How to read the map
How to read a spaghetti map
Every line is one computer forecast of where a storm’s centre will go. Here’s how to turn dozens of lines into an answer in about 30 seconds.
1
Each line is one forecast
A line starts at the storm’s centre today (the L) and follows where one forecast sends it. The dots mark every 12 hours, so you can see when as well as where.
2
Many forecasts, one storm
Weather centres run the same model dozens of times, each with slightly different starting weather. That’s an , and each run is one line. Stormlines draws the ensembles from NOAA, ECMWF and Environment Canada, often more than 150 lines for one storm.
They start together because we know where the storm is now. They drift apart because small differences grow with time.
3
Bunched means sure. Fanned out means unsure.
This is the most important thing on the map. Don’t look for the “right” line; look at the shape of the whole bundle.
- Tight bundle near you: the forecasts agree on the storm’s track there.
- Wide fan: the track is genuinely uncertain. Plan for a range, not a line.
4
The thick line and the cone
The thick black line is the : the middle of the pack. It’s a summary, not a promise.
The outlines where the centre stays in two of every three forecasts. It widens with time because the forecasts disagree more further out.
The cone is about the storm’s centre. Rain, wind and flooding often reach far outside it.
5
The shading: will it pass near me?
The orange shading shows the that the centre passes within 100 miles of a place. Deeper orange is more likely.
“70%” means 7 of every 10 forecasts bring the centre that close. Tap a city on a storm page to see its chance and the .
6
When the lines split in two
Sometimes the forecasts form two separate groups: a storm that stalls, say, versus one that races off. Stormlines calls these and says how many forecasts back each one.
A split means the question isn’t “exactly where?” but “which of two outcomes?”. Watch the next few runs to see which way it tips.
7
Colours show who made the forecast
Under “All forecasts” each line is coloured by the centre that ran it:
- NOAA (American)
- ECMWF (European)
- Environment Canada
- dashed:
When different centres agree, trust the picture more. When one centre sits apart from the rest, that disagreement is the story.
Five mistakes to avoid
- Chasing one line. The single line that hits your town is one forecast out of many. Read the bundle.
- Reading the line as the storm’s size. A nor’easter or hurricane can be hundreds of miles across. The line is only its centre.
- Treating the cone as the danger zone. Impacts reach well outside it, especially on the storm’s stronger side.
- Trusting day 7 like day 2. Lines far out are least reliable; that’s why they spread.
- Reacting to every run. One run can wobble. A shift that holds over several runs is a trend.
Your 30-second checklist
- Where do most lines go?
- How tight is the bundle near you?
- When does the storm get closest? Follow the dots, or press Play.
- What’s your chance within 100 miles?
- Has it shifted since the last run? Storm pages say “Shifted about 25 mi SW since the last run”.
Questions
What is a spaghetti model?
A map of many computer forecasts of a storm’s track drawn on top of each other. Forecast centres run the same model dozens of times with slightly different starting conditions (an ensemble); each run is one line. Together they look like spaghetti.
Which spaghetti line is right?
None of them individually. Read the bundle: where most lines go is the likely path, and how widely they spread shows how uncertain it is. The single line that lands on your town is not a forecast for your town.
Why do the lines spread out over time?
Small differences in the starting weather grow as the forecast runs, so forecasts agree for the next day or two and disagree more by days four to seven. Wider spread means less confidence.
What does the cone mean?
The area where the storm’s centre stays in two of every three forecasts. It is about the centre only: rain, wind and coastal flooding can reach far outside the cone.
What does “chance within 100 miles” mean?
The share of all forecasts that bring the storm’s centre within 100 miles of a place. 70% means 7 in 10 forecasts pass that close.
How often do spaghetti models update?
The global ensembles run every 6 or 12 hours, and each run takes several hours to arrive. Stormlines adds each model as it lands, so the map changes through the day.
Words on this page link to the glossary.