Glossary
What the forecast words mean
Short definitions for the models, maps and numbers on Stormlines. Every underlined word on the site links here.
Forecast models
The forecasts Stormlines draws come from these global models.
- American ensemble (GEFS)
- NOAA’s Global Ensemble Forecast System: the US weather service runs its global model 31 times to show the range of possible outcomes. Physics-based. Updated 4× a day, forecasts out to 16 days.
- American model (GFS)
- NOAA’s main single forecast (Global Forecast System). One run, 4× a day. It’s the “best guess” that the GEFS ensemble spreads around.
- European ensemble (ECMWF ENS)
- The European Centre for Medium-Range Weather Forecasts runs its model 51 times; widely regarded as the leading global ensemble. Physics-based. 00z and 12z runs go out to 15 days; 06z and 18z to 6 days.
- European model (ECMWF)
- ECMWF’s main single forecast (its control run). Often called “the Euro” by forecasters.
- European AI ensemble (AIFS ENS)
- ECMWF’s artificial-intelligence ensemble: a machine-learning model trained on decades of weather, run 51 times. Operational since July 2025. Fast and skilful on tracks; AI models can underestimate how deep storms get.
- European AI model (AIFS)
- ECMWF’s single AI forecast.
- American AI ensemble (AIGEFS)
- NOAA’s AI ensemble, built on Google DeepMind’s GraphCast model, run 31 times. Operational since December 2025.
- American AI model (AIGFS)
- NOAA’s single AI forecast (GraphCast-based).
- Canadian ensemble (GEPS)
- Environment and Climate Change Canada’s Global Ensemble Prediction System, run 21 times. Physics-based. 2× a day (00z, 12z), out to 16 days.
- Canadian model (GDPS)
- Canada’s main single global forecast.
- Google AI ensemble (WeatherNext)
- Google DeepMind’s AI ensemble (64 runs). Not yet included.
- Physics models
- Models that simulate the atmosphere with the equations of fluid motion. Comparing physics and AI models is one of the most useful checks.
- AI models
- Models that learn weather patterns from historical data and run in minutes. Comparing physics and AI models is one of the most useful checks.
Forecast ideas
The words on our maps and charts.
- Ensemble
- Running the same forecast many times with slightly different starting conditions, to see how sure the forecast is. When the runs agree, confidence is high; when they fan out, the future is uncertain.
- Forecast / member
- One run in an ensemble. Each line on the map is one forecast of the storm’s centre. Also called an ensemble member.
- Control run
- The ensemble member started from the best estimate of today’s weather, with no nudges.
- Spaghetti plot
- A map with every forecast track drawn at once. It looks like spaghetti, hence the name.
- Most likely path
- The track through the middle of the main group of forecasts. It’s the median, not a single model. When forecasts split, each scenario gets its own path.
- Cone
- Where the storm’s centre is likely to go: about 2 in 3 forecasts stay inside it. It is not the size of the storm. Wind, rain and snow can reach far outside it.
- Chance within 100 miles
- The share of forecasts that bring the storm’s centre within 100 miles of a place. Our “strike probability”. Nerd mode lets you change the distance.
- Scenario
- A group of forecasts that take a similar path, e.g. “hugs the coast” vs “out to sea”. Shown when the forecasts split into distinct outcomes.
- Agreement
- How tightly the forecasts cluster, 0–100. High means the forecasts are close together; low means the outcome is still uncertain.
- Run / cycle (00z, 06z, 12z, 18z)
- Models start 4 times a day at these UTC times. “12z” is the noon-UTC run (8 a.m. Eastern in summer). New runs appear on Stormlines a few hours later as each model is published.
- UTC / Z
- Coordinated Universal Time, the clock meteorologists use worldwide. “Z” (Zulu) means UTC.
- Central pressure (mb / hPa)
- The air pressure at the storm’s centre. Lower means a stronger storm. Millibars (mb) and hectopascals (hPa) are the same unit. A strong nor’easter is often below 980 mb.
- Bomb cyclone
- A storm whose pressure falls very fast, about 24 mb in 24 hours at New England’s latitude. Also called bombogenesis.
- Nor’easter
- A storm that tracks up the US East Coast with strong northeast winds, most common from October to April.
- Closest approach
- When and how close the storm’s centre passes a place.
- Range and bearing
- Distance and compass direction from a place to the storm’s centre at closest approach. Nerd mode shows this in nautical miles (nm).
- nm / kt
- Nautical miles (1 nm ≈ 1.15 miles) and knots (nautical miles per hour).
- Isobars
- Lines of equal air pressure. Tightly packed isobars mean strong wind.
- Stall
- When the storm slows to a crawl, prolonging wind, rain and coastal flooding.
- Coastal jump (Miller B)
- When a storm’s centre re-forms off the coast as the original low fades inland. A classic nor’easter pattern that’s hard to forecast.