The easy way (which will generate $WX[] variables) is:
1) edit WeatherCat-webtags.txt (it's in the WCT-plugin.zip distribution).
It is TAB delimited like
# 71392 bytes in WeatherCat_all_tags.html read in.
# WeatherCat-tags.txt generated by gen-wct-alltags.php
wvar CT$ Current temperature, temperature units and hourly change in brackets.
wvar CD$ Current dew point, dew point units and hourly change in brackets.
wvar CWC$ Current wind chill, wind chill units and hourly change in brackets.
wvar CX$ Current wind direction and units.
wvar CW$ Current average wind speed and units.
Add new lines to the file (near the beginning if you like) that have
wvar <tab> <WeatherCat-variable-name> <tab> <description of the variable>
Save the file and upload to your site.
2) run gen-WCTtags.php on your site. Save the output file to replace your current template for WCTtags.php in WeatherCat.
3) use the newly defined variables as $WX['<variable-name-minus-trailing-$>'] in your PHP site on any wx....php page.
See
http://saratoga-weather.org/wxtemplates/maint.php for additional info.
Best regards,
Ken