There are many ways that we forecast daily and weekly temperatures. Many factors come into play. The first things to look at are obviously climatological factors and what season it is! In some areas (for example tropical climates) persistance forecasting works very well. Since temperatures do not vary that greatly in those climates, there is a good chance that the temperature will be the same as the day before (unless another factor comes into play, for example rain or more cloud cover). As far as Mid-Latitude climates, the temperature is based on whats going on with the weather. Is there a cold front coming through? Cloud cover can play a big role also. If the day starts off with overcast skies, the daily max temp will usually be lower than if there were clear skies. This is because there is less sunlight and hence heat reaching the ground. HOWEVER, if the day starts off clear, then becomes overcast at night, the minimum temperature may be higher than normal. This is because all the heat that reached the ground during the day cannot as easily escapr through the clouds at night. It is essentially "trapped". Rain can lower temperature as well. the temp tends to dip a few degrees during a shower. This is because evaporation is a cooling process. A good way to think of this is when you sweat. The actual act of a person sweating is not what cools off your body. It is when the sweat begins to evaporate, then your body will cool down. So basically many factors come into play when forecasting a temperature. and it honestly can be one of the hardest elements to forecast! I hope this helped a bit!