GOES GLM + NOAA LightningCast Explained — Satellite Lightning Imagery

GOES GLM + NOAA LightningCast Explained — Satellite Lightning ImageryHow to read GOES-19 GLM (Geostationary Lightning Mapper) and NOAA LightningCast — satellite-detected strikes plus 60-minute lightning probability forecast. Color scales, how they compare to ground networks (NLDN, ENTLN), and how spotters use them.BloomWX surfaces two lightning products on the Charts tab — both free, both satellite-derived, but answering different questions. GLM shows where strikes happened in the last few minutes. LightningCast predicts where strikes will happen in the next 60 minutes.GOES Geostationary Lightning Mapper (GLM)GLM is an instrument on GOES-19 (and GOES-18 for the West) that continuously photographs the Earth in a narrow optical band where lightning emits brightly (777.4 nm). It detects both cloud-to-ground AND in-cloud strikes — the latter being where most of the actual lightning in a thunderstorm lives.

Bright yellow / red: High flash density in the last minute. The storm core.

Orange: Moderate flash density — active convection but not the most-intense cell.

Sparse points: Isolated strikes. Storm starting up, decaying, or anvil-only.

No color: No detected flashes (or the storm is below GLM's detection threshold).

How GLM compares to ground networks

Detects more total flashes than NLDN / ENTLN — GLM sees in-cloud strikes that ground networks miss.

Lower spatial precision — ~8 km at nadir vs sub-kilometer for ground networks.

1-2 minute latency from strike to image. Ground networks publish in seconds.

Free + global — no subscription. Ground networks like Vaisala NLDN are subscription-only.

Best use: "is there lightning in this region?" Not "exactly where is the next strike going to be?"

NOAA LightningCast (60-min probability forecast)LightningCast is a NOAA / CIMSS / NSSL AI model that takes the current GOES satellite imagery (visible, near-IR, long-wave IR) and predicts the probability of lightning at each pixel over the next 60 minutes. It's been used operationally by NWS forecasters since 2021.

Yellow contour (≥10%): Lightning possible somewhere in the next hour.

Orange (≥40%): Lightning likely. Most NWS offices issue Special Weather Statements at this threshold for outdoor venues.

Red (≥75%): Lightning near-certain. Take cover if outdoors.

How spotters / EMA actually use them

Storm initiation: LightningCast often signals 15–45 min BEFORE the first strike. Useful for outdoor-event evacuation calls.

Storm intensity: GLM flash rate jumping from sparse to a bright cluster usually precedes severe weather (hail spike, intensifying updraft).

Storm decay: GLM going dark = storm collapsing. The downburst hazard often peaks during this transition.

Anvil lightning: Strikes well outside the rain core ("bolt from the blue") — GLM catches these; you're at risk under a bright anvil even on a dry sidewalk.

Outdoor event "30/30" rule: Suspend if thunder within 30 sec of flash (~6 mi). LightningCast lets you anticipate; GLM lets you confirm.

Sources: NESDIS GOES-19 GLM at star.nesdis.noaa.gov, CIMSS LightningCast viewer at cimss.ssec.wisc.edu/probsevere/lc/. For genuine strike-by-strike data (life-safety grade), the commercial ground networks (Vaisala NLDN, Earth Networks ENTLN) are the operational standard — BloomWX recommends RadarScope or RadarOmega Pro tiers, which embed those feeds.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.