Hi Tom,
You have that prtoblem because an exceptional situation occur
in your station. Usually, forecast are from 0 to 26, but in rare
situation, station could give negative value (-1 or -26) -1 stand
for 'exceptional' good weather and -26 for 'excepional' worst
weather.
So, you could simply strech your icons set by taking 1.png file
and copy it to -1.png and same thing with 26.png icon.
Hope this help!
Regards,
Jacques