Understanding Meteograms for Beginners: 10-Day, 15-Day, 45-Day, and AI-Based Forecasts

Author: Chris Nitsopoulos

Posted on: Sunday, December 15th, 2024, 12:53:46 PM

Hi folks,

Welcome to Weather IQ, where we help improve your weather knowledge. Today, we’re diving into a tool called the Meteogram, a type of graph produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) and other forecast agencies. Meteograms show forecast data over time in a visual format, making it easier to understand how weather may change day-to-day. These charts are available publicly through www.ecmwf.int and through our subscription service from the "State Pages" as one of the many analytical tools we provide to increase your Weather IQ.

We’ll start with how to read a 10-day meteogram, then move on to the 15-day version, and finally spend extra time on the 45-day meteogram. We’ll also take a quick look at the 10-day AI meteogram. By the end, you’ll be more confident in interpreting these charts to plan your day, farm, tourism operations, or just to impress friends with your newfound Weather IQ!

In the examples below we use Mackay as the example city.

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Part A: Reading the 10-Day Meteogram

What is a 10-day Meteogram?
A 10-day meteogram gives you a 6 hourly look at how the weather is likely to unfold over the next week and a half. It typically shows parameters like:

Cloud Cover often measured in oktas where the sky is broken up into 8 equal parts and then we estimate how many of those 8 parts are cloudy higher numbers mean more cloud cover - useful for spear fisherman, astronomers, pilots etc
Precipitation (rainfall) - This is the accumulated rainfall for the previous 6 hour period leading into 00z, 06z, 12z, 18z
Wind Speed and Direction - in m/s (multiply by 2 for knots) - This is a 10 min average wind speed at 00z, 06z, 12z, 18z - note gusts will be much stronger.
Temperature at ground level (2 m above the surface) - This is an instantaneous value at that time i.e. the temperature at 00z, 06z, 12z, 18z

How is the Information Displayed?
A common format for the meteogram is a series of box-and-whisker plots for each day or time period. Each day is broken up into four lots of six hour chunks based on the UTC (The new day starts at 00z which is equivalent to 10AM QLD time). Here’s what you need to know about those shapes:

The Box (the colored rectangle): The box represents the range where most model forecasts (from an ensemble of many "what if" simulations) agree. The top and bottom of the box show the “middle” portion of all the forecasts.
The Horizontal Line Inside the Box: This often shows the median forecast (the value right in the middle of all predictions).
The Whiskers (thin lines extending above and below the box): These lines show the full spread from the lowest forecasts to the highest. The whiskers help you understand how uncertain the forecast is. Longer whiskers mean models are more split on what might happen.
The Time - 6 hour blocks based in UTC (00z, 06z, 12z, 18z) showing the average/totals values over the previous 6 hour period leading up to the time graduation.
The blue line - is the High resolution forecast chart - the one you see on most websites.

Why Use a 10-Day Meteogram?
This shorter-range look helps you plan for upcoming events—maybe a weekend barbecue or a short trip. It shows you how the weather might change day-to-day and gives a sense of confidence or uncertainty. If the boxes and whiskers are tight (short), it usually means the forecast is more certain. If they’re spread out, the weather is trickier to predict.

From the 10-Day Mackay Meteogram (above)

Notice the Cloud Cover Boxes Over the Wednesday to Saturday period: In the forecast, the box-and-whisker plots for total cloud cover are relatively “tight,” meaning the ensemble models agree closely that the period between Wednesday and Saturday will likely be cloudy and overcast in Mackay. This suggests higher confidence in the amount of cloud expected during those initial days.
A Spike in Predicted Rainfall Mid-Forecast: Around the middle of the 10-day period, the precipitation box gets taller and the whiskers longer, indicating a possible rainy day but with higher uncertainty about rainfall amounts. This could mean some models predict heavy rain, while others expect only light showers or even dry conditions. Take not of some whiskers showing rainfalls as high as 214mm in 6 hrs indicating that there is high uncertainty in Mackay's rainfall with the possibility of extreme rainfall embedded in the overall rainy period.

Part B: Reading the 15-Day Meteogram

What is a 15-day Meteogram?
The 15-day meteogram extends the view beyond the typical short-range forecast. By looking over two weeks, you get a broader picture of potential weather trends, not just the immediate conditions.

What Does It Show?
Similar to the 10-day, you’ll see parameters like cloud cover, precipitation, wind (speed is a 10 min average over the entire day and daily wind direction distribution - direction is shown by a wind rose which shows the likely direction the winds will be coming from on that day), and minimum/maximum temperature. However, because we’re looking further into the future, expect a bit more uncertainty. The boxes may be larger and the whiskers longer as models struggle more to pinpoint exact conditions many days ahead. Just a reminder - the Meteorological day goes from 00z to 23:59z i.e. the meteorological day starts at 10AM QLD time.

How to Interpret It?

