Why Do Weather Apps Disagree—and Why Does the Forecast Keep Changing?

You check one weather app and it says rain tomorrow. Another says partly cloudy. A third gives you a 60% chance of showers, while the first has quietly changed its forecast since you looked at it this morning.

It can feel as though somebody must be getting the weather wrong. But disagreement between forecasts—and forecasts that change as a day gets closer—is a normal part of predicting something as complicated as the atmosphere.

Weather apps don't all start with exactly the same information, use the same forecasting models or update at the same time. And even when they do use some of the same data, they may interpret and present it differently.

The result is something we've all seen: two perfectly respectable weather apps telling you two different things.

Weather Apps Aren't Actually Predicting the Weather Themselves

Most weather apps aren't independently running their own complete global forecasting systems. They rely on weather data and forecasts produced by government agencies, private weather companies, computer models and other sources.

Those sources can include surface weather stations, satellites, radar, weather balloons, aircraft, ships and ocean buoys. All of those observations help describe what the atmosphere is doing right now. The technology behind those observations has changed enormously over time, as weather forecasting evolved from telegraphs to satellites, computer models and AI.

Computer models then use that starting information to calculate how the atmosphere is likely to change. But there isn't just one weather model.

There are many.

Different Weather Models Can Produce Different Answers

You may have heard forecasters talk about the “American model” or the “European model,” particularly when a hurricane or major winter storm is approaching.

These are different numerical weather prediction systems. They use mathematical equations to simulate the atmosphere, but they don't necessarily divide the atmosphere into the same size grid cells, process observations in exactly the same way or represent clouds, precipitation and other atmospheric processes identically.

Small differences at the beginning of a forecast can become larger as the models project farther into the future. That's one reason there are practical limits to how far ahead we can reliably predict the weather.

One model might predict that a storm will pass directly over your town on Saturday afternoon. Another might place it 75 miles north. A third might slow the system down enough that most of the rain doesn't arrive until Saturday night.

To you, those differences could mean the difference between “rain,” “cloudy” and “mostly dry” on your phone.

Your Weather App Has to Turn a Complicated Forecast Into an Icon

Behind that little rain cloud on your screen is much more information than the icon can possibly show.

A forecast may contain probabilities, expected rainfall amounts, temperature ranges, wind speeds and uncertainty about the timing and location of precipitation. An app has to convert all of that into something you can understand at a glance.

Different apps may do that differently.

Suppose scattered thunderstorms are possible during a six-hour period. One app might display a thunderstorm icon for the entire afternoon. Another might show partly cloudy conditions with a 40% chance of rain. Yet another might show rain only during the hour when its data suggests storms are most likely.

The underlying forecasts may not actually be as different as they appear.

Why Does the Forecast Keep Changing?

Because the atmosphere keeps changing—and meteorologists keep getting new information about it.

Weather forecasting is an ongoing process. Satellites continue observing clouds and moisture. Weather stations report changing conditions. Balloons sample the atmosphere above us. Radar tracks developing precipitation. Aircraft collect additional atmospheric measurements.

Computer models are then run again using updated information.

Imagine that yesterday's forecast predicted a storm would pass just north of your area. New observations today may show the system developing slightly farther south than expected. Put that new information into a model, and tomorrow's forecast for your town may suddenly become wetter.

That's not necessarily evidence that the earlier forecast was bad. It's often evidence that the newer forecast has more information.

Update Times Matter Too

Two apps can disagree simply because one is working with newer information.

Major weather models are run on schedules, and the data they produce doesn't instantly appear everywhere. Weather services need time to process model output, combine it with other information and distribute updated forecasts. Apps then have their own schedules for refreshing what you see.

So at a particular moment, one app may have incorporated a newer model run while another is still displaying an earlier forecast.

Check again later and the two may suddenly look much more alike.

Why Forecasts Change More Several Days Out

The farther ahead you're looking, the more opportunity there is for small uncertainties to grow.

The atmosphere is a chaotic system, meaning tiny differences in its starting conditions can eventually produce substantially different outcomes. Meteorologists can measure the atmosphere extraordinarily well, but they can't measure the temperature, pressure, humidity and motion of every bit of air everywhere on Earth.

A small uncertainty that barely matters in tomorrow's forecast may matter considerably a week from now.

This is why a forecast seven or ten days away can bounce around. The app isn't necessarily malfunctioning when Saturday changes from sunny to rainy and back to partly cloudy over several days. Different model runs may be exploring slightly different ways the atmosphere could develop.

As Saturday gets closer, those possibilities usually narrow.

Why Rain Is Especially Difficult

Temperature forecasts can change too, but rainfall often produces the most noticeable disagreements.

That's partly because rain can be extremely local. A large, steady storm covering several states may be relatively straightforward to forecast. Scattered summer thunderstorms are another matter entirely.

Meteorologists may be confident that thunderstorms will develop somewhere in a region but much less certain about exactly where individual storms will form.

If one develops over your neighborhood, you might receive an inch of rain while a location a few miles away gets almost nothing. In fact, rainfall can vary dramatically over surprisingly short distances. Shift the predicted storm track slightly in a computer model and your app's forecast can change dramatically.

That doesn't necessarily mean the entire weather prediction changed. Sometimes your location simply moved from one side of an uncertain boundary to the other.

Your Exact Location Can Make a Difference

Weather apps also have to translate forecasts covering an area into a forecast for the little blue dot representing you.

Computer weather models divide the atmosphere into grids. Even sophisticated models aren't literally predicting conditions independently for every backyard, street or house.

Local geography complicates things further. Elevation, mountains, valleys, coastlines, large lakes and urban areas can all influence weather. Rain shadows are one example of how terrain can influence where rain falls. Two nearby locations can experience different temperatures, winds and rainfall even when they're represented by similar regional forecasts.

This is one reason your weather app can be very useful without being a perfect description of what's happening outside your window.

Which Weather App Is Right?

Sometimes one app really will be closer than another. But a single day's result doesn't necessarily tell you which service is consistently better.

If App A predicts rain and App B predicts no rain, and your house stays dry, it may appear that App B “won.” Meanwhile, a thunderstorm may have dropped an inch of rain five miles away.

For everyday planning, it can be more useful to look beyond the main weather icon. Check the probability of precipitation, expected timing, rainfall amount and hourly forecast. When storms are already developing, radar can provide valuable information about what's actually happening nearby. If you've ever wondered what those radar rainfall estimates actually mean, here's how weather radar estimates how much rain is falling.

For potentially dangerous weather, watches and warnings from official weather agencies matter more than whether your favorite app shows a sun or cloud icon.

A Changing Forecast Can Be a Better Forecast

It would certainly be convenient if a weather forecast issued on Monday remained unchanged through Saturday. But that's not how good forecasting works.

New observations arrive. Storms develop differently than expected. Computer models are rerun. Uncertainty decreases. A responsible forecast changes when the evidence changes.

In fact, an app that stubbornly kept displaying an old forecast despite new information wouldn't be more reliable. It would simply be out of date.

The closer you get to the day you're interested in, the more attention you can generally pay to the details. Several days out, think in terms of overall patterns and possibilities. The night before and morning of an event, updated hourly forecasts and radar become much more useful.

And once the storm is over, even the best weather app still can't tell you everything that happened in your particular yard. Radar may estimate rainfall and a nearby weather station may report a total, but rain can vary considerably over surprisingly short distances. That's why your rain gauge may not match your weather app—and why both measurements can still be useful.

That's where an old-fashioned measurement still has a job. The forecast tells you what meteorologists think may happen. A rain gauge tells you how much rain actually fell right where you live.