Peek down a few more lines of code.
// overrides from Settings.php if available
global $SITE;
if(isset($SITE['cacheFileDir'])) { $cacheFileDir = $SITE['cacheFileDir']; }
$outFileName = $cacheFileDir.$cacheName;
if(isset($SITE['realtimefile'])) { $outFileName = $SITE['realtimefile']; }
If you have "realtimefile" set in the $SITE array (in Settings-weather.php), the "MBconditions.txt" file name never gets used.