Focus on Trends, Not Exact Numbers: By 15 days out, it’s less about precise values and more about whether it looks like there may be a rainy period or a dry spell, warmer-than-usual weather or a cool stretch.
Compare Early Days to Later Days: Notice that forecasts a day or two ahead might have smaller boxes and whiskers (more certainty) than those a week or two away. You'll also notice on major rain events especially in the tropics you'll have very large boxes and whiskers due to the unpredictability of tropical rainfall events.

When Is It Useful?
If you’re planning a trip, a big family gathering, or some minor farm/garden work a week or more in advance, the 15-day meteogram offers an early heads-up. It’s not exact but gives you a sense of what might be around the corner.

From the 15 day Meteogram example above for Mackay

Growing Uncertainty in Cloud Cover Further Out: In the 15-day chart, the cloud cover boxes toward the end of the forecast get much larger. This shows that while we know clouds are possible, the models differ quite a bit on how much cloudiness there will be a week or two down the line.
Precipitation highly likely Tuesday to Saturday: We have excellent model agreement that there will be rainfall in Mackay from Tuesday until and including Saturday. Some models suggest that very heavy falls are possible on Thursday and Friday.
SE winds will dominate the Mackay area from Thursday until Tuesday : The wind rose shows that south-easterly winds will dominate this location for a number of days in the mid forecast period.

Part C: Reading and Using the 45-Day Meteogram for Sub-Seasonal Planning

What is a 45-day Meteogram?
Now we’re looking at what’s sometimes called a sub-seasonal forecast. The 45-day meteogram goes well beyond the usual weather forecast period. It helps identify broad patterns rather than precise day-to-day details.

What Does It Show?

Long-Term Averages & Trends: Instead of just a single run, these plots often show weekly average and they compare that to a the most recent 20 years of climatology.
Anomalies Relative to “Normal”: Some 45-day products might not just show raw values; they might indicate how much warmer, cooler, wetter, or drier it could be compared to a typical pattern for that time of year.
Boxes and Whiskers: As always, the boxes still represent the core “cluster” of forecasts, and the whiskers show the outer ranges. Because we’re looking so far ahead, you’ll often see bigger boxes and longer whiskers, indicating greater uncertainty.

How to Interpret It:

Look for Big Picture Patterns: The 45-day meteogram is not about whether it will rain at 3 pm on a Tuesday four weeks from now. Instead, you might see a general trend towards wetter conditions in one particular week or a likely warm period compared to normal.
Don’t Expect Detail, Embrace the concept of Probability forecasting: These charts help you see if there’s a strong signal for conditions to be notably different than usual or if it’s all rather uncertain.

Who is this chart most useful for?
For farmers, graziers, large scale structural project managers and for tourism operators knowing what might happen weeks in advance can help with planning. For example:

Crop Planning: If a drier-than-average period is likely in three weeks, you might adjust irrigation plans or planting schedules.
Livestock Management: Warmer-than-usual weeks might mean earlier preparation for heat stress in animals, or planning when to move them to different pastures.
Resource Allocation: If it looks like an unusually wet period is on the horizon, you might prepare for potential flooding or take steps to protect crops.
Structural project managers - can strategically allocate their resources/man hours and plan/schedule certain outdoor weather dependant parts of the prject on weeks that look drier than the climatological norm.

In short, the 45-day meteogram helps users think ahead in a general sense, rather than just reacting to changes day-by-day.

From the Mackay example above

Pressure is much lower than climatology in the week ending 23rd December: We see excellent model agreement that the week ending the 23rd December will see well below average surface pressures (MSLP) likely indicating significant troughing, a LOW or Tropical Cyclone could impact the location. We also see some scenarios later int he forecast around mid January where pressures could be much lower than normal signifying a return to troughing/LOW or potentially some TC risks although the uncertainty for mid January is high as shown by the length of the box and whiskers.
Rainfall is high in week 1 compared to the expected climatology: There is a high degree of confidence that Mackay will see significantly more rain (about a median value of 100mm more rain) than the climatological average for this week based on the past 20 seasons. We see a return to mostly near normal rainfall patterns after week 1 with a possible rainy period redeveloping in week 6.

Part D: The 10-Day AI Meteogram

What is the 10-Day AI Meteogram?
This is a newer type of product that uses artificial intelligence to blend together various weather models and historical data to provide a forecast. It covers a similar time frame as the standard 10-day meteogram but uses advanced techniques to estimate where the weather is heading.

How to Interpret the AI Meteogram:

Similar Symbols, Similar Meaning: You’ll still see boxes, whiskers, and median lines. These represent a range of outcomes and the middle-ground forecast.
Confidence and Uncertainty: Just like before, tighter clusters suggest higher confidence. The presence of AI doesn’t mean perfect forecasts—it just means there’s another layer of analysis behind the scenes.

Why Consider the AI Version?
The AI meteogram might sometimes show narrower boxes if the algorithm has found patterns that increase confidence. On the other hand, if conditions are tricky, it might look similar to standard meteograms. For beginners, treat it like any other meteogram—just another forecast tool to help guide your decisions. For advanced users, look for the AI Meteogram to reinforce or perhaps discredit the traditional model meteograms to increase or decrease your confidence in a particular forecast outcome.

