Weather plumes: how to read one for your town

What a plume shows

A forecast map shows one model across a whole region at one moment. A plume flips that around. It holds the place still and shows how a variable, such as temperature or total snow, changes over the coming days in every forecast at once. Each thin line is one forecast, so you can see at a glance whether that big snow total on social media has company or is standing alone.

How to read a plume, step by step

1. Check the place, the variable and the units. A plume answers for one point, and precipitation and snow lines usually add up over time.
2. Find where the lines start to spread. That time is roughly where the forecast stops being settled.
3. Read the middle of the fan first, then its edges. The middle is a reasonable starting point, and the edges show the range to plan around.
4. Count the lines behind any big number. If two of 31 members show a foot of snow, that is a possibility worth watching and not the forecast.
5. Look for clusters. Two groups usually mean two storm scenarios, and it helps to find out what separates them on a map.
6. Compare two ensembles. WPC notes that a multi-model ensemble may verify better than one from a single model, so GEFS and ECMWF EPS landing in the same range is more convincing than either alone.
7. Check again after the next run. A fan that keeps narrowing around the same value means more than one sharp-looking run.

How to read a snow plume

A snow plume climbs when snow is falling and goes flat when it stops, so the slope gives you timing and the top of each line gives you a storm total. The width at the end of the storm is your range of totals.

Snow takes more care than rain because a model has to turn liquid into depth. The National Weather Service office in La Crosse calls 10:1 an old rule of thumb and notes that real ratios vary widely, even within one storm. Models also handle the change between rain and snow differently, so read the precipitation plume beside the snow plume. If precipitation lines agree and snow lines do not, the argument is probably about temperature and not about the storm itself.

Remember that a point on a model grid is an average over the grid box. In the mountains or near a lake, your own spot can differ a lot from the nearest grid point, even when every line agrees.

Ensemble plumes and recent-run plumes

Plumes come in two kinds, and they answer different questions. They look alike, so check which one you are reading.

Recent runs are not an ensemble of equals, because the newest run started from fresher data. They are still a quick way to catch a model that keeps changing its mind.

Which models have plumes in 4070

RRFS took the place of the NAM 3 km Nest in Plumes on November 3, 2026. The AI-GFS does not forecast snowfall, so it has no snow lines. The hybrid ensemble contains the GEFS members, so it is not independent of GEFS.

Where to see plumes for your town

In 4070, Plumes puts all of these models on one time chart, so two runs that started at different times still line up at the same moment.

1. Open Plumes from Models, on the web or in the app.
2. On the web, click the map, use a saved place, search for an address or type a latitude and longitude. In the app, change the location at the top.
3. Choose Temperature, Precip or Snow.
4. Use the model buttons to look at one model at a time, or leave all of them on to compare.
5. Select Table to read the same forecasts as a color-coded table by day or by 3 or 6 hours.

Plumes is part of 4070 Pro, which starts with a seven-day free trial. Public agencies also publish related charts at no cost. ECMWF’s open charts include ENS meteograms, which show the European ensemble at a location as box and whisker plots. NOAA’s Model Analysis and Guidance site has GEFS maps, and NOAA’s Climate Prediction Center publishes spaghetti plots. These are useful references, though they do not put several models on one chart for your address.

Common mistakes

• Treating the highest line as the forecast. It is often the one that gets shared, and it is rarely the most likely outcome.
• Treating the middle line as a promise. WPC found that the high-resolution control is the best member only about 5 to 7 percent of the time, so no single line should be trusted just because it looks detailed.
• Assuming tight lines mean certainty. Members of one ensemble share the same model, so they can miss together.
• Reading a plume far past its useful range. Late in a forecast the fan is often a picture of how little anyone knows, which is honest and still useful.
• Forgetting the wider setup. A plume tells you how much and when at one point. It takes a map, such as a 500 mb map, to see why.

Plumes show model output, not a WDS or National Weather Service forecast. Official National Weather Service forecasts, watches and warnings remain the authority for safety decisions.