System design diagram for the question:

here is a general outline of the steps that you might follow to write a code for a weather forecasting system in an object$-$oriented language:

Define the classes that will be used to represent the different components of the system. This might include classes for representing locations, weather data, and forecasts.

Write the code for each class, including the attributes $($data fields$)$ and methods $($functions$)$ that will be used to represent and manipulate the data.

Define any relationships between the classes, such as inheritance or aggregation, as needed.

Write the code to implement the algorithms and logic that will be used to analyze the data and make forecasts. This might include code to retrieve data from a database, process the data, and generate a forecast.

Write the code to create the GUI and handle user input. This might include code to display the forecast data to the user, as well as code to handle user actions such as login or entering location information.

Test and debug the code to ensure that it is functioning correctly.

This is just a general outline, and the specific code for your weather forecasting system will depend on the requirements and design of the system.