In the Mackay Example above

The AI ensemble above gives me more confidence of Mackay's rainfall beginning Tuesday, peaking on Thursday/Friday and ending on Saturday morning because it reinforces the 10 day Meteogram's from Part A of this blog.
The AI ensemble above helps reinforce confidence from part A of this blog that Mackay's winds will pick up from Thursday and remain fresh until at least Saturday.
The AI ensemble above tells me that Mackay's MSLP will be quite low over the next few days indicating a slow moving trough or LOW in the area, followed by a rise in pressure on Friday to Sunday showing me that a ridge builds after the trough or LOW goes away but that the ridge doesn't last very long because we then see a little sharp pressure drop again on Monday potentially indicating a fast moving trough coming through on Monday before much stronger ridging establishes in time for a beautiful fine Christmas.

Some Limitations of the AI Meteogram
1 - Forecast resolution - the ensemble uses a primitive coarse grid of 100km this is a much larger horizontal grid than the other ensembles. So we're not using AI ensembles for accurate point rainfall estimates - but we can still use it for trend, general area wide rainfall and certainty/uncertainty forecasting.

2 - 10 day 12 hourly timesteps - unlike the actual ensemble 10 day meteogram which utilises 6 hourly timesteps the AI modelling only shows 12 hourly timesteps making it less useful compared to the current NWP ensemble offered by the ECMWF for things like event planning etc

Why the Boxes and Whiskers Matter

In all these meteograms, the boxes and whiskers are crucial because they visually represent forecast uncertainty. Weather models don’t produce a single “yes or no” outcome. Instead, meteorologists run multiple versions (ensembles) of the model with slightly different starting points or assumptions. The range of these results forms the “spread” of possible outcomes.

Boxes (Interquartile Ranges): Show where the central bulk of these “what if” forecasts lie.
Whiskers: Show the extremes, so you know the wild cards.
Median Lines: Show the midpoint, giving you a sense of the most common or “middle” expectation.

This statistical approach helps you understand not just what could happen, but how likely each scenario might be. Think of it like planning a trip: you might expect 25°C, but you know it might be as hot as 30°C or as cool as 20°C, depending on how things evolve. The meteogram shows you these possibilities at a glance.

Understanding the Time Axis on Meteograms

The time axis, runs along the bottom (x-axis) of the chart.

UTC (Coordinated Universal Time):
All times shown on these meteograms are typically given in UTC. This is a standard global time reference that doesn’t change with local daylight savings or your local time zone. If you need to relate it to your local time, you’ll have to adjust by adding or subtracting your local time difference. In QLD for example the meteorological day starts at 10AM or 00z and then key points of reference are 06z which is 4PM, 12z which is 10PM, 18z which is 4AM.

Breaking the Forecast into 6-Hour Chunks:
For a 10-day meteogram, the forecast is often broken down into 6-hour segments—four chunks per day. These might be at meteorological midnight (00 UTC - which remember was 10AM QLD time), early morning in meteorology time (06 UTC or 4PM in QLD time), midday of the meteorological day (12 UTC - which remember is 10PM QLD time), and early evening meteorological time (18 UTC - which is 4AM in QLD time). Each point on the meteogram usually represents conditions for the previous 6-hour period, not just a single moment. For example, a point at 12 UTC might show data averaged or forecasted over the time from 06 UTC (4PM QLD time) to 12 UTC (10PM QLD time).

Why Break It Down This Way?

Smoother Trends: By grouping the forecast into 6-hour periods, you’re seeing a blend or summary of conditions rather than just one moment’s measurement. This can make it easier to spot general trends—like warming afternoons or overnight rain—without getting lost in tiny fluctuations.
Consistency Across the Chart: Using uniform 6-hour segments across the entire 10-day period makes it easier to compare conditions day to day. Each day is divided the same way, which helps you quickly scan across the chart and see how weather might evolve over time.

When you look at 15-day or 45-day meteograms, the same principle applies, although sometimes the time intervals are shown differently or in larger blocks (like daily on the 15 day or weekly averages for the 45 day forecasts). The main idea is that the x-axis always moves forward in time, showing you how weather predictions stack up as you go further into the future.

Wrapping Up

Meteograms—from the 10-day all the way to the 45-day and AI-based versions—are powerful tools for growing your Weather IQ. They help everyone from casual weather-watchers to farmers make informed decisions. The key is learning to read the boxes and whiskers and understanding that these charts show not just a forecast, but the range of possible futures.

As you explore the ECMWF’s meteograms at www.ecmwf.int or through our “State Pages” subscription service, remember: focus on trends, uncertainty, and likelihood rather than just one specific number. With practice, you’ll become more comfortable using these tools for short-term planning and long-term strategies.

For Eyewall Pro users, this article will be included in our upcoming Weather Forecasting booklet which is scheduled for release Dry Season 2025. The booklet will combine how to use models, indices, model strong points/weak points and some cheat codes on how to use the Weather Centre in combination with other analytical tools to create forecasts you can be confident in. Only Eyewall Pro Users will have access to this booklet once it is released.

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