As you can see in the XML Code the data for forecast is a datastring divided by HTML-Tags
<b>Forecast:</b>
Tue - Mostly Sunny. High: 77 Low: 55
Wed - Partly Cloudy. High: 68 Low: 48
Thu - Partly Cloudy. High: 66 Low: 50
Fri - Mostly Sunny. High: 68 Low: 48
Sat - Sunny. High: 69 Low: 47
Therefore it isn't possible to retrieve a value for HIGH by using finder.getValue()
It needs a more complex function to read the single values.
In this case I would use a PHP-Script which reads the data and puts them in a simple xml stream. In the second step you can use your Arduino board to read the output of the xml stream and display them in the serial monitor or on a display.