate (slower, because condensation releases latent heat)
- The parcel rises until it becomes cooler than the surrounding air — this is the Equilibrium Level
If the surface parcel doesn’t find an EL (e.g., a surface inversion blocks it), the algorithm tries the Most Unstable parcel — the level with the highest equivalent potential temperature in the lowest 300 hPa. This catches elevated convection that can form above inversions, especially at night.
How the Threat Score Is Calculated
The Threat Score uses a weighted composite of four atmospheric indices, adjusted for model agreement and atmospheric suppression. Here’s the breakdown.
Step 1: Convert Each Index to a Signal Score (0–1)
Each atmospheric index is mapped to a 0–1 signal based on meteorologically verified ranges.
Step 1: Convert Each Index to a Signal Score (0–1)
CAPE Signal (Weight: 50%)
Convective Available Potential Energy — the energy available to fuel storm updrafts.
Lifted Index Signal (Weight: 30%)
Atmospheric stability — negative values mean unstable air that wants to rise.
K-Index Signal (Weight: 20%)
Thunderstorm probability from temperature and moisture profiles. Automatically excluded at airports above 5,000 ft where the formula is unreliable.
Step 2: Agreement Multiplier & CIN Suppression
The raw score is adjusted based on whether indices agree and whether an atmospheric cap is holding storms down.
Step 2: Agreement Multiplier & CIN Suppression
Agreement Multiplier
If CAPE, Lifted Index, and K-Index all agree on high risk, confidence is high (multiplier near 1.0). If they disagree — say CAPE is high but Lifted Index is stable — the score is reduced. Missing data is treated as neutral, not as disagreement.
If K-Index is unavailable (e.g. high elevation), its weight is redistributed to CAPE and Lifted Index proportionally.
CIN (Convective Inhibition) Suppression
CIN measures the atmospheric “cap” holding storms down. High CIN means storms are unlikely to fire even if energy (CAPE) is high. The system uses CIN as a multiplier that reduces the score when a strong cap is present.
“Loaded Gun” Safety Floor
When extreme instability is present (very high CAPE + indices in strong agreement), PlaneWX will not let CIN suppress the score below 40%, even if the cap is strong. This is a “loaded gun” scenario — the atmosphere is primed. If a cold front, dryline, or other trigger breaks the cap, explosive storms will develop rapidly. You need to know about this risk.
Step 3: The CAPE Gate
A hard minimum that prevents false alarms when there simply isn’t enough energy for storms.
Step 3: The CAPE Gate
If the maximum CAPE across all models at a sample point is below 100 J/kg, the threat is automatically set to “None” regardless of what the other indices say. Without sufficient energy, thunderstorms simply cannot form.
This prevents false positives at high-elevation airports (like Mexico City at 7,300 ft) where K-Index and Lifted Index might look threatening but there isn’t enough atmospheric energy to produce storms.
Step 4: Multi-Model Consensus
How three weather models are reconciled into a single assessment.
Step 4: Multi-Model Consensus
HRRR, GFS, and ECMWF often report different CAPE values because they use different parcel types (surface-based, mixed-layer, most-unstable). To handle this:
- •Median CAPE is used for the actual score — this resists outliers from any single model
- •Maximum CAPE is used for the safety gate — if any model shows energy, we don’t dismiss it
This “safety-first” approach prevents a single model from over-warning you (the median keeps the score grounded) while also preventing all models from silencing a real threat (the max keeps the gate honest).
Final Composite Score
Threat Score = (Weighted Signal Score) × CIN Suppression × Agreement Multiplier
Weighted Signal = (CAPE Signal × 50%) + (LI Signal × 30%) + (K-Index Signal × 20%)
Horizon-Scaled Penalties
Convective forecasts become dramatically more accurate as departure approaches. A thunderstorm forecast 36 hours out is far less certain than the same forecast 3 hours before departure. PlaneWX accounts for this by scaling convective WX Score penalties by the forecast horizon.
Horizon Multiplier
Built-In Safety Guards
CAPE Gate (100 J/kg)
Below 100 J/kg of CAPE, thunderstorms cannot form. The system short-circuits to “None” regardless of other indices, preventing false alarms at high-elevation or cold-weather airports.
Elevation Guard (K-Index)
The K-Index formula uses temperature at 850 hPa and 700 hPa. At airports above 5,000 ft, these pressure levels can be below the surface, making K-Index mathematically invalid. PlaneWX automatically excludes K-Index at high-elevation points and relies on CAPE and Lifted Index instead.
“Loaded Gun” Floor
When extreme energy is present and multiple indices agree, CIN suppression is capped at 0.40× to ensure pilots are warned about “capped but dangerous” environments. If the cap breaks, explosive convection follows.
WPC Geographic Filter
WPC text is filtered to your route’s geographic region. A flight from California won’t see convective language about the Southeast. International flights outside the US bypass WPC entirely and rely solely on model data and TAFs.
Model-Aware METAR Suppression
Some automated weather stations (especially AO2 sensors in the Southeast) routinely report distant lightning (“LTG DSNT”) even when no convective weather threatens your route. A single station’s distant lightning report can create a false-positive score deduction on an otherwise perfect VFR day.
To prevent this, PlaneWX cross-references METAR convective signals against the three NWP models (HRRR, GFS, ECMWF). When all three models unanimously confirm zero convective potential along your route (CAPE = 0, no threatened segments), low-severity signals like distant lightning are suppressed from the WX Score calculation. The observation still appears in your briefing text so you’re aware of it.
Active thunderstorm observations (TS, VCTS, tornado, hail) are never suppressed, and neither are signals from three or more stations — widespread reports always count regardless of what models say.
Time-of-Day Awareness
Surface-based convection is heavily driven by solar heating. Thunderstorms are far more likely during peak afternoon heating (15–19 local) than early morning. PlaneWX applies a diurnal multiplier to the composite threat score based on local solar time at your departure, reducing scores for flights outside the peak convective window without ever fully suppressing warnings.
Storm Type & Bulk Wind Shear
PlaneWX computes 0–6 km bulk wind shear from model sounding data to determine whether convective conditions favor short-lived pulse storms or dangerous organized severe weather. This context appears in the Convective Watch card narrative.
Convective SIGMETs & SPC Day 1 Outlook
Convective SIGMETs
Active Convective SIGMETs (WST) that intersect your route are automatically parsed from SIGMET data and displayed prominently in the Convective Watch card’s “Convective SIGMETs” section with red indicators. These indicate confirmed severe convection observed or expected along your flight path.
SPC Day 1 Categorical Outlook
For CONUS flights, PlaneWX fetches the Storm Prediction Center Day 1 categorical outlook and checks whether your route passes through any risk area. The maximum risk level encountered is displayed as a metric cell on the Convective Watch card.
TAF Time-Window Filtering
En-route TAFs are now filtered against your actual flight window. Convective hazards in TAF groups that don’t overlap with your departure-to-arrival window are excluded, preventing false alarms from storms forecast hours before or after your flight.