Weather forecasts change because new information about the atmosphere is constantly becoming available. Meteorologists regularly update forecasts using observations from weather stations, satellites, radar, weather balloons, aircraft, and other sources, while computer weather models are rerun using the latest data.
The atmosphere is also constantly changing. Small differences in temperature, pressure, moisture, or wind can affect how a weather system develops, particularly several days into the future. As an event gets closer, meteorologists usually have better information and can narrow down the most likely outcome.
A changing forecast does not necessarily mean the original forecast was wrong. Often, it means the forecast is becoming more precise.
How Often Are Weather Forecasts Updated?
Weather forecasts can be updated several times per day as new observations and model guidance become available.
Computer weather models are run on regular schedules. Each new model run begins with updated observations of the atmosphere and calculates how conditions are expected to develop.
Meteorologists compare these new results with previous forecasts, other models, current observations, and their own analysis before making adjustments.
Read More: How Do Weather Models Work?
Why Can Tomorrow’s Forecast Suddenly Change?
Even short-range forecasts can change when the atmosphere develops differently than expected.
For example, a storm may move slightly farther north than originally predicted. That relatively small shift could put a location into a different part of the storm, changing the expected temperature, wind, precipitation, or snowfall.
This is especially important near boundaries between different conditions. During a winter storm, a few degrees of temperature difference could help determine whether precipitation falls as snow, freezing rain, ice pellets, or rain.
The closer your location is to one of these boundaries, the more likely the forecast is to change as meteorologists determine where it will actually set up.
Why Do Forecasts Change More Several Days Away?
Forecast uncertainty increases the further into the future meteorologists try to predict.
Weather models start with the best available picture of the atmosphere, but it is impossible to measure atmospheric conditions at every location and altitude. Small uncertainties in those starting conditions can grow as a model calculates further into the future.
As a result, tomorrow’s forecast is generally much more dependable than one for 7 or 10 days from now.
A storm appearing in a forecast a week away may eventually arrive earlier, later, farther north or south, or with a different intensity than initially predicted.
Read More: How Far in Advance Can Weather Be Predicted?
Why Do Weather Models Disagree?
Different weather models do not always predict the same outcome.
Models can differ in how they process observations, represent atmospheric processes, divide the atmosphere into grids, and perform their calculations. These differences can result in separate predictions for the same weather system.
For example, one model may predict that a storm passes directly over a region while another places it farther south. Those different tracks could produce completely different local forecasts.
Meteorologists therefore look at multiple models and ensemble forecasts rather than assuming one model will always be correct.
Why Does the Chance of Rain Keep Changing?
Precipitation probabilities can change as meteorologists gain confidence about where and when precipitation will develop.
A forecast showing a 30% chance of rain several days away might increase to 70% as models increasingly agree that rain will affect the area. It can also move in the opposite direction if newer information suggests the system will miss the location.
The same applies to snow and other precipitation. The expected timing, amount, and location become clearer as the event approaches.
Why Do Snow Forecasts Change So Much?
Snow forecasts are particularly sensitive to changes in storm track, temperature, moisture, wind, and precipitation intensity.
A small shift in a winter storm can move a heavy snow band into or away from a community. Temperature changes can also turn expected snow into freezing rain, ice pellets, or rain.
This is why snowfall totals several days in advance should be treated as preliminary. Expected accumulation usually becomes more reliable as the storm gets closer.
Read More: How Do Meteorologists Predict Snow?
Why Do Different Weather Apps Have Different Forecasts?
Two weather apps can show different forecasts because they may not use the same weather models, data sources, update schedules, or forecasting methods.
One service might predict rain while another shows cloudy conditions because each is interpreting the available data differently.
Differences between forecasts tend to become more noticeable further into the future, when there is greater uncertainty about how the atmosphere will develop.
Does a Changing Forecast Mean Meteorologists Got It Wrong?
Not necessarily.
A forecast represents the most likely weather based on the information available at that time. When new observations show that the atmosphere is developing differently, updating the forecast is more useful than continuing to publish an older prediction.
Forecasts should therefore be viewed as evolving rather than fixed.
This is particularly important several days before a major storm. Early forecasts can identify the potential for significant weather, while later forecasts determine the track, timing, precipitation type, and expected impacts with greater confidence.
Read More: How Accurate Are Weather Forecasts?
When Should You Check the Forecast?
For everyday planning, checking the forecast several days ahead can give you a good idea of the expected weather pattern. Check again the day before and, when timing matters, on the day itself.
For potentially hazardous weather, forecasts should be checked more frequently as the event approaches. Official weather watches, warnings, and advisories should also take priority when severe or dangerous conditions are expected.
The Bottom Line
Weather forecasts change because the atmosphere changes and the information available to meteorologists improves over time. New observations are collected, weather models are rerun, and predictions are adjusted as the most likely outcome becomes clearer.
