How can you make a weather prediction with justification based upon the weather data from the 3 days previous?

What you will look for are clues as to what is going on and experience from the area you are forecasting. Here are some clues that may help you make an educated guess as to what may be occurring.

1. Temperature. If it is higher than normal...you are more likely to be under a high pressure ridge. If it is lower than normal...you are more likely be under a low pressure trough.

2. Barometric Pressure will tell you pressure changes. U:nder a normal day, gradual pressure measurements will rise and fall normally even when there is no significant changes in the weather. What is important is when you detect faster or slower change in daily pressure. The greater rate in change with time, the better chance you will experience a change in weather. For example: if the pressure suddenly changes rapidly on day three and it is falling, expect weather to change and become more unstable on day 4.

3. Wind speed has almost a direct relationship with pressure gradient (the higher the pressure gradient...the more difference in surface pressure you will experience with a given distance). This will indicate that things maybe becomeing more unstable. Also, knowing the fact that wind blows from areas of higher pressure to lower pressure would give you a clue as to how the pressure fields are positioned in your surrounding areas.

4. Cloud type, level, and amount per time will give you important information on moisture available and the stability of the air mass above you. If clouds are there during the day and they are low to medium level clouds, then the airmass is somewhat unstable in the afternoon and evening and that some low level moisture is available. If precipitation forms, it will likely be in a showery format. If the cloud completely fills the sky and the bases of the clouds are low to mid level, if precipitation forms, it will tend to be a steady type of rain that can occur for long periods of time. If they are just high clouds, it may be fair weather. If the high clouds are invading the sky, this may indicate a system coming in soon. If low and/or middle level clouds are present, then there is a possiblility that there is enough moisture available for precipitation to form is the air mass becpmes unstable enough with time..

These are some clues that can tell you what may be going on around you. There are much more clues to look for that I did not list above. By looking at the changes that had occurred over a 3 day period, and using the combination of clues you are getting from the weather observations, you can make an educated guess that will likely be more accurate then a random guess. The important thing is that the guess you will make must be made using all the available data and that the changes you are forecasting will match the changes you are seeing with the three days worth of data.

Most of this clues I have learned comes comes from many years of experience so it would be tough to teach with one writing on Yahoo Answers. Good luck!